Ridge filters and methods for designing ridge filters in PBS treatment systems

Abstract

The present invention relates to a method for designing a ridge filter for a charged particle accelerator, the ridge filter being used to deposit a specific dose (specific dose) into a specific position within a treatment zone of tissue containing tumor cells by means of an accelerated particle beam according to a predetermined treatment plan by means of a single-layer pencil beam scanning, the method comprising the following steps: defining a spot array defining a base of a cylindrical sub-zone defining the treatment zone; the sub-zone being divided into N cells. The ridge filter is designed to include the same number of energy degradation units as the spots. Each energy degradation unit is formed by N cylindrical sub-degradation units having a length and an area. The length (Lij) of each sub-degradation unit (11.ij) is calculated as Lij=Wij / Wu, and Wij=W0-dij, wherein Wij is the required sub-unit water equivalent thickness (Wij), Wu is the sub-unit water equivalent thickness per unit length (Wu), W0 is the maximum beam range, and dij is the required position of the Bragg peak along the irradiation axis (X).

Description

Field of the Invention

[0001] The present invention relates to a ridge filter for depositing a predetermined dose (Dij) over the entire treatment volume (V) by irradiating the entire treatment volume (V) with an accelerated particle (preferably proton) beam by pencil beam scanning (PBS) in a single plotting layer. In particular, the present invention relates to a method for designing such a ridge filter, optimizing its dimensions to accurately deposit the dose (Dij) according to a pre-established treatment plan. The ridge filter of the present invention is particularly suitable for flash irradiation of a treatment volume (V) or a portion thereof with an ultra-high dose deposition rate (HDR) by PBS. Background of the Invention

[0002] Radiation therapy using particles or waves (such as electron beams, proton beams, heavy ion beams, X-rays, gamma rays, etc.) has become an essential tool for treating cancer patients.

[0003] Since both tumor cells and healthy cells contained in the zone are damaged by such radiation, the first challenge in cancer treatment is to define the treatment plan to ensure that a defined dose is deposited into tumor cells to effectively destroy or kill the tumor cells, while limiting the dose deposited into healthy cells to minimize harm to healthy cells. The second challenge is to actually deposit the defined dose into tumor cells, while actually depositing a limited dose into healthy cells, especially those adjacent to tumor cells.

[0004] The treatment plan must ensure that at the end of treatment, a total target dose sufficient to kill the tumor cells is delivered to the region while minimizing the degradation of healthy cells adjacent to the tumor cells. Different radiations deposit their energy in different patterns. For example, X-rays deposit most of their energy near the skin level, and the deposited energy decreases as the penetration depth into the tissue increases. Therefore, healthy tissue located upstream of the tumor cell target region receives a higher dose than the tumor cells in the target region. In contrast, as shown in Figures 2(a) and 2(b), charged particle beams, particularly protons, deposit most of their energy near the end of their beam path, forming the so-called Bragg peak.

[0005] Pencil beam scanning (PBS) is a technique that involves steering a charged particle beam along a corresponding beam axis (Xi) toward individual spots of a grid of spots (Sij) defining a target area including tumor cells. Thus, a predetermined target dose is deposited in cells aligned with each spot. The beam is steered along the corresponding beam axis (Xi), and the dose deposition is performed according to a treatment plan that defines the dose (Dij) to be deposited into each cell aligned with a given spot along the beam axis (Xi), and the scanning sequence of the spot irradiation. PBS reduces unnecessary radiation exposure to surrounding non-cancerous cells by shaping the treated area to reflect the geometry of the tumor. In addition to the geometry of the target, PBS also allows for local adjustment of the intensity of the beam according to the location of the spot within the target.

[0006] The grid usually comprises several plotted layers (Tj=T1 to TN) perpendicular to the irradiation axis (X) with the beam axis (Xi) centered on it. The spots (Sij, Si(j+1)...) are arranged in a 2D array on the corresponding upstream plane of each layer (Tj). Each plotted layer defines a number of cells (Cij, C(i+1)j...), which are defined as generalized cylinders with the corresponding spot (Sij) as the base and the generatrix parallel to the corresponding beam axis (Xi). The cell (Cij) has the same thickness as the corresponding layer (Tj). The superposition of the plotted layers (Tj) defines the entire treatment zone (V). The spots (Sij) of the upstream layer (Tj) are not necessarily aligned along the corresponding beam axis (Xi) with the corresponding spots (Si(j+1), Si(j+2)...) of the downstream layer (T(j+1), T(j+2)...).

[0007] The terms "upstream" and "downstream" are defined relative to the direction of the charged particle beam (100.i). Unless otherwise specified, the terms "generalized cylinder", "cylinder" and their derivatives herein refer to a surface consisting of all points on all lines parallel to the busbar and passing through the periphery of the substrate, the substrate being contained in a plane not parallel to the busbar. The substrate can have any planar geometry. In the special case where the substrate is circular, the substrate defines a cylinder. If the substrate is a polygon, the substrate forms a prism. A positive cylinder is a cylinder whose base is perpendicular to the busbar.

[0008] PBS is very advantageous because it optimizes the geometric distribution of dose deposition to match the geometry of the treatment volume (V) surrounding the tumor. However, PBS can be long because the beam must scan each spot (Sij) and each layer (Tj). It takes several milliseconds to move the beam from one beam axis (Xi) to another beam axis (X(i+1)). Changing the energy of a given beam parallel to a given beam axis (Xi) to deposit the desired dose (Dij) into cells (Cij) in different layers (Tj) is even more time consuming, requiring approximately 500 milliseconds. Therefore, the number of layers (Tj) has a significant impact on the duration of treatment.

[0009] In the case of an accelerated proton beam, a given beam parallel to a given beam axis (Xi) can sequentially deposit predetermined charges into corresponding cells (Cij) aligned along the given beam axis (Xi) of each drawing layer by superimposing several Bragg peaks staggered in depth at each drawing layer along the beam axis (Xi). This results in a spread Bragg peak (SOBP) extending over all cells (Cij, Ci(j+1)...) aligned along the given beam axis (Xi). However, this operation requires sequentially varying the energy of the given beam so that the corresponding Bragg peak is centered on the corresponding cell (Cij). This operation is time consuming. Just like drawing, applying several layers or sequentially varying the energy of a given beam is time consuming, and it is better to be able to deposit all doses (Dij) on all layers with one beam of constant energy along the axial beam (Xi) centered on a given spot (Sij), i.e., applying a single drawing layer.

[0010] Saving treatment time reduces the time each patient occupies the particle accelerator. It is also more comfortable for patients. It is also advantageous that the treatment plan includes FLASH irradiation, in which the dose is deposited into the cells at an ultra-high deposition rate (HDR) of at least 1 Gy / s. Depositing a given dose with HDR has been shown to not harm healthy cells compared to depositing the same dose at a lower deposition rate (CDR). What is particularly noteworthy about FLASH irradiation is that a given dose deposited into tumor cells has the same killing effect, whether deposited with HDR or CDR. However, depositing the dose (Dij) layer by layer into the treatment zone (V) by PBS significantly reduces the rate of deposition into the cells, because cells located upstream of the zone (V) must be repeatedly irradiated several times until all cells (Cij) aligned along a given beam axis (Xi) have received their corresponding doses (Dij).

[0011] The use of a ridge filter allows the deposition of a predetermined dose (Dij) into the treatment area through a single layer of PBS. The ridge filter requires that the spots (Sij, Si(j+1)...) of each layer (Tj) be aligned along the corresponding beam axis (Xi). Ridge filters comprising energy degradation units in the form of smooth or stepped pyramids or crests have been described in the art. For example, Simeonov et al. describe a ridge filter in Phys. Med. Biol. 62 (2017) 7075, which comprises a plurality of energy degradation units in the form of smooth pyramids extending along the beam axis (Xi) corresponding to each spot (Sij). Sakae et al. describe a multilayer ridge filter in Med. Phys. 27, 2, (2000) 368, in which the layers are stacked one on top of the other, and each layer comprises parallel linear crests having a stepped pyramidal cross-section. JP 2019136167 describes a ridge filter comprising a conical or triangular pyramidal protrusion in the form of a plate positioned upstream of a Bragg peak expansion filter, the plate comprising a plurality of through holes for increasing the width of the SOBP along the corresponding beam axis (Xi).

[0012] The principle of the ridge filter is that part of the beam (100.i) with a given energy directed along the corresponding beam axis (Xi) passes through different material thicknesses of the filter, generating Bragg peaks with different ranges, the superposition of which leads to a uniform SOBP that extends along the corresponding beam axis (Xi) over the columnar area defined by the spot (Si1) in the first layer (T1) across the entire depth of the treatment zone (V) to the corresponding spot (SiN) in the last layer (TN) downstream of the first layer (T1).

[0013] The design and sizing of the energy degradation units of ridge filters remains a challenge. Simeonov et al. describe a complex method for sizing the pyramidal protrusions that form ridge filters, which involves adapting the function (DSOBP-D(z)) 2 minimized, where DSOBP defines a uniform dose distribution, and where B(z) is the measured raw Bragg peak, Δz is the step size between successive Bragg peaks, the weights ωi determine the contribution of each peak i to the SOBP, and D(z) is the resulting depth dose distribution. The solution of this equation requires an empirical fit function. Sakae et al. do not provide much information about how they designed their ridge filter other than using Monte Carlo calculations.

[0014] Therefore, there is a need for a simple, reliable and repeatable method for determining the size of the energy degradation unit of a ridge filter. An alternative ridge filter design with the required accuracy that is easier to produce is proposed. These and other advantages will be described in more detail below. SUMMARY OF THE INVENTION

[0015] The present invention relates to a method for designing a ridge filter for a treatment system, the treatment system comprising a charged particle accelerator (preferably a proton accelerator), a beam line nozzle, a collimator, etc., collectively referred to herein as an accelerator, the ridge filter being used to deposit a specific dose (Dij) to a specific location within a treatment area of ​​tissue comprising tumor cells by means of an accelerated particle beam. The present invention relates to deposition by spot-by-spot pencil beam scanning (PBS) according to a predetermined treatment plan (TP) in a single mapping layer defining the entire treatment area. The beam extends along a corresponding beam axis (Xi) that is substantially parallel to the irradiation axis (X), the deviation from parallelism to the irradiation axis (X) being an angle comprised within ±5°, preferably within ±3°. The tissue is characterized by a maximum beam range (W0), which is defined as the water equivalent distance at which the beam stops propagating through the tissue. The method comprises the following steps.

[0016] The expression "water equivalent thickness" (=WET) is well known in the art and is defined as the thickness of water that causes the same energy degradation of a particle beam as the passage of the particle beam through one or more materials of a given thickness.

[0017] The boundary inscribed in the treatment zone is defined by defining an area (Aj) on an upstream plane (Y,Z)j of N slices (Tj=T1-TN) with thickness (dxj), wherein the plane (Y,Z)j is perpendicular to the irradiation axis (X). The shortest water equivalent thickness (d0) and the longest water equivalent thickness (d1) to the patient's skin are defined as the points of the boundary closest to and farthest from the skin, respectively, measured along the irradiation axis (X).

[0018] An array of sub-zones (Vi) is defined, each sub-zone extending from the patient's skin to the corresponding farthest water equivalent thickness parallel to the corresponding beam axis (Xi), and the projection of each sub-zone on a plane (Y, Z) perpendicular to the irradiation axis (X) defines a spot array covering the entire area of ​​the projection of the zone on the plane (Y, Z).

[0019] For each slice (Tj) of the N slices (T1-TN) contained in the sub-region (Vi), the cell is defined as a portion of the sub-region (Vi) contained in the corresponding slice (Tj). For each cell of a given sub-region (Vi), the cell water equivalent thickness from the skin to the geometric center of the cell is determined. The beam weight (ωij) is attributed to each cell a specific dose (Dij) that needs to be deposited into the cell according to the TP. The beam weight (ωij) is proportional to the number of charged particles under the cell water equivalent thickness.

[0020] The ridge filter can be designed to have an energy degradation unit group, wherein each energy degradation unit is configured to reduce the initial energy (E0) of a corresponding charged particle beam having a beam diameter coaxial with the corresponding beam axis (Xi) and the sub-region (Vi) to reduce the energy (Eij) so as to deposit a specific dose (Dij) into the corresponding cell contained in the sub-region (Vi) with a cell water equivalent thickness according to TP. The energy degradation unit of a given sub-region (Vi) is designed as follows.

[0021] For each cell of the sub-region (Vi), the size of the sub-degradation unit is determined to have a generalized cylindrical geometry, which has a base with an area (Aij) perpendicular to the corresponding beam axis (Xi) and a generatrix with a length (Lij) parallel to the corresponding beam axis (Xi). The sub-degradation unit is made of a material having a sub-unit water equivalent thickness (Wu) per unit length along the corresponding beam axis (Xi), and wherein the length is determined such that the sub-unit water equivalent thickness (Wij=Wu xLij) of the sub-degradation unit is equal to the product of the sub-unit water equivalent thickness (Wu) per unit length and the length (Lij). The sum of the sub-unit water equivalent thickness (Wij) and the cell water equivalent thickness (dij) is equal to the maximum beam range (W0) (i.e., W0=Wij+dij).

[0022] As defined above, the terms "generalized cylinder", "cylinder" and their derivatives herein refer to a surface consisting of all points on all lines parallel to the generatrix and passing through the perimeter of the base, the base being contained in a plane not parallel to the generatrix. The base may have any planar geometry. In the special case where the base is circular, the base defines a cylinder. If the base is polygonal, the base forms a prism. A regular cylinder is a cylinder whose base is perpendicular to the generatrix.

[0023] The area of ​​the sub-degraded unit (Aij) is calculated by normalizing the bundle weight (ωij / ∑ j ω ij ) is determined by being equal to the ratio of the integral of the fluence (F(y,z)) over the subunit base area (Aij) to the integral of this fluence over the base area (Abi) of the degradation unit (11.i),

[0024]

[0025] where the fluence F(y,z) is the charge per unit area of ​​the beam (100.i) at position (y,z) of the beam, and where the base area (Abi) is equal to the sum of the subunit areas (Aij) (i.e., Abi = ∑ j Aij).

[0026] The N sub-degradation units are combined to obtain an energy degradation unit designed to degrade the energy of the beam in order to deposit the desired dose (Dij) into the sub-area (Vi). The energy degradation units corresponding to all remaining sub-areas (Vi) may be designed as defined above.

[0027] In a preferred embodiment, a specific dose (Dij) will be deposited according to the treatment plan into at least some selected specific location in a specific location within a region of tissue at an ultra-high dose deposition rate (HDR). An ultra-high dose deposition rate (HDR) is defined as a dose deposition rate with HDR ≥ 1 Gy / s.

[0028] The spots in the spot array may be separated from each other by a distance (ds) that is less than or equal to 1.8 times the standard deviation (σ) of the fluence (Fi(y,z)) of the beam at a single spot (i.e., ds≤1.8σ), preferably less than or equal to 1.5σ. Under these conditions, the fluence (F(y,z)) of the beam passing through the substrate area (Abi) is approximately constant over all values ​​of the plane (Y,Z)j defining the boundary inscribed in the zone.

[0029] Alternatively, the spots in the spot array may be separated from each other by a distance (ds) greater than 1.2 times, preferably greater than 1.5 times (i.e., ds>1.2σ) the standard deviation (σ) of the fluence (Fi(y,z)) of the beam at a single spot. Under these conditions, the fluence (Fi(y,z)) of the beam across the substrate area (Abi) approximates a Gaussian distribution,

[0030]

[0031] where (yi,zi) are the coordinates of the position of the maximum value (Ai) of the fluence of the spot (Si) on the (Y,Z) plane, and where, in the case of a circular spot, then σ y =σ z =σ.

[0032] In a preferred configuration of the present invention, the energy degradation unit is in the form of orifices arranged side by side according to an array of spots in a supporting substrate having a thickness (Bi) measured along the beam axis (Xi). Each orifice extends from a hole opening at the surface of the supporting substrate and penetrates to a given depth measured along the corresponding beam axis (Xi). Each energy degradation unit (11.i) is formed by one or more sub-degradation units in the form of orifices having a generalized cylindrical geometry of a cross-sectional area (Axij) and extending a length (Lsij) from a hole in the supporting substrate along the corresponding beam axis (Xi) such that Lij=Bi-Lsij. The sub-degradation units as defined above are arranged within the substrate area (Abi).

[0033] In the energy degradation unit of this configuration, at least two sub-units may be arranged within the base area (Abi) in one of a series configuration, a parallel configuration, and a hybrid configuration.

[0034] In the series configuration, the sub-degradation units are arranged in descending order of length (Lsij) along the corresponding beam axis (Xi), preferably coaxially, and the aperture with the longest length (Lsi3) is positioned at the center. The sub-unit base area (Aij) of a given sub-degradation unit is equal to the cross-sectional area difference (Axij-Axi(j+1)) between the cross-sectional area of ​​the given degradation unit (Axij) and the cross-sectional area of ​​the degradation unit defined within the given degradation unit (Axi(j+1)).

[0035] In the parallel configuration, the sub-degradation units are arranged side by side within the base area (Abi) with no spacing between two sub-degradation units or with spacing between two adjacent sub-degradation units.

[0036] In a hybrid configuration of parallel and series, three or more sub-degradation units are arranged both in series and in parallel. One or more structures formed by two or more sub-degradation units arranged in series along the corresponding beam axis (Xi), and optionally one or more individual sub-degradation units are arranged side by side within the base area (Abi).

[0037] In an alternative preferred configuration of the present invention, the energy degradation unit is in the form of pins arranged side by side according to the spot array and supported on a supporting substrate having a thickness (Bi) measured along the beam axis (Xi). Each pin extends from the supporting substrate along the corresponding beam axis (Xi). In this configuration, each energy degradation unit is formed by one or more sub-degradation units having a generalized cylindrical geometry of a cross-sectional area (Aij) and extending a length (Lsij) from the supporting substrate along the corresponding beam axis (Xi) such that Lij=Bi+Lsij. These sub-degradation units are arranged within the substrate area (Abi).

[0038] In the energy degradation unit of this configuration, at least two sub-units may be arranged within the base area (Abi) in one of a series configuration, a parallel configuration, and a hybrid configuration.

[0039] In the series configuration, the sub-degradation units are arranged in descending order of length (Lsij) along the corresponding beam axis (Xi), preferably coaxially, and the pin with the longest length (Lsi1) is positioned at the center. The sub-unit base area (Aij) of a given sub-degradation unit is equal to the cross-sectional area difference (Axij-Axi(j-1)) between the cross-sectional area (Axij) of the given sub-degradation unit and the cross-sectional area (Axi(j-1)) of the sub-degradation unit (11.i(j-1)) defined within the given sub-degradation unit.

[0040] In a hybrid configuration of parallel and series, three or more sub-degradation units are arranged both in series and in parallel. One or more structures formed by two or more sub-degradation units arranged in series along the corresponding beam axis (Xi), and optionally one or more individual sub-degradation units are arranged side by side within the base area (Abi).

[0041] In a preferred embodiment, at least the first sub-degradation unit of the first energy degradation unit can be made of a first material, which is different from the second material of the second sub-degradation unit of the first energy degradation unit or the second energy degradation unit. The first material has a sub-unit water equivalent thickness (Wu) value per unit length that is different from the second material, so as to change, preferably reduce, the length value of the first sub-degradation unit compared to the length of the corresponding second sub-degradation unit made of the second material (L11=W11 / Wu). By using different materials with different sub-unit water equivalent thickness (Wu) values ​​per unit length, the length (L11) of the first sub-degradation unit can be kept within ±20% of the length (Lij) of the second sub-degradation unit, so as to make the energy degradation unit more compact. The lengths (Lij) of all sub-degradation units of the energy degradation unit preferably have the same length (Lij) within a variation range of ±20% of the average length (Lm,ij) (i.e., ). BRIEF DESCRIPTION OF THE DRAWINGS

[0042] In these figures,

[0043] Fig. 1(a) schematically shows a treatment area (V) divided into sub-areas (Vi) extending parallel to the corresponding beam axis (Xi), each sub-area (Vi) being divided into cells (Cij) in which a predetermined dose (Dij) is to be deposited. The corresponding SOBP is shown along different beam axes (Xi) of the corresponding energy degradation unit passing through the ridge filter.

[0044] Fig. 1(b) shows a side view of the treatment zone (V) of Fig. 1(a) where the SOBP is obtained from three beams (100.i, 100.k, 100.m) after passing through a ridge filter according to the present invention.

[0045] FIG. 1( c ) shows a SOBP obtained with a beam ( 100 . i ) extending along a beam axis (Xi) coaxial with the irradiation axis (X).

[0046] FIG. 1( d ) shows a SOBP obtained with a beam ( 100 . m ) extending along the beam axis ( Xm ).

[0047] Figure 2(a) shows a typical Bragg peak depth dose curve for a beam of a given energy. The horizontal axis represents the depth in water. The maximum beam range in water (W0) is defined as the depth beyond the Bragg peak maximum and the corresponding energy equal to 80% of the Bragg peak maximum, where W0 must be at least equal to or greater than dij (dij <W0)。

[0048] FIG. 2( b ) shows the depth dose profile of the Bragg peak of the beam of FIG. 2( a ) where the energy degrading cell intersects the path of the beam.

[0049] Fig. 3(a) shows beams (100.i, 100.(i+1)...) extending along respective beam axes (Xi, X(i+1)...) parallel to each other and to the irradiation axis (X) through the ridge filter.

[0050] FIG. 3( b ) shows beams ( 100 . i , 100 . ( i + 1 ) . . . ) extending along respective beam axes ( Xi , X ( i + 1 ) . . . ) fanning out around an irradiation axis ( X ) passing through a ridge filter.

[0051] FIG. 3( c ) schematically shows a ridge filter comprising energy degradation units in the form of pins.

[0052] Fig. 3(d) shows two energy degradation units, where the beams (100.i, 100.(i+1)) extend along beam axes (Xi, X(i+1)) that are not parallel to each other (the (Xi, X(i+1))-angle is exaggerated).

[0053] Figure 3(e) shows the zone elements of the treatment zone (V), including several sub-zones (Vi) extending parallel to the irradiation axis (X) and divided into cells (Cij). The SOBP obtained in the sub-zone (V5) is shown.

[0054] Fig. 3(f) shows the zone elements of the treatment zone (V), comprising several sub-zones (Vi) extending parallel to the corresponding beam axes (Xi). The beam axes (Xi) are neither completely parallel to each other nor to the irradiation axis (X). For the sake of clarity, the deviation angles are exaggerated.

[0055] FIG. 4( a ) shows an embodiment of a ridge filter including energy degradation units formed by gaps.

[0056] Fig. 4(b) shows a side cross-sectional view of the gap of the energy degradation unit forming the ridge filter of Fig. 4(a), which is composed of three sub-degradation units. The depth dose curve of the SOBP generated by the degradation unit is shown.

[0057] Fig. 4(c) shows a side cross-sectional view of the first sub-degradation unit of the energy degradation unit of Fig. 4(b). Two examples of possible cross-sections of the first degradation unit are shown: circular or polygonal (hexagonal). The Bragg peak generated by the first degradation unit has a maximum at depth di1.

[0058] Figure 4(d) shows a side cross-sectional view of the second sub-degradation unit of the energy degradation unit of Figure 4(b). Two examples of possible cross-sections of the first degradation unit are shown: circular or polygonal (hexagonal). The Bragg peak generated by the first degradation unit has a maximum at depth di2.

[0059] Figure 4(e) shows a side cross-sectional view of the second sub-degradation unit of the energy degradation unit of Figure 4(b). Two examples of possible cross-sections of the first degradation unit are shown: circular or polygonal (hexagonal). The Bragg peak generated by the first degradation unit has a maximum at depth di3.

[0060] FIG. 5( a ) shows an embodiment of a ridge filter comprising energy degradation units formed by protruding pins.

[0061] Figure 5(b) shows a side cross-sectional view of a protruding pin of an energy degradation unit forming the ridge filter of Figure 5(a), which is composed of three sub-degradation units. The depth dose curve of the SOBP produced by the degradation unit is shown.

[0062] Figure 5(c) shows a side cross-sectional view of the first sub-degradation unit of the energy degradation unit of Figure 5(b). Two examples of possible cross-sections of the first degradation unit are shown: circular or polygonal (hexagonal). The Bragg peak generated by the first degradation unit has a maximum at depth di3.

[0063] Figure 5(d) shows a side cross-sectional view of the second sub-degradation unit of the energy degradation unit of Figure 5(b). Two examples of possible cross-sections of the first degradation unit are shown: circular or polygonal (hexagonal). The Bragg peak generated by the first degradation unit has a maximum at depth di2.

[0064] Figure 5(e) shows a side cross-sectional view of the second sub-degradation unit of the energy degradation unit of Figure 5(b). Two examples of possible cross-sections of the first degradation unit are shown: circular or polygonal (hexagonal). The Bragg peak generated by the first degradation unit has a maximum at depth di1.

[0065] FIG. 6( a ) shows a side cross-sectional view of the energy degradation unit ( 11 . i ) of FIG. 4( b ) comprising N=3 sub-degradation units ( 11 . ij ) formed by successive gaps and the corresponding SOBP.

[0066] FIG6(b) shows a variation of the energy degradation unit (11.i) of FIG6(a) including N=6 sub-degradation units (11.ij) and the corresponding SOBP.

[0067] Figure 6(c) shows Figure 6(a) and 6(b) A variant of an energy degradation unit (11.i) comprising N→∞ sub-degradation units (11.ij) forming a continuous generalized frusto-conical gap and a corresponding SOBP.

[0068] FIG6(d) shows an energy degradation unit (11.i) in the form of a stepped pin including N=6 sub-degradation units (11.ij) and the corresponding SOBP.

[0069] FIG6(e) shows a variation of the energy degradation unit (11.i) of FIG6(d) including N→∞ sub-degradation units (11.ij) forming a continuous generalized truncated cone and the corresponding SOBP.

[0070] FIG. 7( a ) shows the shift of the Bragg peak, characterizing the dose deposition by a beam passing through an energy degradation unit in the form of a protruding pin, which includes two sub-degradation units of different lengths and cross-sectional areas and arranged coaxially.

[0071] FIG. 7( b ) shows the shift of the Bragg peak, characterizing the dose deposition by a beam passing through an energy degradation unit in the form of a gap, which includes two sub-degradation units of different lengths and cross-sectional areas.

[0072] FIG. 7( c ) shows the shift of the Bragg peak, characterizing the dose deposition by a beam passing through an energy degradation cell in the form of concentric elements comprising two sub-degradation cells of different density and cross-sectional area (the density of the inner element is lower than that of the peripheral element).

[0073] FIG7(d) shows the shift of the Bragg peak, characterizing the dose deposition by a beam passing through an energy degradation cell in the form of concentric elements comprising two sub-degradation cells of different density and cross-sectional area (the density of the inner element is greater than that of the peripheral element).

[0074] FIG. 7( e ) shows the shift of the Bragg peak, characterizing the dose deposition by a beam passing through an energy degradation unit in the form of a protruding pin, which includes two sub-degradation units of different lengths and cross-sectional areas and arranged side by side.

[0075] Figure 7(f) shows a perspective view of an energy degradation unit (11.i) of the type shown in Figure 7(a).

[0076] Figure 7(g) shows a perspective view of an energy degradation unit (11.i) of the type shown in Figure 7(e).

[0077] Fig. 7(h) shows a perspective view of an embodiment of an energy degradation unit (11.i) comprising sub-degradation units arranged in series and in parallel in a hybrid configuration.

[0078] Fig. 7(i) shows a perspective view of an alternative embodiment of an energy degradation unit (11.i) comprising sub-degradation units arranged in series and in parallel in a hybrid configuration. DETAILED DESCRIPTION

[0079] The present invention relates to a method for designing a ridge filter (11) for a charged particle accelerator, preferably a proton accelerator. The ridge filter (11) of the present invention is configured to deposit a specific dose (Dij) to a specific location within a treatment volume (V) of tissue including tumor cells (3t) by an accelerated particle beam (100.i) according to a predetermined treatment plan (TP) by pencil beam scanning (PBS) of an array of spots one by one (Si). The ridge filter allows PBS to be performed in a single mapping layer defining the entire treatment volume (V). In the PBS, a narrow pencil beam is deflected to scan each spot (Si) of the array. Although the pencil beam is a single beam that is deflected, each beam (100.i) aimed at a corresponding spot (Si) and extending along a corresponding beam axis (Xi) is hereinafter regarded as a separate beam different from a beam (100.k) aimed at a different spot (Sk, k≠i) and extending along a second beam axis (Xk). As shown in Figures 1(b) and 3(a), the beam axis (Xi) is substantially parallel to the irradiation axis (X). In practice, since the "individual" beam (100.i) is a single beam deflected from a common accelerator exit towards an individual spot (Si) in the array, the beam axis (Xi) deviates from parallelism to the irradiation axis (X) by an angle comprised within ±5°, preferably within ±3°, depending in particular on the size of the treatment zone (V) and the distance separating the accelerator exit from the treatment zone (V).

[0080] The tissue traversed by the beam (100.i) absorbs a portion of the energy of the beam, determining the penetration depth of the Bragg peak position along the beam axis (Xi). The penetration depth of the Bragg peak position in a given tissue can be characterized by the maximum beam range (W0) in water, defined as the "water equivalent thickness" (=WET), which defines the position where the beam stops propagating in water. The expression "water equivalent thickness" (=WET) is defined as the thickness of water that causes the same energy degradation of the particle beam as the particle beam passing through one or more materials of a given thickness. The maximum beam range (W0) can be directly related to the penetration depth of the same beam through the tissue. It can be seen that WET and the penetration depth of the Bragg peak position through the tissue can be used interchangeably, the former (WET) being of course easier to test and measure experimentally.

[0081] The method for designing a ridge filter (11) according to the present invention comprises the following steps:

[0082] a map defining the treatment volume (V) and indicating the location and dose to be deposited into that treatment volume, and

[0083] • Design and size the ridge filter accordingly.

[0084] As schematically shown in Figures 1(a), 1(b), 3(a) and 3(b), the definition of the treatment zone first includes defining a boundary inscribed in the treatment zone (V). This operation includes defining an area on an upstream plane (Y, Z) j of N slices (Tj = T1-TN) with a thickness (dxj), wherein the plane (Y, Z) j is perpendicular to the irradiation axis (X). The upstream plane (Y, Z) j is the plane (Y, Z) of the slice (Tj) first hit by the beam (100.i). The depth of the treatment zone (V) measured along the irradiation axis (X) is contained between the shortest water equivalent thickness (d0) and the longest water equivalent thickness (d1) measured from the patient's skin (3s), the shortest water equivalent thickness and the longest water equivalent thickness corresponding to the points of the boundary measured along the irradiation axis (X) closest to and farthest from the skin (3s), respectively.

[0085] Secondly, as shown in Figures 1(a), 1(b), 3(e) and 3(f), the treatment area (V) inscribed within the boundary is divided by defining an array of sub-areas (Vi). Each sub-area (Vi) extends from the patient's skin to the corresponding farthest water equivalent thickness (d1) parallel to the corresponding beam axis (Xi), and the projection of each sub-area on the plane (Y, Z) perpendicular to the irradiation axis (X) defines an array of spots (Si) covering the entire area of ​​the projection of the area (V) on the plane (Y, Z) (see Figure 1(a)).

[0086] Third, the sub-area (Vi) itself is divided into cells (Cij) as follows. For each slice (Tj) of the N slices (T1-TN) contained in the sub-area (Vi), the cell (Cij) is defined as the portion of the sub-area (Vi) contained in the corresponding slice (Tj), as shown in Figures 1(a), 3(a) and 3(b). For each cell (Cij) of a given sub-area (Vi), the cell water equivalent thickness (dij) is determined from the skin (3s) to the geometric center (Cij) of the cell. The beam weight (ωij) is attributed to each cell (Cij) according to the specific dose (Dij) that needs to be deposited into the cell (Cij) according to the TP. The beam weight (ωij) corresponds to the number of charged particles (e.g., protons) to be delivered within the spot (Cij). Therefore, the beam weight (ωij) is proportional to the number of charged particles under the cell water equivalent thickness (dij).

[0087] Once the treatment area has been divided into sub - regions (Vi) and cells (Cij), and once the beam weights (ωij) required to deposit the dose (Dij) into each cell (Cij) according to the treatment plan have been determined, the ridge filter (11) can be designed and sized accordingly as follows.

[0088] The ridge filter (11) includes a group of energy degradation units (11.i) configured to reduce the initial energy (E0) of a charged particle beam (100.i) having a beam diameter (D100.i) co - axial with a corresponding beam axis (Xi) and sub - region (Vi) to reduce the energy (Eij) such that a specific dose (Dij) is deposited in a corresponding cell (Cij) contained within the sub - region (Vi) in terms of cell water equivalent thickness (dij) according to the TP. This principle is shown in FIGS. 2(a) and 2(b).

[0089] FIG. 2(a) plots the dose (Dij) deposited by a beam (100.i) of a given energy (E0) along the beam axis (Xi) as a function of the WET. As discussed above, the maximum beam range in water (WET) is defined as W0, corresponding to the depth beyond the maximum of the Bragg peak and corresponding to an energy equal to 80% of the maximum of the Bragg peak. This means that a beam (100.i) of a given energy cannot penetrate deeper into the tissue than the corresponding water equivalent thickness W0 (i.e., d1 ≤ W0). If d1 > W0, i.e., the treatment area (V) extends deeper than the Bragg peak, a beam of higher energy must be used. FIG. 2(b) shows the displacement of the Bragg peak when a sub - degradation unit (11.ij) is inserted along the path of the same beam (100.i) of a given energy. It can be seen that the WET of the Bragg peak is now at a depth (dij < W0) from the skin (3s). This is because a portion (E0 - Eij) of the energy of the beam (100.i) is absorbed by the sub - degradation unit (11.ij) that the beam must pass through. FIG. 2(b) shows how a Bragg peak can be shifted from WET = W0 to the desired WET = dij by inserting a sub - degradation unit (11.ij). Since the SOBP can be formed by superposing several Bragg peaks distributed at a given depth along the irradiation axis (X), in order to deposit the desired dose (Dij) into the entire sub - region (Vi) through a single painted layer PBS, several sub - degradation units (11.ij) can be superposed to form an energy degradation unit (11i), where the size of each sub - degradation unit (11.ij) is determined to shift the Bragg peak from WET = W0 to the corresponding cell water equivalent thickness (dij) to ensure that the desired dose (Dij) is deposited in each cell (Cij) of the sub - region (Vi) irradiated by the beam (100.i). This is in Figure 6(a) to Figure 6(e), showing energy degradation units (11.i) and corresponding SOBPs, each energy degradation unit consisting of N=3, N=6 and N→∞ sub-degradation units (11.ij). The number of Bragg peaks required to produce the desired SOBP according to TP and their corresponding cell water equivalent thickness (dij) must be determined. The number of Bragg peaks in a given beam (100.i) and therefore the number of sub-degradation units (11.ij) of the corresponding energy degradation unit (11.i) is equal to the number of slices (T1-TN) N. Each Bragg peak of a given beam (100.i) has a corresponding energy degradation unit (11.i), the sub-degradation units (11.ij) of which can be designed to produce the desired SOBP and thus deposit the desired dose (Dij) into the corresponding cell (Cij). When there are N→∞ sub-degradation units (11.ij) as shown in Figures 6(c) and 6(e), the step size of the stepped configuration of the energy degradation unit (11.i) becomes smaller until it approaches zero, thereby producing a smooth geometric shape of a generalized truncated cone shown in Figures 6(c) and 6(e), where the generalized cone can have a base of any geometric shape, not limited to a circle.

[0090] Each sub-degradation unit (11.ij) has a generalized cylindrical geometry (ie not necessarily a cylinder) with a base with area (Aij) perpendicular to the corresponding beam axis (Xi) and a generatrix with length (Lij) parallel to the corresponding beam axis (Xi). Figure 4(a) to Figure 4(e) and Figure 5(a) to Figure 5(e) The sub-degradation units shown in have circular or polygonal cross-sections. Clearly, other cross-sections are also possible. However, it is preferred that the cross-section has a certain degree of symmetry in order to facilitate the calculation of the size of the area (Aij). The sub-degradation units are made of a material having a water equivalent thickness (Wu) per unit length of the sub-unit along the corresponding beam axis (Xi). The length (Lij) and area (Aij) of each sub-degradation unit (11.ij) can be determined as follows.

[0091] The length (Lij) of the sub-degradation unit (11.ij) is determined so that the sub-degradation unit (11.i) has a sub-unit water equivalent thickness (Wij=Wu x Lij) equal to the product of the sub-unit water equivalent thickness per unit length (Wu) and the length (Lij). Considering that the sum of the sub-unit water equivalent thickness (Wij) and the cell water equivalent thickness (dij) must be equal to the maximum beam range (W0) (i.e., W0=Wij+dij), the length (Lij) of the sub-degradation unit (11.ij) is defined as (W0-dij), where the factor (W0-dij) is graphically shown in Figure 2(b). The length of the sub-degradation unit is thus fully defined.

[0092] The area (Aij) of the sub-degraded unit (11.ij) is calculated by normalizing the bundle weight (ωij / ∑ j ω ij ) is determined by being equal to the ratio of the integral of the fluence (F(y,z)) over the subunit base area (Aij) to the integral of this fluence over the base area (Abi) of the degradation unit (11.i),

[0093]

[0094] where the fluence F(y,z) is the charge per unit area of ​​the beam (100.i) at position (y,z) of the beam, and where the base area (Abi) is equal to the sum of the subunit areas (Aij) (i.e., Abi = ∑ j Aij), as shown in Figure 4(a) and Figure 5(a). Figure 7(a) to Figure 7(e) As shown in , the fluence of a charged particle beam generally has a Gaussian distribution. The diameter (D100) of the beam can be defined as the distance 4σ, where σ is the standard deviation of the Gaussian distribution that characterizes the fluence of the beam. Approximately 95% of the proton spots are located within this 4σ diameter.

[0095] like Figure 4(b) to Figure 4(e) , Figure 4(g) to Figure 4(j) , Figure 5(b) to Figure 5(e) and Figure 5(g) to Figure 5(j) As shown in , three sub-degradation units (11.ij) of an energy degradation unit (11.i) are dimensioned and can be coaxially combined to obtain an energy degradation unit (11.i) designed to degrade the energy of a beam (100.i) so as to deposit a desired dose (Dij) into a sub-area (Vi).

[0096] The same exercise is repeated to design energy degradation units (11.i) corresponding to all remaining sub-regions (Vi) as defined above.

[0097] Spot (Si) Array

[0098] As illustrated in FIG. 1( a ), the array of spots (Si) is defined as covering an area projected parallel to the irradiation axis (X) of the treatment zone (V) onto a projection plane (Y, Z) perpendicular to the irradiation axis (X).

[0099] The oncologist characterizes the geometry and topography of the tumor region based on images of the tumor region obtained by computer tomography (= CT scan). As shown in FIG1( b ), in order to reach the treatment zone (V), the beam (100.i) must pass through the healthy cells separating the treatment zone (V) from the patient's skin (3s), thereby irradiating both healthy cells and tumor cells. The oncologist defines a treatment plan that defines the dose (Dij) to be deposited into the treatment zone (V) and the dose deposition not to be exceeded in healthy cells located outside the treatment zone (in particular upstream).

[0100] The spot has a dimension perpendicular to the irradiation axis (X), which may be equal to the beam diameter discussed above. The distance between adjacent spots, which defines the array density, is an important parameter, because the denser the array (i.e., the closer the adjacent spots are to each other), the greater the overlap dose effect of cells spanned by adjacent spots. Substantial overlap resulting in a uniform lateral dose distribution is observed at a distance between adjacent spots of about 1.5σ.

[0101] In a first embodiment, the spots (Si) in the spot array are separated from each other by a distance (ds) that is less than or equal to 1.8 times the standard deviation (σ) of the fluence (Fi(y,z)) of the beam (100.i) at a single spot (i.e., ds≤1.8σ), preferably less than or equal to 1.5σ. With this configuration, a given sub-region (Vi) receives a dose (Dij) from a beam (100.i) centered on the corresponding beam axis (Xi), but also receives a dose from an adjacent beam centered on an adjacent beam axis and whose fluence extends onto the given sub-region (Vi) and also onto a given degradation unit 11.i. Taking into account the dose deposited into the sub-region (Vi) by the adjacent beam, the fluence (F(y,z)) of the beam (100.i) across the substrate area (Abi) (see equation (1)) is approximately constant over all values ​​of the plane (Y,Z)j of the slice (Tj) defining the boundary inscribed in the region (V).

[0102] In a second embodiment, the spots (Si) in the spot array are separated from each other by a distance (ds) greater than 1.2 times, preferably greater than 1.5 times (i.e., ds>1.2σ) the standard deviation (σ) of the fluence (Fi(y,z)) of the beam (100.i) at a single spot. At such a distance, the fluence of the neighboring beams passing through a given degradation unit (11.i) is negligible. Therefore, the fluence (Fi(y,z)) of the beam (100.i) passing through the substrate area (Abi) is approximately Gaussian,

[0103]

[0104] where (yi,zi) are the coordinates of the position of the maximum value (Ai) of the fluence of the spot (Si) on the (Y,Z) plane, and where, in the case of a circular spot, then σ y =σ z =σ.

[0105] It would seem that increasing the density of the spot (Si) array can always be advantageous over a low density array. However, irradiating a high density array will extend the scan time required to cover the entire treatment volume (V). Furthermore, FLASH deposition (where the dose (Dij) will be deposited in HDR to produce the FLASH effect without harming healthy cells) is more difficult to produce with a high density array due to the dose deposited from adjacent beams, thus extending the deposition time (and correspondingly reducing the deposition rate). Therefore, the density of the spot (Si) array will be determined on a case-by-case basis.

[0106] Degraded unit (11.i)

[0107] The principle of the ridge filter (11) of the present invention is to degrade the energy of the beam (100.i) for each spot (Si) so as to produce the desired SOBP in the corresponding sub-area (Vi), wherein the extended peak extends between the shortest water equivalent thickness (d0) and the longest water equivalent thickness (d1) measured from the patient's skin (3s), defining the portion of the sub-area (Vi) contained within the treatment area. The challenge is to degrade the energy of only a portion of the beam (100.i) so as to shift the Bragg peak of a predetermined beam weight fraction (ωij) of the entire weight (ωi) of the beam (100.i) to the corresponding cell water equivalent thickness (dij), thereby depositing the desired dose (Dij) into the corresponding cell (Cij) of the corresponding sub-area (Vi).

[0108] Three geometries of energy degradation cells (11i) are proposed to achieve this goal, with corresponding methods for determining the dimensions of the sub-degradation cells (11.ij) forming the energy degradation cell (11i). One geometry can be combined with another to achieve the most convenient ridge filter.

[0109] an energy degradation unit in the form of an orifice (11i),

[0110] Energy degradation unit in the form of a pin (11i),

[0111] • Combining energy degradation units (11i) of different materials having different water equivalent thickness per unit length of the sub-unit (Wu).

[0112] Degraded unit (11.i) = orifice

[0113] In this embodiment, if Figure 4(a) to Figure 4(e) , Figure 4(f) to Figure 4(j) and Figure 6(a) to Figure 6(c)As shown in , the energy degradation unit (11.i) is in the form of apertures arranged side by side according to an array of spots (Si) in a supporting substrate (11b), the supporting substrate having a thickness (Bi) measured along the beam axis (Xi). Each aperture extends from a hole opening at the surface of the supporting substrate (11b) and penetrates to a given depth measured along the corresponding beam axis (Xi). Each energy degradation unit (11.i) is formed by one or more sub-degradation units (11.ij, 11.i3, 11.i2, 11.i1) in the form of apertures, which have a generalized cylindrical geometry of a cross-sectional area (Axij) and extend a length (Lsij) from a hole in the supporting substrate (11b) along the corresponding beam axis (Xi), such that Lij=Bi-Lsij. The sub-degraded units (11.ij, 11.i3, 11.i2, 11.i1) are arranged within a diameter slightly greater than or equal to the diameter (D100) of the bundle (100.i), and such that the sum of the sub-unit base areas (Aij) is equal to the base area (Abi) (i.e., ).

[0114] exist Figure 4(a) to Figure 4(e) In the preferred embodiment shown in FIG. 7( b), the sub-degradation units are arranged in a series configuration, wherein the sub-degradation units (11.ij) are arranged in descending order of length (Lsij) along the corresponding beam axis (Xi). The sub-degradation units (11.ij) are preferably arranged coaxially, and the orifice with the longest length (Lsi3) is positioned in the center. These sub-degradation units may also be arranged in other configurations, for example, non-concentrically, and the orifice with the longest length (Li3) is not necessarily centered.

[0115] In an embodiment of the concentric arrangement of the sub-degradation units (11.ij), the sub-unit base area (Aij) of each sub-degradation unit (11.ij) has an annular geometry except for the sub-unit base area (Ai3) of the deepest orifice (11.i3). Therefore, the sub-unit base area (Aij) of a given sub-degradation unit (11.ij) is equal to the cross-sectional area difference (Axij-Axi(j+1)) between the cross-sectional area (Axij) of the given sub-degradation unit (11.ij) and the cross-sectional area (Axi(j+1)) of the sub-degradation unit (11.i(j+1)) defined within the given sub-degradation unit, wherein the cross-sectional area (Axij) of the sub-degradation unit (11.ij) is the cross-sectional area of ​​the orifice perpendicular to the corresponding beam axis (Xij) included within the outer periphery of the cross section of the sub-degradation unit (11.ij).

[0116] exist Figure 4(f) to Figure 4(j)In the alternative embodiment shown in , the sub-degradation units can be arranged in a parallel configuration. In this embodiment, the sub-degradation units are arranged side by side within the base area (Abi), as shown for the pins in Figures 7(e) and 7(g), with no spacing between two sub-degradation units (it is easy to imagine a corresponding design with an orifice), or with a spacing between two adjacent sub-degradation units, as shown in Figures 4(f) and 4(g).

[0117] In yet another alternative embodiment, the sub-degradation units may be arranged in a mixed series and parallel configuration. In this configuration, as shown for the pins in Figures 7(h) and 7(i) (it is easy to imagine the corresponding design with the orifice), three or more sub-degradation units (11.ij) are arranged both in series and in parallel, wherein one or more structures formed by two or more sub-degradation units arranged in series along the corresponding beam axis (Xi), and optionally one or more individual sub-degradation units are arranged side by side within the base area (Abi).

[0118] This embodiment is particularly easy to produce by machining or etching a block forming the support substrate (11b) or forming it by 3D printing technology. Considering that the maximum fluence of the beam (100.i) is at the beam axis (Xi), the manufacturing error of the sub-degradation unit (11.ij) that intersects the beam axis (Xi) at the maximum of the Gaussian distribution of the fluence is therefore more critical than the peripheral position. The advantage of using an orifice instead of a pin is that the weight of the Bragg peak is matched to the fluence of the spot. In fact, the Bragg peak with a larger range is usually the Bragg peak with a larger weight (ωij) in the treatment plan. Therefore, it is best to place the corresponding sub-unit (i.e., with the smallest length Li) at the position where the fluence of the beam is highest (i.e., the center). In addition, the sub-unit with the largest length Li will correspond to the Bragg peak (with the shortest range) that is generally less weighted and therefore smaller in cross-section. Therefore, it is convenient to position the sub-unit with the largest length in the spot area with the smallest fluence. It is easier to form an orifice with the tight tolerance required to produce the desired effect than to form a thin pin with the same tolerance and length (Lij). Offsetting the sub-degraded cells of longest length (Lij) can also help reduce the weight of slight variations in manufacturing.

[0119] For clarity, Figure 4(a) to Figure 4(e) An energy degradation unit (11.i) formed of three sub-degradation units (11.ij) is shown, and FIG7(b) shows an energy degradation unit (11.i) formed of only two sub-degradation units (11.ij). It is clear that the number of sub-degradation units (N) can actually be higher to produce a smoother SOBP plateau. Figure 6(a) to Figure 6(c)Similar energy degradation units (11.i) are shown comprising three, six, an "infinite number" of sub-degradation units (11.ij), the "infinite number" of sub-degradation units forming a frusto-conical aperture. Figure 6(a) to Figure 6(c) All orifices in have an opening area (Axi1) and a bottom area (Axi3), as well as a length (Lsi3) (see Figure 4(a) to Figure 4(e) ).

[0120] exist Figure 4(c) to Figure 4(e) In the figure, each sub-degradation unit is shown separately and the whole thereof forms a generalized hollow cylinder, which are assembled with each other to form the sub-degradation unit of Figure 4 (b). In practice, the sub-degradation unit (11.ij) of Figure 4 (b) can be produced by machining or etching the supporting substrate (11b) to form an orifice or by forming a ridge filter with a corresponding orifice through 3D printing technology.

[0121] Degraded unit (11.i) = pin

[0122] exist Figure 5(a) to Figure 5(e) , Figure 7(a) , Figure 7(e) and 7(f) In the alternative embodiment shown in , the energy degradation unit (11.i) is in the form of pins arranged side by side according to an array of spots (Si) and supported on a supporting substrate (11b), the supporting substrate having a thickness (Bi) measured along the beam axis (Xi). Each pin extends from the supporting substrate along the corresponding beam axis (Xi). Each energy degradation unit (11.i) is formed by one or more sub-degradation units (11.ij, 11.i3, 11.i2, 11.i1), which have a generalized cylindrical geometry of a cross-sectional area (Axij) and extend a length (Lsij) from the supporting substrate along the corresponding beam axis (Xi), so that Lij=Bi+Lsij. The sub-degradation units (11.ij, 11.i3, 11.i2, 11.i1) are arranged within a diameter slightly greater than or equal to the diameter (D100) of the beam (100.i), and so that the sum of the sub-unit substrate areas (Aij) is equal to the substrate area (Abi) (i.e., ).

[0123] exist Figure 5(a) to Figure 5(e) In the preferred embodiment shown in Figure 7 (a), the sub-degradation units (11.ij) are arranged in series. In this configuration, the sub-degradation units are arranged in descending order of length (Lsij) along the corresponding beam axis (Xi). These sub-degradation units are preferably arranged coaxially, and the pin with the longest length (Lsi1) is located in the center.

[0124] In an embodiment of the concentric arrangement of the sub-degradation units (11.ij), the sub-unit base area (Aij) of a given sub-degradation unit (11.ij) is equal to the cross-sectional area difference (Axij-Axi(j-1)) between the cross-sectional area (Axij) of the given sub-degradation unit (11.ij) and the cross-sectional area (Axi(j-1)) of the sub-degradation unit (11.i(j-1)) defined within the given sub-degradation unit. In other words, for a finite number N of sub-degradation units (11.ij), the pin has the shape of a stepped pyramid (see Figure 4(b) to Figure 4(e) ). The area (Aij) of a given sub-degradation unit (11.ij) is the area of ​​the step surface (or flange) of the next sub-degradation unit (11.i(j+1)) of the given sub-degradation unit (11.ij) whose surrounding dimension is smaller than that of the step surface (or flange) of the next sub-degradation unit (11.i(j+1)) of the given sub-degradation unit (11.ij).

[0125] exist Figure 5(f) to Figure 5(j) , in the alternative embodiment shown in FIG. 7 (e) and FIG. 7 (g), the sub-degradation unit can be arranged in a parallel configuration. In this embodiment, the sub-degradation units are arranged side by side within the base area (Abi), as shown in FIG. 7 (e) and FIG. 7 (g), there is no interval between the two sub-degradation units, or there is an interval between two adjacent sub-degradation units, as shown in FIG. 5 (f) and FIG. 5 (g). For example, in the embodiment shown in FIG. 7 (e) and FIG. 7 (g), the sub-degradation units are arranged side by side, wherein the longest sub-degradation unit is preferably offset relative to the corresponding beam axis (Xi). In this way, the longest degradation unit faces a portion of the beam (100.i) with a lower weight, and therefore the cross-sectional area (Aij) can be larger, thereby facilitating the manufacture of the energy degradation unit (11.i). The same configuration can also be applied to the energy degradation unit (11.i) in the form of an orifice discussed above.

[0126] In yet another alternative embodiment, three or more sub-degradation units may be arranged in a mixed configuration of series and parallel. In this configuration, as shown in Figures 7(h) and 7(i), three or more sub-degradation units (11.ij) are arranged both in series and in parallel, wherein one or more structures formed by two or more sub-degradation units arranged in series along the corresponding beam axis (Xi), and optionally one or more individual sub-degradation units are arranged side by side within the base area (Abi).

[0127] For clarity, Figure 5(a) to Figure 5(e)and Figure 7(f) show an energy degradation unit (11.i) formed by three sub-degradation units (11.ij), and Figure 7(a) and Figure 7(e) show an energy degradation unit (11.i) formed by only two sub-degradation units (11.ij). It is clear that the number of sub-degradation units (N) can actually be higher to produce a smoother SOBP plateau. As shown in Figures 5(b), 6(d) and 6(e), the energy degradation unit (11.i) in the form of a pin can include any number (such as three, six, "infinite") of sub-degradation units (11.ij), and the "infinite" sub-degradation units form a truncated cone (see Figure 6(e)). Regardless of the number N of sub-degradation units (11.ij), all pins have a base area (Ai1) and an end area (Ai3), as well as a length (Lsi3) (see the meaning of the symbols for Figure 5(a) to Figure 5(e) ).

[0128] exist Figure 5(c) to Figure 5(e) In the embodiment of the present invention, each sub-degradation unit is shown separately and the whole is assembled with each other to form the sub-degradation unit in Figure 5 (b). In practice, the sub-degradation unit (11.ij) of Figure 5 (b) can be produced by machining, etching or 3D printing the whole energy degradation unit (11.i), preferably using a supporting substrate (11b), or each sub-degradation unit (11.ij) is produced separately and then assembled to form the energy degradation unit (and supporting substrate (11b)). Most preferably, the entire ridge filter is produced in an integral manner (i.e., no assembly step is required).

[0129] Degraded units made of different materials (11.i)

[0130] In the third embodiment shown in Figures 7(c) and 7(d), which can be combined with the above-mentioned embodiments of the energy degradation unit in the form of a cavity or a pin, at least the first sub-degradation unit (11.11) of the first energy degradation unit (11.1) is made of a first material, which is different from the second material of the second sub-degradation unit (11.ij) of the first energy degradation unit or the second energy degradation unit (11.1, 11.2). The first material has a different value of water equivalent thickness per unit length of the sub-unit (Wu) than the second material, so as to change, preferably reduce, the length value of the first sub-degradation unit (11.11) compared to the length of the corresponding second sub-degradation unit (11.ij) made of the second material (L11=W11 / Wu).

[0131] Figure 7(c) shows an embodiment including a first sub-degrading unit and a second sub-degrading unit (11.i1, 11.i2) arranged concentrically. The second sub-degrading unit (11.i2) is enclosed within the annular first sub-degrading unit (11.i1). The first sub-degrading unit (11.i1) is made of a first material having a higher water-equivalent thickness per unit length of sub-units (Wu) than the material forming the second sub-degrading unit (11.i2). Thus, for the same length (Li1 = Li2) measured along the beam axis (Xi), the first sub-degrading unit (11.i1) absorbs more energy from the beam and causes the Bragg peak to shift to cells having a smaller water-equivalent thickness of cells (di1 < di2) that is lower than the water-equivalent thickness of cells (di2) passing through the second sub-degrading unit (11.i2).

[0132] The first sub-degrading unit (11.i1) absorbs more energy from the beam and causes the Bragg peak to shift to cells having a smaller water-equivalent thickness of cells (di1 < di2) that is lower than the water-equivalent thickness of cells (di2) passing through the second sub-degrading unit (11.i2).

[0133] Figure 7(d) shows a similar configuration, but the first sub-degrading unit (11.i1) is made of a first material having a higher value of water-equivalent thickness per unit length of sub-units (Wu) than the second material forming the second sub-degrading unit (11.i2), and the first sub-degrading unit is positioned at the center and surrounded by the second sub-degrading unit (11.i2). Thus, for the portion of the beam passing through the first sub-degrading unit (11.i1) located at the center of the energy-degrading unit (11.i), the Bragg peak appears at a depth (di1 < di2) that is lower than that of the portion passing through the second sub-degrading unit (11.i2), which has the same length (Li1 = Li2) measured along the beam axis (Xi). The sub-degrading units (11.i1, 11.i2) in Figures 7(c) and 7(d) are arranged concentrically. Clearly, other configurations are possible, such as arranging them side by side. Having the same length Li1 = Li2 is also an optimized case. Since it is not possible to select materials that continuously span the entire range of values of water-equivalent thickness per unit length of sub-units (Wu), it is difficult to select materials that produce the desired water-equivalent thickness of cells (dij) and at the same time have the same length (Lij). However, changing the materials allows reducing the height difference between the sub-degrading units, thereby producing a more compact and robust energy-degrading unit (11.i).

[0134] This embodiment of combining different materials having different water-equivalent thicknesses per unit length of sub-units (Wu) can be applied to cavity and pin-shaped energy-degrading units (11.i) to reduce the length (Lij) of the longest sub-degrading unit (11.ij) and increase the length of the shortest sub-degrading unit (11.ij), to produce a shorter ridge filter (11) and facilitate production and compliance with tolerances, and select the most production-friendly dimensions.

[0135] For example, the choice of material for each sub-degradation unit may be driven by the goal of making the length (L11) of the first sub-degradation unit (11.11) within ±20% of the length (Lij) of the second sub-degradation unit, etc. Preferably, the lengths (Lij) of all sub-degradation units (11.ij) of the energy degradation unit (11.i) preferably have the same length (Lij) within a variation range of ±20% of the average length (Lm,ij) (i.e., ). In this way, a compact ridge filter can be obtained.

[0136] Ridge filter (11) and particle accelerator including the ridge filter (11)

[0137] As shown in Figure 3(c), the ridge filter (11) is formed by a plurality of energy degradation units (11.i) arranged side by side on a supporting substrate (11b). The position of each energy degradation unit (11.i) corresponds to the position of the corresponding spot (Si), and the orientation of each energy degradation unit is parallel to the corresponding beam axis (Xi).

[0138] In Figures 1(a), 1(b), 3(a) and 3(e), all beam axes (Xi) are represented as being parallel to each other. This is not entirely correct, since all beam axes (Xi) start from the same point in the center of the scanning magnet in the nozzle and deviate to scan all spots (Si) representing the treatment zone (V). This is illustrated in Figures 3(b), 3(d) and 3(f), where the angle of deviation is exaggerated for clarity. The aperture angle of the cone surrounding all beam axes (Xi) depends on the distance between the scanning magnet and the treatment zone (V) and the size of the treatment zone (V). Figure 3(b) shows a view of the irradiation system corresponding to the view of Figure 3(a), where the beam axes deviate from parallelism taking into account the scanning effect of the beam. The aperture angle is exaggerated in Figure 3(b). Figure 3(c) shows a corresponding ridge filter (11), where the pins extend from a supporting base (11b) and deviate from parallelism relative to each other. Figure 3(d) shows two energy degradation units formed by two corresponding apertures in the supporting substrate (11b). Again, the aperture angles are exaggerated for clarity.

[0139] In practice, the aperture angle is typically within ±5°, preferably within ±3°, and more preferably within ±1° from the irradiation axis (X). For this reason, although not strictly correct, it is an acceptable practical simplification to show the beam axes (Xi) parallel to each other in the figures.

[0140] Fig. 3(e) and Fig. 3(f) show similar cubic elements of a treatment zone (V) comprising several sub-zones (Vi). The sub-zones (V1, V4 and V5) are visible in a side cross-sectional view, showing the positions of the corresponding cells (Cij, where i = 1, 4 and 5, and j = 1 to 4). In Fig. 3(e), the sub-zones (Vi) extend (approximately) parallel to each other and to the irradiation axis (X), while in Fig. 3(f), the sub-zones extend along the divergent beam axis (Xi) (again, the aperture angle is exaggerated for clarity).

[0141] The sub-degradation unit (11.i1) with the longest length (Li1) measured along the corresponding beam axis (Xi) absorbs more energy of the beam (100.i) than shorter sub-degradation units. Therefore, the longest sub-degradation unit (11.i1) determines the shortest cell water equivalent thickness (di1), which defines the position of the Bragg peak closest to the patient's skin (3s). As the length (Lij) of the sub-degradation unit (11.ij) decreases, the corresponding cell water equivalent thickness (dij) increases until the shortest sub-degradation unit (11.iN) with the shortest length (LiN) determines the cell water equivalent thickness (diN) of the Bragg peak farthest from the patient's skin (3s). The superposition formation of all Bragg peaks must be based on the SOBP of the treatment plan (see Figure 4(a) to Figure 4(e) and Figure 5(a) to Figure 5(e) ). The length (Lij) of each sub-degradation unit (11.ij) can be easily determined as discussed above, such as Lij=Wij / Wu=(W0-dij) / Wu (see Figure 2(b)).

[0142] The area (Aij) of each sub-degradation cell (11.ij) must be dimensioned so that the required number of charged particles deposits a predetermined dose (Dij) into the corresponding cell with the corresponding cell water equivalent thickness (dij). Equation (1) is used to determine the value of the area (Aij) of the sub-degradation cell (11.ij),

[0143]

[0144] In equation (1), the normalized bundle weight (ω ij ) is equal to the normalized value of the integral of the beam fluence (F(y,z)) over the area (Aij) whose size is to be determined. Since the area (Aij) defines the boundary for calculating the integral at the numerator of equation (1), the area (Aij) of each sub-degradation unit (11.ij) can be determined. As discussed above, the preferred arrangement of the individual sub-degradation units (11.ij) is to assemble them coaxially to form a corresponding energy degradation unit (11.i) (see Figure 4(a) to Figure 4(e) and Figure 5(a) to Figure 5(e)). In this series arrangement, the area (Aij) of the first sub-degradation unit (11.ij) has an annular geometry, forming an annular step around the sub-degradation unit (11.i(j+1)) coaxially inscribed within the first sub-degradation unit. Figure 4(f) to Figure 4(j) , Figure 5(h) to Figure 5(j) ) and in the parallel arrangement shown in FIG. 7( e ) and FIG. 7( g ), the area (Aij) of the sub-degradation unit (11.ij) is the area at the base of each sub-degradation unit.

[0145] In the case of a dense array of spots (Si), where the distance (ds) separating the spots (Si) from each other is less than or equal to 1.8 times, preferably 1.5 times, the standard deviation (σ) of the fluence (Fi(y,z)) of the beam (100.i), the fluence of the beam across the substrate area (Abi) is approximately constant over all values ​​of the plane (Y,Z)j defining the boundary inscribed in the zone (V). This configuration greatly simplifies the solution of the integral in the numerator of equation (1).

[0146] If the array of spots (Si) is less dense, so that the distance (ds) separating the spots from each other is greater than 1.2 times, preferably 1.5 times (i.e., ds>1.2σ) the standard deviation (σ) of the fluence (Fi(y,z)) of the beam (100.i), then the fluence (Fi(y,z)) of the beam (100.i) passing through the substrate area (Abi) is approximately Gaussian distributed,

[0147]

[0148] where (yi,zi) are the coordinates of the position of the maximum value (Ai) of the fluence of the spot (Si) on the (Y,Z) plane, and where, in the case of a circular spot, then σ y =σ z =σ. The solution of the integral in the numerator of equation (1) is not as easy as in the case of a densely packed spot (Si) array (ie, ds < 1.8σ or < 1.5σ), but it is still at least numerically solvable. If the spots are circular and σ y =σ z =σ, the solution of equation (1) is simplified.

[0149] Conclusion

[0150] The method proposed in this paper for designing a ridge filter (11) and determining its dimensions for single-layer PBS mapping of a treatment zone (V) is simple, reliable and reproducible. The design of an energy degradation unit (11.i) in the form of a cavity is more robust to production tolerances than pins formed by concentric sub-degradation units (11.ij), because the central sub-degradation unit (11.i1) is simultaneously the longest and thinnest of the entire energy degradation unit (11.i), making its precise production more complicated. Configurations other than concentricity are possible, such as stacking, which reduces this problem in some way. For the sub-degradation unit (11.ij) with the longest length (Lij), using a material with a higher water equivalent thickness (Wu) per unit length of the sub-unit is also a solution to reduce the problem of precise production of long and thin pins.

[0151] Starting from the treatment plan, the array of spots (Si) can be defined as discussed above, and the treatment area (V) can be divided into sub-areas (Vi) (one for each spot), and the sub-areas (Vi) can be divided into N cells (Cij) accordingly. The dose (Dij) to be deposited into the cells (Cij) is determined based on the treatment plan.

[0152] The ridge filter is designed to include the same number of energy degradation units (11.i) as spots (Si). Each energy degradation unit (11.i) is formed by N sub-degradation units (11.ij) having a length (Lij) and an area (Aij).

[0153] The length (Lij) of each sub-degradation unit (11.ij) is calculated as the ratio of the required sub-unit water equivalent thickness (Wij) to the sub-unit water equivalent thickness per unit length (Wu) (i.e., Lij=Wij / Wu). The sub-unit water equivalent thickness is defined as Wij=W0-dij, where W0 is the maximum beam range and dij is the desired position of the Bragg peak at the center of the corresponding cell (Cij). The length (Lij) must take into account the thickness (Bi) of the supporting substrate (11b) that supports all the energy degradation units (11.i).

[0154] The area (Aij) of each sub-degradation unit (11.ij) is obtained by determining the area (Aij) over which the integral in the numerator of equation (1) is calculated. This operation can be performed digitally.

[0155] The individual energy degradation units (11.i) are arranged on a support substrate (11b) so as to extend coaxially along the corresponding beam axis (Xi). The ridge filter can be produced by machining a block, by connecting individual pins to the support substrate (11b), by 3D printing technology, etc. The ridge filter (11) produced in this way can be installed between the outlet of the charged particle accelerator and the treatment zone (V) so that each sub-zone (Vi) is coaxial with the corresponding beam axis (Xi). The irradiation of the single-layer PBS drawing can be started.

[0156] The ridge filter (11) designed by the method of the present invention is particularly suitable for treatment plans involving FLASH irradiation of at least a portion of the treatment area (V) requiring the deposition of dose (Dij) into cells (Cij) by PBS with an ultra-high deposition rate (HDR), because the ridge filter allows the entire treatment area to be covered by a single mapping layer, thereby greatly reducing the scanning time required to deposit the dose (Dij) into each slice (Tj).

[0157]

[0158]

Claims

1. A method for designing a ridge filter for a charged particle accelerator, the ridge filter being used to deposit a specific dose (Dij) by means of an accelerated particle beam (100.i) to a specific location within a treatment volume (V) of tissue comprising tumor cells (3t), the deposition being performed in a single mapping layer defining the entire treatment volume (V) by spot-by-spot (Si) pencil beam scanning (PBS) according to a predetermined treatment plan TP, wherein: The beams (100.i) extend along respective beam axes (Xi) substantially parallel to an irradiation axis (X), deviating from parallelism to the irradiation axis (X) by an angle within ±5°, and wherein the tissue is characterized by a maximum beam range (W0) defined as the water equivalent distance at which the beam stops propagating through the tissue, the method comprising the steps of: defining a boundary inscribed in the treatment volume (V) by defining an area (Aj) on an upstream plane (Y,Z)j of N slices with thickness (dxj), wherein the plane (Y,Z)j is perpendicular to the irradiation axis (X), wherein the shortest water-equivalent thickness (d0) and the longest water-equivalent thickness (d1) to the patient's skin are defined as the points of the boundary closest to and farthest from the skin, respectively, measured along the irradiation axis (X), defining an array of sub-areas (Vi), each extending parallel to a corresponding beam axis (Xi) from said skin of said patient to a corresponding farthest water-equivalent thickness (d1), and the projection of each sub-area on a plane (Y, Z) perpendicular to said irradiation axis (X) defining an array of spots (Si) covering the entire area of ​​the projection of said area (V) on said plane (Y, Z), For each slice (Tj) of the N slices contained in a sub-region (Vi), define a cell (Cij) as the portion of said sub-region (Vi) contained in the corresponding slice (Tj), for each cell (Cij) of the given sub-area (Vi), determining the cell water equivalent thickness (dij) from the skin (3s) to the geometric center of the cell (Cij), and assigning the beam weight (ωij) required to deposit the specific dose (Dij) into the cell (Cij) according to the TP, wherein the beam weight (ωij) is proportional to the number of charged particles under the cell water equivalent thickness (dij), The ridge filter (11) is designed as a group of energy degradation units (11.i), wherein each energy degradation unit (11.i) is configured to reduce the initial energy (E0) of a charged particle beam (100.i) having a corresponding beam diameter (D100.i) coaxial with the corresponding beam axis (Xi) and the sub-area (Vi) to reduce the energy (Eij) so as to deposit the specific dose (Dij) into the corresponding cell (Cij) contained in the sub-area (Vi) with the cell water equivalent thickness (dij) according to the TP, and the energy degradation unit (11.i) of a given sub-area (Vi) is designed as follows: o for each cell (Cij) of the sub-region (Vi), determining the size of a sub-degradation unit (11.ij) having a generalized cylindrical geometry, the generalized cylindrical geometry having a base with an area (Aij) perpendicular to the corresponding beam axis (Xi) and a generatrix with a length Lij parallel to the corresponding beam axis (Xi), the sub-degradation unit being made of a material having a sub-unit water equivalent thickness (Wu) per unit length along the corresponding beam axis (Xi), wherein the length Lij is determined so that the sub-unit water equivalent thickness (Wij) of the sub-degradation unit (11.ij) is equal to the product of the sub-unit water equivalent thickness (Wu) per unit length and the length Lij, wherein the sum of the sub-unit water equivalent thickness (Wij) and the cell water equivalent thickness (dij) is equal to the maximum beam range (W0), and The area (Aij) of the sub-degradation unit (11.ij) is determined by making the normalized beam weight equal to the ratio of the integral of the fluence F(y,z) over the sub-unit base area (Aij) to the integral of the fluence over the base area (Abi) of the energy degradation unit (11.i), Among them, ωij / ∑ j ω ij is the normalized beam weight, the fluence F(y,z) is the charge per unit area of ​​the beam at the position (y,z) of the beam (100.i), and wherein the base area (Abi) is equal to the sum of the subunit base areas (Aij), o combining N sub-degradation units (11.ij) to obtain the energy degradation unit (11.i), the energy degradation unit being designed to degrade the energy of the beam (100.i) so as to deposit a desired dose (Dij) into the sub-area (Vi), The energy degradation units (11.i) corresponding to all remaining sub-areas (Vi) are designed as defined above, Therein, the expression "water equivalent thickness" (WET) is defined as the thickness of water that causes the same energy degradation of a particle beam as a particle beam passing through one or more materials of a given thickness.

2. The method according to claim 1, wherein: The specific dose (Dij) will be deposited into at least one selected specific location in the specific location within the region (V) of the tissue at an ultra-high dose deposition rate (HDR) according to the treatment plan, wherein HDR is defined as a dose deposition rate of HDR≥1Gy / s.

3. The method according to claim 1, wherein: The spots (Si) in the spot array are separated from each other by a distance (ds) that is less than or equal to 1.8 times the standard deviation (σ) of the fluence Fi(y,z) of the beam (100.i) at a single spot, and wherein the fluence (F(y,z)) of the beam (100.i) passing through the substrate area (Abi) is approximately constant at all values ​​of the plane (Y,Z)j defining the boundary inscribed in the zone (V).

4. The method according to claim 2, wherein: The spots (Si) in the spot array are separated from each other by a distance (ds) that is less than or equal to 1.8 times the standard deviation (σ) of the fluence Fi(y,z) of the beam (100.i) at a single spot, and wherein the fluence (F(y,z)) of the beam (100.i) passing through the substrate area (Abi) is approximately constant at all values ​​of the plane (Y,Z)j defining the boundary inscribed in the zone (V).

5. The method according to claim 1, wherein: The spots (Si) in the spot array are separated from each other by a distance (ds) greater than 1.2 times the standard deviation (σ) of the fluence Fi(y,z) of the beam (100.i) at a single spot, and wherein the fluence Fi(y,z) of the beam (100.i) passing through the substrate area (Abi) is approximately Gaussian distributed, where (yi,zi) are the coordinates of the position of the maximum value (Ai) of the fluence of the spot (Si) on the (Y,Z) plane, and where, in the case of a circular spot, then σ y =σ z =σ.

6. The method according to claim 2, wherein: The spots (Si) in the spot array are separated from each other by a distance (ds) greater than 1.2 times the standard deviation (σ) of the fluence Fi(y,z) of the beam (100.i) at a single spot, and wherein the fluence Fi(y,z) of the beam (100.i) passing through the substrate area (Abi) is approximately Gaussian distributed, where (yi,zi) are the coordinates of the position of the maximum value (Ai) of the fluence of the spot (Si) on the (Y,Z) plane, and where, in the case of a circular spot, then σ y =σ z =σ.

7. The method according to any one of claims 1 to 6, wherein: The energy degradation unit (11.i) is in the form of apertures arranged side by side according to the array of spots (Si) in a supporting substrate (11b), the supporting substrate having a thickness Bi measured along the beam axis (Xi), each aperture extending from a hole opening at the surface of the supporting substrate (11b) and penetrating to a given depth measured along the corresponding beam axis (Xi), wherein, Each energy degradation unit (11.i), o is formed by one or more sub-degradation units (11.ij, 11.i3, 11.i2, 11.i1) in the form of an aperture having a generalized cylindrical geometry of a cross-sectional area (Axij) and extending from a hole in the supporting substrate (11b) along the corresponding beam axis (Xi) by a length Lsij such that Lij=Bi-Lsij, and wherein, o The sub-degradation units (11.ij, 11.i3, 11.i2, 11.i1) are arranged within the base area (Abi).

8. The method according to claim 7, wherein: The energy degradation unit (11.i) comprises at least two sub-degradation units (11.ij), and the at least two sub-degradation units are arranged in the base area (Abi) in one of the following configurations: In a series configuration, where o the sub-degradation units are arranged in decreasing order of length (Lsij) along the corresponding beam axis (Xi), and the aperture with the longest length (Lsi3) is located at the center, and wherein, o the subunit base area (Aij) of a given sub-degraded unit (11.ij) is equal to the cross-sectional area difference between the cross-sectional area (Axij) of the given sub-degraded unit (11.ij) and the cross-sectional area (Axi(j+1)) of the sub-degraded unit (11.i(j+1)) defined within the given sub-degraded unit, In a parallel configuration, wherein the sub-degradation units are arranged side by side within the base area (Abi), with no spacing between two sub-degradation units, or with spacing between two adjacent sub-degradation units, In a hybrid configuration of parallel and series, wherein three or more sub-degradation units (11.ij) are arranged both in series and in parallel, wherein one or more structures formed by two or more sub-degradation units arranged in series along a corresponding beam axis (Xi), and one or more individual sub-degradation units are arranged side by side within the base area (Abi).

9. The method according to any one of claims 1 to 6, wherein: The energy degradation unit (11.i) is in the form of pins arranged side by side according to the array of spots (Si) and supported on a supporting substrate (11b), the supporting substrate having a thickness Bi measured along the beam axis (Xi), each pin extending from the supporting substrate along the corresponding beam axis (Xi), wherein Each energy degradation unit (11.i), o is formed by one or more sub-degraded units (11.ij, 11.i3, 11.i2, 11.i1) having a generalized cylindrical geometry of a cross-sectional area (Aij) and extending from the supporting substrate along the corresponding beam axis (Xi) by a length Lsij such that Lij=Bi+Lsij, and wherein, o The sub-degradation units (11.ij, 11.i3, 11.i2, 11.i1) are arranged within the base area (Abi).

10. The method according to claim 9, wherein: The energy degradation unit (11.i) comprises at least two sub-degradation units (11.ij), and the at least two sub-degradation units are arranged in the base area (Abi) in one of the following configurations: In a series configuration, where o The sub-degradation units are arranged in descending order of length (Lsij) along the corresponding beam axis (Xi), and the pin with the longest length (Lsi1) is located at the center, and wherein, o the subunit base area (Aij) of a given sub-degraded unit (11.ij) is equal to the cross-sectional area difference between the cross-sectional area (Axij) of the given sub-degraded unit (11.ij) and the cross-sectional area (Axi(j-1)) of the sub-degraded unit (11.i(j-1)) defined within the given sub-degraded unit, In a hybrid configuration of parallel and series, wherein three or more sub-degradation units (11.ij) are arranged both in series and in parallel, wherein one or more structures formed by two or more sub-degradation units arranged in series along a corresponding beam axis (Xi), and one or more individual sub-degradation units are arranged side by side within the base area (Abi).

11. The method according to claim 7, wherein: At least a first sub-degradation unit (11.11) of a first energy degradation unit (11.1) is made of a first material, which is different from a second material of a second sub-degradation unit (11.ij) of the first energy degradation unit or the second energy degradation unit (11.1, 11.2), and the first material has a sub-unit water equivalent thickness (Wu) value per unit length different from that of the second material, so as to change a length value (L11) of the first sub-degradation unit (11.11) compared to a length of a corresponding second sub-degradation unit (11.ij) made of the second material.

12. The method according to claim 9, wherein: At least a first sub-degradation unit (11.11) of a first energy degradation unit (11.1) is made of a first material, which is different from a second material of a second sub-degradation unit (11.ij) of the first energy degradation unit or the second energy degradation unit (11.1, 11.2), and the first material has a sub-unit water equivalent thickness (Wu) value per unit length different from that of the second material, so as to change a length value (L11) of the first sub-degradation unit (11.11) compared to a length of a corresponding second sub-degradation unit (11.ij) made of the second material.

13. The method according to claim 8 or 10, wherein: At least a first sub-degradation unit (11.11) of a first energy degradation unit (11.1) is made of a first material, which is different from a second material of a second sub-degradation unit (11.ij) of the first energy degradation unit or the second energy degradation unit (11.1, 11.2), and the first material has a sub-unit water equivalent thickness (Wu) value per unit length different from that of the second material, so as to change a length value (L11) of the first sub-degradation unit (11.11) compared to a length of a corresponding second sub-degradation unit (11.ij) made of the second material.

14. The method according to claim 11 or 12, wherein: The length (L11) of the first sub-degradation unit (11.11) is within ±20% of the length (Lij) of the second sub-degradation unit.

15. The method according to claim 13, wherein: The length (L11) of the first sub-degradation unit (11.11) is within ±20% of the length (Lij) of the second sub-degradation unit.

16. The method according to claim 1, wherein: The charged particle accelerator is a proton accelerator, and / or wherein the beam (100.i) deviates from parallelism to the irradiation axis (X) by an angle within ±3°.

17. The method according to claim 3 or 4, wherein: The spots (Si) in the spot array are separated from each other by a distance (ds) less than or equal to 1.5σ.

18. The method according to claim 5 or 6, wherein: Spots (Si) in the spot array are separated from each other by a distance (ds) greater than 1.5 times the standard deviation (σ) of the fluence Fi(y,z) of the beam (100.i) at a single spot.

19. The method according to claim 8, wherein: The sub-degradation units are coaxially arranged along the corresponding beam axis (Xi) in descending order of length (Lsij).

20. The method according to claim 10, wherein: The sub-degradation units are coaxially arranged along the corresponding beam axis (Xi) in descending order of length (Lsij).

21. The method according to claim 11 or 12, wherein: The first material has a different value of water equivalent thickness per unit length subunit (Wu) than the second material so as to reduce the length value (L11) of the first sub-degraded unit (11.11) compared to the length of a corresponding second sub-degraded unit (11.ij) made of the second material.

22. The method according to claim 13, wherein: The first material has a different value of water equivalent thickness per unit length subunit (Wu) than the second material so as to reduce the length value (L11) of the first sub-degraded unit (11.11) compared to the length of a corresponding second sub-degraded unit (11.ij) made of the second material.

23. The method according to claim 14, wherein: The lengths (Lij) of all sub-degradation units (11.ij) of the energy degradation unit (11.i) have the same length (Lij) within a variation range of ±20% of the average length (Lm,ij).

24. The method according to claim 15, wherein: The lengths (Lij) of all sub-degradation units (11.ij) of the energy degradation unit (11.i) have the same length (Lij) within a variation range of ±20% of the average length (Lm,ij).

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