Ridge filter in PBS treatment system and method for designing the same

Abstract

To provide a ridge filter in a PBS treatment system, and a method for designing the same.SOLUTION: A method for designing a ridge filter for a charged particle accelerator includes a step for dividing a cylindrical partial volume (Vi) that regulates a treatment volume (V) into N cells (Cij), and the ridge filter includes the same number of energy degrading units (11.i) as the number of existing spots (Si). The respective energy degrading units (11.i) are formed by N cylindrical degrading sub units unit (11.ij) with length (Lij) and area (Aij).SELECTED DRAWING: Figure 3a-3d

Description

[Technical Field]

[0001] The present invention relates to a ridge filter for imparting a predetermined dose (Dij) over an entire treatment volume (V) by irradiating it with a beam of accelerated particles (preferably protons) using a pencil beam scan (PBS) in a single painting layer. In particular, the present invention relates to a method for designing such a ridge filter, optimizing the dimensions of such a ridge filter to accurately impart a dose (Dij) according to a pre-established treatment plan. The ridge filter of the present invention is particularly suited for FLASH irradiation of a treatment volume (V) or a portion thereof at an ultra-high dose rate (HDR) by PBS. [Background technology]

[0002] Radiation therapy using particles or waves such as electron beams, proton beams, heavy ion beams, X-rays, and gamma rays has become an essential means of treating patients with tumors.

[0003] Because such radiation damages both tumor cells and healthy cells within a given volume, the first challenge in cancer treatment is to formulate a treatment plan that ensures the prescribed dose is delivered to tumor cells to effectively destroy or kill them, while limiting the dose delivered to healthy cells to preserve healthy cells as much as possible. The second challenge is to actually deliver the prescribed dose to tumor cells while actually delivering the limited dose to healthy cells, especially healthy cells adjacent to tumor cells.

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

[0005] Pencil beam scanning (PBS) is a technique that involves steering a beam of charged particles along a corresponding beam axis (Xi) toward individual spots in a mesh of spots (Sij) that define a target volume containing tumor cells. This ensures that a predetermined target dose is delivered to cells aligned with the individual spots. The beam is steered along the corresponding beam axis (Xi), and dose delivery proceeds according to a treatment plan and spot irradiation scan sequence that defines the dose (Dij) to be delivered to each cell aligned with a given spot along the beam axis (Xi). PBS reduces unnecessary radiation exposure to surrounding non-cancerous cells by shaping the treatment area to reflect the geometric shape of the tumor. In addition to the target's geometric shape, PBS allows for localized adjustment of the beam intensity depending on the location of the spots within the target.

[0006] The mesh generally includes several painting layers (Tj=T1~TN) perpendicular to the irradiation axis (X) which is the center of the beam axis (Xi). Spots (Sij, Si(j+1)...) are arranged in a two-dimensional array on the plane corresponding to the upstream of each layer (Tj). Each painting layer defines several cells (Cij, C(i+1)j...) defined as generalized cylinders with a corresponding spot (Sij) as the base and a generatrix parallel to the corresponding beam axis (Xi). The cells (Cij) have the same thickness as the corresponding layer (Tj). The superposition of painting layers (Tj) defines the entire treatment volume (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 layers (T(j+1), T(j+2)...).

[0007] The terms “upstream” and “downstream” are defined with respect to the direction of the beam (100.i) of charged particles. In this specification, unless otherwise indicated, “generalized cylinder,” “cylinder,” and its derivatives refer to a surface consisting of all points on all lines passing through the periphery of the base that are contained in a plane parallel to and not parallel to the generatrix. The base can have any flat geometric shape. In particular, if the base is circular, it defines a cylinder. If the base is polygonal, it forms a prism. A right circular cylinder is a cylinder whose base is perpendicular to the generatrix.

[0008] PBS is highly advantageous because it optimizes the geometric distribution of dose delivery to match the geometric shape of the treatment volume (V) surrounding the tumor. However, PBS can be time-consuming because the beam must scan each spot (Sij) and each layer (Tj). Moving the beam from one beam axis (Xi) to another (X(i+1)) takes several milliseconds. Varying the energy of a given beam parallel to a given beam axis (Xi) to deliver the desired dose (Dij) to cells (Cij) in different layers (Tj) takes even longer, on the order of 500 milliseconds. Therefore, the number of layers (Tj) strongly influences the duration of treatment.

[0009] Using an accelerated proton beam, a given beam parallel to a given beam axis (Xi) can successively impart a predetermined charge to the corresponding cell (Cij) in each painting layer aligned along the given beam axis (Xi) by superimposing several Bragg peaks at shifted depths in each painting layer along the beam axis (Xi). As a result, an expanded Bragg peak (SOBP) is obtained over all cells (Cij, Ci(j+1)...) aligned along the given beam axis (Xi). However, this operation requires successively changing 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 painting a picture, layering several layers and successively changing the energy of a given beam takes time. It is preferable that the entire dose (Dij) can be applied across all layers along the beam axis (Xi) centered on a given spot (Sij) by applying a single beam of fixed energy, that is, by applying a single paint layer.

[0010] Reducing treatment time shortens the time each patient occupies the particle accelerator, making it more comfortable for patients. It is also advantageous when the treatment plan includes FLASH irradiation, in which the dose is delivered to cells at an ultra-high dose rate (HDR) of at least 1 Gy / s. A given dose delivered by HDR has been shown to preserve healthy cells compared to the same dose delivered at a lower dose rate (LDR). What makes FLASH irradiation particularly interesting is that a given dose delivered to tumor cells has the same killing effect, regardless of whether it is delivered by HDR or LDR. However, in delivering dose (Dij) layer by layer to the treatment volume (V) using PBS, the delivery rate to cells is considerably reduced because cells located upstream within the volume (V) are inevitably irradiated multiple times until all cells (Cij) aligned along a given beam axis (Xi) receive the corresponding dose (Dij).

[0011] Applying a predetermined dose (Dij) to a therapeutic volume by PBS in a single layer can be achieved by using a ridge filter. When using a ridge filter, it is necessary that the spots (Sij, Si(j+1)...) in each layer (Tj) be aligned along the corresponding beam axis (Xi). Ridge filters having energy degrading units in the form of flat pyramidal or stepped pyramidal or ridge surfaces have been shown in the art. For example, (Non-Patent Document 1) describes a ridge filter having multiple energy degrading units in the form of flat pyramidal surfaces extending along the beam axis (Xi) corresponding to each spot (Sij). (Non-Patent Document 2) describes a multilayer ridge filter in which these layers overlap each other, and each layer has parallel linear ridges having a stepped pyramidal cross-section. (Patent Document 1) describes a ridge filter having conical or triangular pyramidal projections positioned upstream of a plate-shaped Bragg peak widening filter with multiple through holes for widening the SOBP along the corresponding beam axis (Xi).

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

[0013] Designing and dimensioning the energy degrading unit for ridge filters remains a challenge. (Non-patent document 1) describes the function (DSOBP-D(z)). 2 The document describes a complex method for determining the dimensions of the pyramidal protrusions forming the ridge filter, including minimizing the , and the DSOBP specifies a uniform dose distribution. The equation is TIFF2023074477000002.tif6170, where B(z) is the original Bragg peak to be measured, Δz is the step size between consecutive Bragg peaks, the weight ωi determines the contribution of each peak i to SOBP, and D(z) is the resulting depth dose distribution. Solving this equation requires an experimentally fitted function. Non-patent document 2 provides little information on how its ridge filter is designed, other than that Monte Carlo calculations are used. [Prior art documents] [Patent Documents]

[0014] [Patent Document 1] Japanese Patent Publication No. 2019-136167 [Non-patent literature]

[0015] [Non-Patent Document 1] Simeonov et al.,Phys.Med.Biol.62(2017)7075 [Non-Patent Document 2] Sakae et al.,Med.Phys.27,2,(2000)368 [Overview of the Initiative]

[0016] Therefore, a simple, reliable, and repeatable method is needed for determining the dimensions of the energy degrading unit of a ridge filter. An alternative ridge filter design that is easier to manufacture with the required precision is proposed. These and other advantages are described in more detail below.

[0017] The present invention relates to a method for designing a ridge filter of a treatment system including a charged particle accelerator (preferably a proton accelerator), a beam line nozzle, a collimator, etc. (collectively referred to as an accelerator herein) for applying a specific dose (Dij) to a specific location within a treatment volume of tissue containing tumor cells using a beam of accelerated particles. The present invention relates to dose delivery by pencil beam scanning (PBS) for each spot according to a predetermined treatment plan (TP) in a single painting layer defining the entire treatment volume. The beam is substantially parallel to the irradiation axis (X) and extends along a corresponding beam axis (Xi) that deviates from the parallel with the irradiation axis (X) by an angle within ±5°, preferably within ±3°. The tissue is characterized by a maximum beam range (W0) defined as the water equivalent distance at which the beam ceases to propagate through the tissue. The method includes the following steps.

[0018] 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 given particle beam as a given thickness of one or more materials traversed by the particle beam.

[0019] The boundary inscribed in the treatment volume is defined by defining an area (Aj) over a plane (Y,Z)j upstream of N slices (Tj = T1 to TN) of thickness (dxj). 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) with respect 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).

[0020] An array of sub-volumes (Vi) is defined, each sub-volume extending from the patient's skin parallel to the corresponding beam axis (Xi) to the corresponding farthest water equivalent thickness, and the projection of the array of sub-volumes (Vi) onto a plane (Y,Z) perpendicular to the irradiation axis (X) defines an array of spots that cover the entire area of the volume projection onto the plane (Y,Z).

[0021] For each slice (Tj) of the N slices (T1 to TN) contained within the partial volume (Vi), the cell is defined as a portion of the partial volume (Vi) contained within the corresponding slice (Tj). For each cell of a given partial volume (Vi), the water equivalent thickness of the cell is determined from the skin to the geometric center of the cell. A beam weight (ωij) is determined for each cell to impart a specific dose (Dij) to the cell according to TP. The beam weight (ωij) is proportional to the number of charged particles at the water equivalent thickness of the cell.

[0022] The ridge filter is designed to comprise a set of energy degradation units, and each energy degradation unit is configured to reduce the initial energy (E0) of a beam of charged particles of a certain beam diameter, which is coaxial with the corresponding beam axis (Xi) and the partial volume (Vi), to a reduced energy (Eij) such that a specific dose (Dij) is imparted at the water equivalent thickness of the corresponding cell within the partial volume (Vi) according to TP. The energy degradation unit of a given partial volume (Vi) is designed as follows.

[0023] For each cell of the partial volume (Vi), a degradation sub-unit having a generalized cylindrical geometry with a base area (Aij) perpendicular to the corresponding beam axis (Xi) and a generatrix length (Lij) parallel to the corresponding beam axis (Xi) is dimensioned. The degradation sub-unit is made of a material having a water equivalent thickness (Wu) of the sub-unit per unit length along the corresponding beam axis (Xi), and the length is determined such that the degradation sub-unit has a water equivalent thickness (Wij = Wu × Lij) of the sub-unit equal to the product of the water equivalent thickness (Wu) of the sub-unit per unit length and the length (Lij). The sum of the water equivalent thickness (Wij) of the sub-unit and the water equivalent thickness (dij) of the cell is equal to the maximum beam range (W0) (i.e., W0 = Wij + dij).

[0024] In this specification, as defined above, “generalized cylinder,” “cylinder,” and its derivatives refer to a surface consisting of all points on all lines passing through the periphery of the base that are contained in a plane parallel to the generatrix and not parallel to the generatrix. The base can have any flat geometric shape. In particular, if the base is circular, it defines a cylinder. If the base is polygonal, it forms a prism. A right circular cylinder is a cylinder whose base is perpendicular to the generatrix.

[0025] The area (Aij) of the degrading subunit is equal to the normalized beam weight (ωij / Σ j ωij) is determined by equaling the ratio of the integral of the fluence (F(y,z)) over the base area (Aij) of the subunit to the same integral over the base area (Abi) of the degrading unit (11.i), TIFF2023074477000003.tif13170 Here, the fluence F(y,z) is the number of charges per unit area of ​​the beam (100.i) at beam position (y,z), and the base area (Abi) is equal to the sum of the subunit areas (Aij) (i.e., Abi = Σ j Aij).

[0026] N degrading subunits are combined to obtain an energy degrading unit designed to degrade the beam energy to impart the required dose (Dij) to a partial volume (Vi). Energy degrading units corresponding to all remaining partial volumes (Vi) can be designed as defined above.

[0027] In a preferred embodiment, a specific dose (Dij) is delivered at a very high dose rate (HDR) to at least selected specific locations within the tissue volume according to the treatment plan. The very high dose rate (HDR) is defined as a dose rate with HDR ≥ 1 Gy / s.

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

[0029] Instead, the spots in the array of spots may be separated from each other by a distance (ds) greater than 1.2 times the standard deviation (σ) of the beam fluence (Fi(y,z)) at a single spot (i.e., ds > 1.2σ), preferably greater than 1.5 times. Under these conditions, the beam fluence (Fi(y,z)) passing through the base area (Abi) is Gaussian-distributed. It is approximated as TIFF2023074477000004.tif10170, where (yi,zi) is the coordinate of the position of the maximum value (Ai) of the fluence of the spot (Si) in the (Y,Z) plane, and in the case of a circular spot, σ y =σ z =σ.

[0030] In a preferred configuration of the present invention, the energy degrading unit is in the form of orifices arranged in a row according to an array of spots in a support base of thickness (Bi) measured along the beam axis (Xi). Each orifice extends from an opening in the surface of the support base and penetrates to a given depth measured along the corresponding beam axis (Xi). Each energy degrading unit (11.i) is formed by one or more degrading subunits in the form of orifices having a generalized cylindrical geometry of cross-sectional area (Ai) and extending over a length (Lsij) such that Lij = Bi - Lsij along the corresponding beam axis (Xi). The degrading subunits defined above are arranged within the base area (Abi).

[0031] In this configuration of energy degrading unit, at least two subunits can be arranged within the base area (Abi) in one of the following structures: series, parallel, or mixed.

[0032] In a series configuration, the degrading subunits are aligned along the corresponding beam axis (Xi) in order of decreasing length (Lsij), preferably coaxially, such that the orifice with the longest length (Lsi3) is located in the center. The base area (Aij) of a given degrading subunit is equal to the difference in cross-sectional area (Axij - Axi(j+1)) between the cross-sectional area (Axij) of a given degrading unit and the cross-sectional area (Axi(j+1)) of a degrading unit circumscribed within the given degrading unit.

[0033] In a parallel structure, degrading subunits are arranged side by side within the base area (Abi), either with no space between two degrading subunits or with space between two adjacent degrading subunits.

[0034] In mixed parallel and series structures, three or more degrading subunits are arranged in both series and parallel configurations. One or more structures are formed by two or more degrading subunits aligned in series along the corresponding beam axis (Xi), and optionally, one or more individual degrading subunits are arranged side by side within the base area (Abi).

[0035] In an alternative preferred configuration of the present invention, the energy degrading units are in the form of pins arranged in a line according to the arrangement of spots and supported on a support base of thickness (Bi) measured along the beam axis (Xi). Each pin extends from the support base along the corresponding beam axis (Xi). In this configuration, each energy degrading unit is formed by one or more degrading subunits having a generalized cylindrical geometry of cross-sectional area (Aij) and extending from the support base along the corresponding beam axis (Xi) over a length (Lsij) such that Lij = Bi + Lsij. These degrading subunits are arranged within the base area (Abi).

[0036] In this configuration of energy degrading unit, at least two subunits can be arranged within the base area (Abi) in one of the following structures: series, parallel, or mixed.

[0037] In a series configuration, the degrading subunits are arranged along the corresponding beam axis (Xi) in order of decreasing length (Lsij), preferably coaxially, and such that the pin with the longest length (Lsi1) is located in the center. The base area (Aij) of a given degrading subunit is equal to the difference in cross-sectional area (Axij - Axi(j-1)) between the cross-sectional area (Axij) of a given degrading unit and the cross-sectional area (Axi(j-1)) of a degrading unit circumscribed within the given degrading unit.

[0038] In mixed parallel and series structures, three or more degrading subunits are arranged in both series and parallel configurations. One or more structures are formed by two or more degrading subunits aligned in series along the corresponding beam axis (Xi), and optionally, one or more individual degrading subunits are arranged side by side within the base area (Abi).

[0039] In a preferred embodiment, at least the first degrading subunit of the first energy degrading unit may be made of a first material different from the second material of the first energy degrading unit or the second degrading subunit of the second energy degrading unit. The first material has a different value of water equivalent thickness (Wu) per unit length of the subunit from the second material, such that it changes, preferably decreases, the value of the length of the first degrading subunit (L11 = W11 / Wu) compared to, for example, the length of the corresponding first energy subunit made from the second material. By using different materials with different values ​​of water equivalent thickness (Wu) per unit length of the subunit, the length of the first degrading subunit (L11) can be kept within ±20% of the length of the second degrading subunit (Lij) in order to make the energy degrading unit more compact. The lengths (Lij) of all degrading subunits of an energy degrading unit are preferably the same length (Lij) with a variation of ±20% of the average length (Lm,ij) (i.e., Lij = Lm,ij ± 20% ∀j). [Brief explanation of the drawing]

[0040] [Figure 1a] Figure 1(a) schematically shows the treatment volume (V). The treatment volume (V) is divided into partial volumes (Vi) extending parallel to the corresponding beam axis (Xi), and each partial volume (Vi) is divided into cells (Cij), to which a predetermined dose (Dij) is assigned. The corresponding SOBP is shown along different beam axes (Xi) passing through the corresponding energy degrading units of the ridge filter. [Figure 1b] Figure 1(b) shows a side view of the treatment volume (V) in Figure 1(a) in a state where SOBP is obtained by the three beams (100.i, 100.k, 100.m) after passing through the ridge filter according to the present invention. [Figure 1c]FIG. 1(c) shows the SOBP obtained in a state where the beam (100.i) extends along the beam axis (Xi) coaxial with the irradiation axis (X). [Figure 1d] FIG. 1(d) shows the SOBP obtained in a state where the beam (100.m) extends along the beam axis (Xm). [Figure 2a] FIG. 2(a) shows the depth-dose profile of the typical Bragg peak of a beam of a given energy. The horizontal axis represents the depth in water. The maximum beam range (W0) in water is the depth corresponding to the energy that exceeds the maximum value of the Bragg peak and is equal to 80% of the maximum value of the Bragg peak, and W0 is defined to be at least dij or more (dij < W0). [Figure 2b] FIG. 2(b) shows the depth-dose profile of the Bragg peak of the beam in FIG. 2(a) in a state where the energy degrading unit intersects the path of the beam. [Figure 3a] FIG. 3(a) shows the beams (100.i, 100.(i + 1)...) that extend along their respective beam axes (Xi, X(i + 1)...) parallel to each other and to the irradiation axis (X) and pass through the ridge filter. [Figure 3b] FIG. 3(b) shows the beams (100.i, 100.(i + 1)...) that spread fanwise around the irradiation axis (X) and extend along their respective beam axes (Xi, X(i + 1)...) and pass through the ridge filter. [Figure 3c] FIG. 3(c) schematically shows a ridge filter provided with an energy degrading unit in the form of a pin. [Figure 3d] FIG. 3(d) shows two energy degrading units in a state where the beams (100.i, 100.(i + 1)) extend along their respective beam axes (Xi, X(i + 1)) that are not parallel to each other (the angle formed by Xi and X(i + 1) is exaggerated). [Figure 3e]Figure 3(e) shows the volume components of the treatment volume (V), which extends parallel to the irradiation axis (X) and includes several partial volumes (Vi) divided into cells (Cij). The SOBP obtained in the partial volume (V5) is also shown. [Figure 3f] Figure 3(f) shows the volume elements of the treatment volume (V), which include several partial volumes (Vi) extending parallel to the corresponding beam axis (Xi). The beam axes (Xi) are not perfectly parallel to each other, nor are they perfectly parallel to the irradiation axis (X). The angular deviations have been exaggerated for clarity. [Figure 4a] Figure 4(a) shows one embodiment of a ridge filter that includes an energy degrading unit formed by gaps. [Figure 4b] Figure 4(b) shows a cross-sectional side view of the gaps forming the energy degrading unit of the ridge filter in Figure 4(a), which consists of three degrading subunits. The depth dose profile of the SOBP generated by the degrading unit is also shown. [Figure 4c] Figure 4(c) shows a cross-sectional side view of the first degrading subunit of the energy degrading unit shown in Figure 4(b). Two examples of possible cross-sectional shapes of the first degrading subunit are shown: circular or polygonal (hexagonal). The Bragg peak generated by the first degrading subunit is maximum at depth di1. [Figure 4d] Figure 4(d) shows a cross-sectional side view of the second degrading subunit of the energy degrading unit shown in Figure 4(b). Two examples of possible cross-sectional shapes for the second degrading subunit are shown: circular or polygonal (hexagonal). The Bragg peak generated by the second degrading subunit is maximum at depth di2. [Figure 4e]Figure 4(e) shows a cross-sectional side view of the third degrading subunit of the energy degrading unit shown in Figure 4(b). Two examples of possible cross-sectional shapes for the third degrading subunit are shown: circular or polygonal (hexagonal). The Bragg peak generated by the third degrading subunit is maximum at depth di3. [Figure 4f] Figure 4(f) shows an alternative embodiment of a ridge filter comprising an energy degrading unit formed by gaps. [Figure 4g] Figure 4(g) shows a cross-sectional side view of the gaps forming the energy degrading unit of the ridge filter in Figure 4(f), which consists of three degrading subunits. The depth dose profile of the SOBP generated by the degrading unit is also shown. [Figure 4h] Figure 4(h) shows a cross-sectional side view of the first degrading subunit of the energy degrading unit shown in Figure 4(g). Examples of possible cross-sectional shapes of the first degrading subunit are shown. The Bragg peak generated by the first degrading subunit is maximum at depth di1. [Figure 4i] Figure 4(i) shows a cross-sectional side view of the second degrading subunit of the energy degrading unit in Figure 4(g). Two examples of possible cross-sectional shapes for the second degrading subunit are shown: circular or polygonal (hexagonal). The Bragg peak generated by the second degrading subunit is maximum at depth di2. [Figure 4j] Figure 4(j) shows a cross-sectional side view of the third degrading subunit of the energy degrading unit in Figure 4(g). Examples of possible cross-sectional shapes of the third degrading subunit are shown. The Bragg peak generated by the third degrading subunit is maximum at depth di3. [Figure 5a]Figure 5(a) shows one embodiment of a ridge filter that includes an energy degrading unit formed by protruding pins. [Figure 5b] Figure 5(b) shows a cross-sectional side view of the protruding pins that form the energy degrading unit of the ridge filter in Figure 5(a), which consists of three degrading subunits. The depth dose profile of the SOBP generated by the degrading unit is shown. [Figure 5c] Figure 5(c) shows a cross-sectional side view of the first degrading subunit of the energy degrading unit shown in Figure 5(b). Two examples of possible cross-sectional shapes of the first degrading subunit are shown: circular or polygonal (hexagonal). The Bragg peak generated by the first degrading subunit is maximum at depth di3. [Figure 5d] Figure 5(d) shows a cross-sectional side view of the second degrading subunit of the energy degrading unit shown in Figure 5(b). Two examples of possible cross-sectional shapes for the second degrading subunit are shown: circular or polygonal (hexagonal). The Bragg peak generated by the second degrading subunit is maximum at depth di2. [Figure 5e] Figure 5(e) shows a cross-sectional side view of the third degrading subunit of the energy degrading unit shown in Figure 5(b). Two examples of possible cross-sectional shapes for the third degrading subunit are shown: circular or polygonal (hexagonal). The Bragg peak generated by the third degrading subunit is maximum at depth di1. [Figure 5f] Figure 5(f) shows an alternative embodiment of a ridge filter comprising an energy degrading unit formed by protruding pins. [Figure 5g]Figure 5(g) shows a cross-sectional side view of the protruding pins that form the energy degrading unit of the ridge filter in Figure 5(f), which consists of three degrading subunits. The depth dose profile of the SOBP generated by the degrading unit is also shown. [Figure 5h] Figure 5(h) shows a cross-sectional side view of the first degrading subunit of the energy degrading unit shown in Figure 5(g). Examples of possible cross-sectional shapes of the first degrading subunit are shown. The Bragg peak generated by the first degrading subunit is maximum at depth di3. [Figure 5i] Figure 5(i) shows a cross-sectional side view of the second degrading subunit of the energy degrading unit in Figure 5(g). Two examples of possible cross-sectional shapes of the second degrading subunit are shown: circular or polygonal (hexagonal). The Bragg peak generated by the second degrading subunit is maximum at depth di2. [Figure 5j] Figure 5(j) shows a cross-sectional side view of the third degrading subunit of the energy degrading unit in Figure 5(g). Examples of possible cross-sectional shapes of the third degrading subunit are shown. The Bragg peak generated by the third degrading subunit is maximum at depth di1. [Figure 6a] Figure 6(a) shows a cross-sectional side view of the energy degrading unit (11.i) of Figure 4(b) and the corresponding SOBP, comprising N=3 degrading subunits (11.ij) formed by a continuous gap. [Figure 6b] Figure 6(b) shows a modified form of the energy degrading unit (11.i) of Figure 6(a) and the corresponding SOBP, comprising N=6 degrading subunits (11.ij). [Figure 6c]Figure 6(c) shows a modified form of the energy degrading unit (11.i) of Figures 6(a) and 6(b), and the corresponding SOBP, which includes a degrading subunit (11.ij) that forms a continuous truncated generalized conical gap as N→∞. [Figure 6d] Figure 6(d) shows an energy degrading unit (11.i) and its corresponding SOBP in the form of a stepped pin, comprising N=6 degrading subunits (11.ij). [Figure 6e] Figure 6(e) shows a modified form of the energy degrading unit (11.i) of Figure 6(d) and the corresponding SOBP, which includes a degrading subunit (11.ij) as N→∞ forms a continuous truncated generalized cone. [Figure 7a] Figure 7(a) shows the Bragg peak shift characterizing dose delivery by a beam passing through an energy degrading unit in the form of a protruding pin, which comprises two coaxially arranged degrading subunits with different lengths and cross-sectional areas. [Figure 7b] Figure 7(b) shows the Bragg peak shift characterizing dose delivery by a beam passing through an energy degrading unit in the form of a gap, comprising two degrading subunits with different lengths and cross-sectional areas. [Figure 7c] Figure 7(c) shows the Bragg peak shift characterizing dose delivery by a beam passing through an energy degrading unit in the form of a concentric element, which has two degrading subunits with different densities and cross-sectional areas (the density of the inner element is lower than that of the surrounding element). [Figure 7d] Figure 7(d) shows the Bragg peak shift characterizing dose delivery by a beam passing through an energy degrading unit in the form of a concentric element, which has two degrading subunits with different densities and cross-sectional areas (the density of the inner element is higher than that of the surrounding element). [Figure 7e]Figure 7(e) shows the Bragg peak shift characterizing dose delivery by a beam passing through an energy degrading unit in the form of a protruding pin, which comprises two side-by-side degrading subunits with different lengths and cross-sectional areas. [Figure 7f] Figure 7(f) shows a perspective view of the energy degrading unit (11.i) of the type shown in Figure 7(a). [Figure 7g] Figure 7(g) shows a perspective view of the energy degrading unit (11.i) of the type shown in Figure 7(e). [Figure 7h] Figure 7(h) shows a perspective view of one embodiment of an energy degrading unit (11.i) comprising degrading subunits arranged in a mixed configuration of both series and parallel structures. [Figure 7i] Figure 7(i) shows a perspective view of an alternative embodiment of the energy degrading unit (11.i) comprising degrading subunits arranged in a mixed configuration of both series and parallel structures. [Modes for carrying out the invention]

[0041] 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 use a beam (100.i) of accelerated particles to deliver a specific dose (Dij) to each spot (Si) in an array of spots at specific locations within a treatment volume (V) of tissue containing tumor cells (3t) by pencil beam scanning (PBS), according to a predetermined treatment plan (TP). By using a ridge filter, PBS can be performed in a single painting layer that defines the entire treatment volume (V). In PBS, a narrow pencil beam is deflected to scan each spot (Si) in an array. Although a single beam is deflected, each beam (100.i) that is successively directed to a corresponding spot (Si) and extends along the corresponding beam axis (Xi) is treated as a separate beam from a beam (100.k) that is directed to a different spot (Sk, k≠i) and extends along a second beam axis (Xk). As shown in Figures 1(b) and 3(a), the beam axis (Xi) is approximately parallel to the irradiation axis (X). In practice, since each "individual" beam (100.i) is a single beam deflected from a common accelerator exit point toward individual spots (Si) in the array, the beam axis (Xi) deviates by an angle within ±5°, preferably within ±3°, of parallelism with the irradiation axis (X), depending in particular on the size of the treatment volume (V) and the distance from the treatment volume (V) to the accelerator exit.

[0042] The structure through which the beam (100.i) traverses absorbs a portion of the beam's energy, thereby determining the penetration depth at the location of the Bragg peak along the beam axis (Xi). The penetration depth at the location of the Bragg peak in a given structure can be defined as the "water equivalent thickness" (=WET), that is, it can be characterized by the maximum beam range in water (W0), which defines the location where the beam propagation stops in water. 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 the particle beam as one or more materials of a given thickness through which the particle beam traverses. The maximum beam range (W0) can be directly related to the penetration depth of the same beam through the structure. Therefore, WET and the penetration depth at the location of the Bragg peak through the structure can be used synonymously, and naturally, the former (WET) is easier to test and measure experimentally.

[0043] The design method for the ridge filter (11) according to the present invention is: The steps include defining a treatment volume (V) and a map showing the location and dose assigned to the treatment volume (V), • The steps to design and define the dimensions of the ridge filter accordingly. Includes.

[0044] The definition of the treatment volume primarily involves defining the inscribed boundary of the treatment volume (V), as schematically shown in Figures 1(a), 1(b), 3(a), and 3(b). This operation involves defining the area over the upstream plane (Y,Z)j of N slices (Tj=T1~TN) of thickness (dxj), where 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) to which the beam (100.i) first strikes. The depth of the treatment volume (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), where these water equivalent thicknesses correspond to the closest and furthest boundary points, respectively, relative to the skin (3s) measured along the irradiation axis (X).

[0045] Secondly, as shown in Figures 1(a), 1(b), 3(e), and 3(f), the inscribed treatment volume (V) within the boundary is divided by defining an array of partial volumes (Vi). Each partial volume extends parallel to the corresponding beam axis (Xi) from the patient's skin to the corresponding farthest equivalent water thickness (d1), and the projection of the array of partial volumes (Vi) onto a plane (Y,Z) perpendicular to the irradiation axis (X) defines an array of spots (Si) that cover the entire area of ​​the projection of volume (V) onto the plane (Y,Z) (see Figure 1(a)).

[0046] Thirdly, the partial volume (Vi) itself is divided into cells (Cij) as follows: For each slice (Tj) of the N slices (T1~TN) contained within the partial volume (Vi), a cell (Cij) is defined as a portion of the partial volume (Vi) contained within the corresponding slice (Tj). This is shown in Figures 1(a), 3(e), and 3(f). For each cell (Cij) of a given partial volume (Vi), the water equivalent thickness (dij) of the cell from the skin (3s) to the geometric center of the cell (Cij) is determined. A beam weight (ωij) is determined for each cell (Cij) to impart a specific dose (Dij) to the cell (Cij) according to TP. The beam weight (ωij) corresponds to the number of charged particles (e.g., protons) delivered to the spot (Cij). Therefore, the beam weight (ωij) is proportional to the number of charged particles in the water equivalent thickness (dij) of the cell.

[0047] Once the treatment volume is divided into partial volumes (Vi) and cells (Cij), and the beam weights (ωij) required to assign a dose (Dij) to each cell (Cij) according to the treatment plan are determined, the ridge filter (11) can be designed and sized accordingly as follows.

[0048] The ridge filter (11) comprises a set of energy degrading units (11.i) configured to reduce the initial energy (E0) of a beam (100.i) of charged particles having a beam diameter (D100.i) to a reduced energy (Eij), coaxial with the corresponding beam axis (Xi) and partial volume (Vi), and having a beam diameter (D100.i), such that a specific dose (Dij) is applied to the corresponding cell (Cij) contained within the partial volume (Vi) according to TP at the water equivalent thickness (dij) of the cell. The principle is shown in Figures 2(a) and 2(b).

[0049] Figure 2(a) plots the dose (Dij) imparted as a function of the water equivalent thickness (WET) along the beam axis (Xi) by a beam (100.i) of a given energy (E0). As described above, the maximum beam range (WET) in water 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., if the treatment volume (V) extends deeper than the Bragg peak, a beam of higher energy must be used. Figure 2(b) shows the displacement of the Bragg peak when a degrading subunit (11.ij) is inserted into the path of the same beam (100.i) of a given energy. In this case, it can be seen that the WET of the Bragg peak is at a depth (dij < W0) from the skin (3s). This is because a part (E0 - Eij) of the energy of the beam (100.i) is absorbed by the degrading subunit (11.ij) through which the beam must pass. Figure 2(b) shows a method of displacing one Bragg peak with WET = W0 to the desired WET = dij by interposing the degrading subunit (11.ij). Since the SOBP can be formed by superimposing several Bragg peaks distributed over a given depth along the irradiation axis (X) so as to impart the required dose (Dij) to the entire partial volume (Vi) by PBS in a single painting layer, several degrading subunits (11.ij) can be superimposed to form an energy degrading unit (11i), and each degrading subunit (11.ij) is dimensioned to shift the Bragg peak from WET = W0 to the equivalent thickness (dij) of the corresponding cell, ensuring that the required dose (Dij) is imparted to each cell (Cij) of the partial volume (Vi) irradiated by the beam (100.i).This is shown in Figures 6(a) to 6(e), which show the energy degrading unit (11.i) and its corresponding SOBP, each consisting of N=3, N=6, and N→∞ degrading subunits (11.ij). The number of Bragg peaks required to obtain the desired SOBP according to TP and the corresponding water equivalent thickness (dij) of the cell must be determined. The number of Bragg peaks in a given beam (100.i), and therefore the number of degrading subunits (11.ij) in the corresponding energy degrading unit (11.i), is equal to the number of slices (T1 to TN), N. Each Bragg peak in a given beam (100.i) has a corresponding energy degrading unit (11.i), and these degrading subunits (11.ij) can be designed to obtain the required SOBP and, consequently, to impart the required dose (Dij) to the corresponding cell (Cij). In the case of the degrading subunit (11.ij) as N→∞ as shown in Figures 6(c) and 6(e), the step in the stepped configuration of the energy degrading unit (11.i) becomes small enough to be almost zero, resulting in a flat geometric shape on the surface of the truncated generalized cone shown in Figures 6(c) and 6(e), and the generalized cone can have a base of any geometric shape, not limited to a circle.

[0050] Each degrading subunit (11.ij) has a generalized cylindrical geometry consisting of 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) (i.e., it is not necessarily a cylinder). The degrading subunits shown in Figures 4(a) to 4(e) and Figures 5(a) to 5(e) have circular or polygonal cross-sections. It is clear that other cross-sections are also possible. However, it is desirable that the cross-section has some degree of symmetry to facilitate the calculation of the area (Aij) dimension. The degrading subunits are made of a material having a water equivalent thickness (Wu) per unit length of the subunit along the corresponding beam axis (Xi). The length (Lij) and area (Aij) of each degrading subunit (11.ij) can be dimensionally defined as follows:

[0051] The length (Lij) of the degrading subunit (11.ij) is determined such that the degrading subunit (11.i) has a subunit water equivalent thickness (Wij = Wu × Lij) equal to the product of the subunit water equivalent thickness (Wu) per unit length and its length (Lij). Considering that the sum of the subunit 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 degrading subunit (11.ij) is: The code is specified as TIFF2023074477000005.tif7170, and the coefficient (W0-dij) is illustrated in Figure 2(b). In this way, the length of the degrading subunit is fully defined.

[0052] The area (Aij) of the degrading subunit (11.ij) is equal to the normalized beam weight (ωij / Σ j ωij) is determined by making equal to the ratio of the integral of the fluence (F(y,z)) over the base area (Aij) of the subunit to the integral of the fluence (F(y,z)) over the base area (Abi) of the degrading unit (11.i), TIFF2023074477000006.tif15170 Here, the fluence F(y,z) is the number of charges per unit area of ​​the beam (100.i) at beam position (y,z), and the base area (Abi) is equal to the sum of the subunit areas (Aij) as shown in Figures 4(a) and 5(a) (i.e., Abi = Σ j Aij). As shown in Figures 7(a) to 7(e), the fluence of a charged particle beam generally has a Gaussian distribution. The beam diameter (D100) is defined as a distance of 4σ, where σ is the standard deviation of the Gaussian distribution that characterizes the beam's fluence. Approximately 95% of the protons in the spot are located within this diameter of 4σ.

[0053] As shown in Figures 4(b) to 4(e), 4(g) to 4(j), 5(b) to 5(e), and 5(g) to 5(j), the three degrading subunits (11.ij) of the energy degrading unit (11.i) can be coaxially assembled in a dimensionally defined manner to obtain an energy degrading unit (11.i) designed to degrade the energy of the beam (100.i) to impart the required dose (Dij) to a partial volume (Vi).

[0054] The same operation is repeated to design the energy degrading units (11.i) corresponding to all remaining partial volumes (Vi) as defined above.

[0055] Arrangement of spots (Si) As shown in Figure 1(a), an arrangement of spots (Si) is defined on a projection plane (Y,Z) perpendicular to the irradiation axis (X), covering a region of the treatment volume (V) projected parallel to the irradiation axis (X).

[0056] The oncologist determines the geometric shape and topography characteristics of the tumor region based on images of the tumor region obtained by computed tomography (CT scan). As shown in Figure 1(b), in order to reach the treatment volume (V), the beam (100.i) must pass through healthy cells separating the treatment volume (V) from the patient's skin (3s), thereby irradiating both healthy cells and tumor cells. The oncologist defines a treatment plan that specifies the dose (Dij) to be delivered to the treatment volume (V) and the dose that must not be exceeded in healthy cells located outside (especially upstream) the treatment volume.

[0057] The spot has dimensions perpendicular to the irradiation axis (X), which can be equal to the beam diameter discussed above. The higher the array density (i.e., the closer adjacent spots are to each other), the greater the effect of dose overlap from adjacent spots on spread cells; therefore, the distance between adjacent spots, which defines the array density, is an important parameter. When the distance between adjacent spots is approximately 1.5σ, sufficient overlap is observed that allows for a uniform dose distribution in the lateral direction.

[0058] In the first embodiment, the spots (Si) in the spot array are separated from each other by a distance (ds) of 1.8 times or less the standard deviation (σ) of the fluence (Fi(y,z)) of a beam (100.i) in a single spot (i.e., ds ≤ 1.8σ), preferably 1.5σ or less. In this configuration, a given partial volume (Vi) receives a dose (Dij) that spreads over a given partial volume (Vi) and over a given degrading unit 11.i, not only from the beam (100.i) centered on the corresponding beam axis (Xi), but also from adjacent beams centered on adjacent beam axes, whose fluence extends over the given partial volume (Vi). Considering the dose imparted to the partial volume (Vi) by adjacent beams, the fluence (F(y,z)) of the beam (100.i) passing through the base area (Abi) (see equation (1)) is approximated to be constant over all values ​​of the plane (Y,Z)j of the slice (Tj) that defines the boundary inscribed in the volume (V).

[0059] In the second embodiment, the spots (Si) of 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) in a single spot (100.i), i.e., ds > 1.2σ, preferably greater than 1.5 times. At such a distance, the fluence of adjacent beams passing through a given degradation unit (11.i) can be ignored. Therefore, the fluence (Fi(y,z)) of the beam (100.i) passing through the bottom area (Abi) is approximated as a Gaussian distribution as in TIFF2023074477000007.tif11170, where (yi,zi) are the coordinates of the position of the maximum value (Ai) of the fluence of the spot (Si) in the (Y,Z) plane, and in the case of a circular spot, σ y =σ z =σ.

[0060] Increasing the density of the spot (Si) array may sometimes seem to be always more advantageous than a less dense array. However, irradiating a high-density array increases the scan time required to cover the entire treatment volume (V). Furthermore, FLASH delivery of the dose (Dij) at HDR to obtain the FLASH effect for sparing healthy cells is difficult to achieve with a high-density array because the delivery time becomes longer (and accordingly the delivery rate decreases) since the dose is delivered from adjacent beams. Therefore, the density of the spot (Si) array is determined individually.

[0061] Degradation unit (11.i) The principle of the ridge filter (11) of the present invention is to degrade the energy of the beam (100.i) such that, for each spot (Si), the desired SOBP is obtained in the corresponding partial volume (Vi), with the expanded peak extending between the shortest water equivalent thickness (d0) and the longest water equivalent thickness (d1) measured from the patient's skin (3s), with the portion of the partial volume (Vi) contained within the treatment volume as the boundary. The challenge is to degrade the energy of only a portion of the beam (100.i) such that the desired dose (Dij) is applied to the corresponding cell (Cij) in the corresponding partial volume (Vi), by shifting the Bragg peak of a predetermined beam weight portion (ωij) of the total weight (ωi) of the beam (100.i) to the water equivalent thickness (dij) of the corresponding cell.

[0062] To achieve the above objective, three geometric shapes of the energy degrading unit (11i) are proposed here, along with corresponding methods for determining the dimensions of the degrading subunits (11.ij) that form the energy degrading unit (11i). One geometric shape can be combined with another to realize the most convenient ridge filter. • Orifice-type energy degrading unit (11.i) • Pin-type energy degrading unit (11.i) • Energy degrading unit (11i) combining different materials having different water equivalent thickness (Wu) per unit length of subunits.

[0063] When the degrading unit (11.i) is an orifice In the embodiments shown in Figures 4(a) to 4(e), Figures 4(f) to 4(j), and Figures 6(a) to 6(c), the energy degrading unit (11.i) is in the form of orifices arranged in a line according to an array of spots (Si) in a support base (11b) of thickness (Bi) measured along the beam axis (Xi). Each orifice extends from an opening in the surface of the support base (11b) and penetrates to a given depth measured along the corresponding beam axis (Xi). Each energy degrading unit (11.i) is formed by one or more degrading subunits (11.ij, 11.i3, 11.i2, 11.i1) in the form of orifices having a generalized cylindrical geometry of cross-sectional area (Axij) and extending over a length (Lsij) such that Lij = Bi - Lsij along the corresponding beam axis (Xi). The degrading subunits (11.ij, 11.i3, 11.i2, 11.i1) are such that their diameters are slightly larger than or equal to the diameter (D100) of the beam (100.i), and the sum of the base areas (Aij) of the subunits is equal to the base area (Abi) (i.e., It is configured as follows: TIFF2023074477000008.tif7170.

[0064] In the preferred embodiments shown in Figures 4(a) to 4(e) and Figure 7(b), the degrading subunits are arranged in a series configuration, with the degrading subunits (11.ij) aligned along the corresponding beam axis (Xi) in order of decreasing length (Lsij). The degrading subunits (11.ij) are preferably arranged coaxially, with the longest orifice (Lsi3) positioned at the center. The degrading subunits (11.ij) can also be arranged in other configurations, such as a non-concentric configuration, with the longest orifice (Li3) not necessarily at the center.

[0065] In an embodiment in which the degrading subunits (11.ij) are arranged concentrically, the portion of the subunit's base area (Aij) of each degrading subunit (11.ij), excluding the base area (Ai3) of the deepest orifice (11.i3), has an annular geometric shape. As a result, the base area (Aij) of a given degrading subunit (11.ij) is equal to the difference in cross-sectional area (Axij - Axi(j+1)) between the cross-sectional area (Axij) of a given degrading unit (11.ij) and the cross-sectional area (Axi(j+1)) of a degrading unit circumscribed within the given degrading unit. The cross-sectional area (Axij) of the degrading subunit (11.ij) is the cross-sectional area of ​​the orifice perpendicular to the corresponding beam axis (Xij) contained within the outer circumference of the cross-section of the degrading subunit (11.ij).

[0066] In the alternative embodiments shown in Figures 4(f) to 4(j), the degrading subunits may be arranged in a parallel configuration. In this embodiment, the degrading subunits are arranged side by side within the base area (Abi) either with no space between two degrading subunits, as shown with respect to the pins in Figures 7(e) and 7(g) (it is easy to imagine the corresponding design with an orifice), or with space between two adjacent degrading subunits, as shown in Figures 4(f) and 4(g).

[0067] In further alternative embodiments, the degrading subunits may be arranged in a hybrid configuration of both series and parallel structures. In this configuration shown with respect to the pins in Figures 7(h) and 7(i) (it is easy to imagine the corresponding design with an orifice), three or more degrading subunits (11.ij) are arranged in both series and parallel configurations, and one or more structures are formed by two or more degrading subunits aligned in series along the corresponding beam axis (Xi), and optionally, one or more individual degrading subunits are arranged side by side within the base area (Abi).

[0068] This embodiment is particularly easy to manufacture by machining or etching the block forming the support base (11b), or by forming it using a three-dimensional printing technique. Given that the maximum fluence of the beam (100.i) is at the beam axis (Xi), the manufacturing error of the degrading subunit (11.ij) that intersects the beam axis (Xi) at the maximum value of the Gaussian distribution of fluence is more important than at peripheral locations. Using an orifice instead of a pin has the advantage that the weight assigned to the Bragg peak matches the fluence of the spot. In fact, Bragg peaks with greater range are usually Bragg peaks with greater weight (ωij) in the treatment plan. Therefore, the corresponding subunit (i.e., the subunit with the shortest length Li) can be placed at the location of the beam's highest fluence (i.e., the center). In addition, the subunit with the longest length Li usually corresponds to a Bragg peak with a small weight (the Bragg peak with the shortest range) and therefore has a small cross-section. Therefore, it is convenient for the subunit to be placed in the region of the spot with the smallest fluence. Forming the orifice with tight tolerances is easier than forming a thin pin of the required length (Lij) to achieve the desired effect with the same tolerances. Offsetting the longest length (Lij) degrading subunit can also help reduce the weight of small deviations in manufacturing.

[0069] For clarity, Figures 4(a) to 4(e) show an energy degrading unit (11.i) formed by three degrading subunits (11.ij), and Figure 7(b) shows an energy degrading unit (11.i) formed by only two degrading subunits (11.ij). It is clear that the number of degrading subunits (N) can actually be much larger to obtain a smoother SOBP plateau. Figures 6(a) to 6(c) show similar energy degrading units (11.i) with three, six, and an "infinite number" of degrading subunits (11.ij), in the case of an infinite number, forming a truncated triangular pyramidal orifice. All orifices in Figures 6(a) to 6(c) are orifices with an opening area (Axi1) and a base area (Axi3), and a length (Lsi3) (see Figures 4(a) to 4(e) for the meaning of the symbols).

[0070] In Figures 4(c) to 4(e), each degrading subunit is shown separately, and the whole forms a generalized hollow cylinder, which is then fitted together to form the degrading subunit in Figure 4(b). In practice, the degrading subunit (11.ij) in Figure 4(b) can be manufactured by machining or etching the support base (11b) to form an orifice, or by forming a ridge filter together with the corresponding orifice using a three-dimensional printing technique.

[0071] If the degrading unit (11.i) is a pin In the alternative embodiments shown in Figures 5(a) to 5(e), Figure 7(a), Figure 7(e), and Figure 7(f), the energy degrading units (11.i) are in the form of pins supported on a support base (11b) of thickness (Bi) measured along the beam axis (Xi), arranged in a line according to the arrangement of spots (Si). Each pin extends from the support base along the corresponding beam axis (Xi). Each energy degrading unit (11.i) is formed by one or more degrading subunits (11.ij, 11.i3, 11.i2, 11.i1) having a generalized cylindrical geometry of cross-sectional area (Axij) and extending from the support base along the corresponding beam axis (Xi) over a length (Lsij) such that Lij = Bi + Lsij. The degrading subunits (11.ij, 11.i3, 11.i2, 11.i1) are such that their diameters are slightly larger than or equal to the diameter (D100) of the beam (100.i), and the sum of the base areas (Aij) of the subunits is equal to the base area (Abi) (i.e., It is configured as follows: TIFF2023074477000009.tif6170.

[0072] In the preferred embodiments shown in Figures 5(a) to 5(e) and Figure 7(a), the degrading subunits (11.ij) are arranged in series. In this configuration, the degrading subunits are aligned along the corresponding beam axis (Xi) in order of decreasing length (Lsij). Preferably, the degrading subunits are arranged coaxially, with the pin having the longest length (Lsi1) positioned in the center.

[0073] In an embodiment in which the degrading subunits (11.ij) are arranged concentrically, the base area (Aij) of a given degrading subunit (11.ij) is equal to the difference in cross-sectional areas (Axij - Axi(j-1)) between the cross-sectional area (Axij) of a given degrading unit (11.ij) and the cross-sectional area (Axi(j-1)) of a degrading unit circumscribed within the given degrading unit. In other words, for a finite number of N degrading subunits (11.ij), the pins have the shape of a stepped pyramidal pyramid (see Figures 4(b) to 4(e)). The area (Aij) of a given degrading subunit (11.ij) is the area of ​​the stair tread (or flange) formed by the given degrading subunit (11.ij) enclosing the next degrading subunit (11.i(j+1)) which is smaller in dimensions than the given degrading subunit (11.ij).

[0074] In the alternative embodiments shown in Figures 5(f) to 5(j), 7(e), and 7(g), the degrading subunits may be arranged in a parallel structure. In this embodiment, the degrading subunits are arranged side by side within the base area (Abi), either with no space between two degrading subunits as shown in Figures 7(e) and 7(g), or with space between two adjacent degrading subunits as shown in Figures 5(f) and 5(g). For example, in the embodiments shown in Figures 7(e) and 7(g), the degrading subunits are arranged side by side, and it is preferable that the longest degrading subunit is offset with respect to the corresponding beam axis (Xi). In this way, the longest degrading unit faces a portion of the lighter beam (100.i), and therefore the cross-sectional area (Aij) can be increased, which facilitates the fabrication of the energy degrading unit (11.i). The same configuration can also be applied to the energy degrading unit (11.i) in the form of an orifice discussed above.

[0075] In a further alternative embodiment, three or more degrading subunits may be arranged in a hybrid configuration of both series and parallel arrangements. In this configuration, shown in Figures 7(h) and 7(i), three or more degrading subunits (11.ij) are arranged in both series and parallel configurations, with one or more structures formed by two or more degrading subunits aligned in series along the corresponding beam axis (Xi), and optionally, one or more individual degrading subunits arranged side by side within the base area (Abi).

[0076] For clarity, Figures 5(a) to 5(e) and 7(f) show an energy degrading unit (11.i) formed by three degrading subunits (11.ij), while Figures 7(a) and 7(e) show an energy degrading unit (11.i) formed by only two degrading subunits (11.ij). It is clear that to obtain a smoother SOBP plateau, the number of degrading subunits (N) can actually be much larger. As shown in Figures 5(b), 6(d), and 6(e), a pin-shaped energy degrading unit (11.i) can have any number of degrading subunits (11.ij), such as three, six, and an "infinite number," forming a truncated cone in the case of an infinite number (see Figure 6(e)). Regardless of the number N of degrading subunits (11.ij), all pins have a base area (Ai1) and a tip area (Ai3), and a length (Lsi3) (see Figures 5(a) to 5(e) for the meaning of the symbols).

[0077] In Figures 5(c) to 5(e), each degrading subunit is shown separately, and the whole, when fitted together, forms the degrading subunit of Figure 5(b). In practice, the degrading subunit (11.ij) of Figure 5(b) can be manufactured by machining, etching, or 3D printing a monolithic energy degrading unit (11.i) preferably with a support base (11b), or by individually machining, etching, or 3D printing each degrading subunit (11.ij) and assembling them to form the energy degrading unit (and support base (11b)). Most preferably, the entire ridge filter is produced monolithically (i.e., no assembly process is required).

[0078] Degrading units made from different materials (11.i) In the third embodiment shown in Figures 7(c) and 7(d), the above embodiments of the cavity or pin-shaped energy degrading unit discussed above can be combined with the third embodiment, wherein at least the first degrading subunit (11.11) of the first energy degrading unit (11.1) is made of a first material different from the second material of the second degrading subunit (11.ij) of the first energy degrading unit (11.1) or the second degrading unit (11.2). The first material has a different water equivalent thickness (Wu) value per unit length of the subunit than the second material in order to change, preferably decrease, the value of the length (L11 = W11 / Wu) of the first degrading subunit (11.11) compared to the length of the corresponding first energy subunit (11.11) made from the second material.

[0079] Figure 7(c) shows one embodiment comprising a first degrading subunit (11.i1) and a second degrading subunit (11.i2) arranged concentrically. The second degrading subunit (11.i2) is enclosed within the annular first degrading subunit (11.i1). The first degrading subunit (11.i1) is made of a first material having a higher water equivalent thickness (Wu) per unit length of the subunit than the material forming the second degrading subunit (11.i2). Thus, for the same length measured along the beam axis (Xi) (Li1=Li2), the first degrading subunit (11.i1) absorbs more energy from the beam, and the water equivalent thickness (di1) of a smaller cell is lower than the water equivalent thickness (di2) of the cell crossing the second degrading subunit (11.i2). <di2)のセルへブラッグピークをシフトさせる。

[0080] Fig. 7(d) also shows a similar configuration, where the first degradation subunit (11.i1) is made of a first material with a higher value of the water equivalent thickness (Wu) of the subunit per unit length than the second material from which the second degradation subunit (11.i2) is made, and is surrounded by the second degradation subunit (11.i2) and located at the center. As a result, the depth (di1 < di2) at which the Bragg peak of a part of the beam crossing the first degradation subunit (11.i1) located at the center of the energy degradation unit (11.i) occurs is lower than the ratio of the second degradation subunit (11.i2) having the same length (Li1 = Li2) measured along the beam axis (Xi). The degradation subunits (11.i1, 11.i2) in Figs. 7(c) and 7(d) are arranged concentrically. It is obvious that other configurations are also possible, such as arranging them side by side. Having the same length Li1 = Li2 is also an optimized situation. Since it is not possible to continuously select materials over the entire range of values of the water equivalent thickness (Wu) of the subunit per unit length, it is difficult to select a material that has the necessary water equivalent thickness (dij) of the cell and at the same time has the same length (Lij). However, by changing the material, the height difference between the degradation subunits can be reduced, and a more compact and robust energy degradation unit (11.i) can be realized.

[0081] This embodiment of combining different materials with different water equivalent thicknesses (Wu) of the subunits per unit length is applied to both the cavity-shaped and pin-shaped energy degradation units (11.i) so as to shorten the length (Lij) of the longest degradation subunit (11.ij) and lengthen the length of the shortest degradation subunit (11.ij), enabling a shorter ridge filter (11) to be obtained and facilitating production from the perspective of tolerance, while being able to select the dimensions most suitable for production.

[0082] For example, the selection of materials for each degrading subunit may aim to keep the length (L11) of the first degrading subunit within ±20% of the length (Lij) of the second degrading subunit. Preferably, the lengths (Lij) of all degrading subunits (11.ij) of the energy degrading unit (11.i) are the same (Lij) with a variation of ±20% of the average length (Lm,ij) (i.e., Lij = Lm,ij ± 20% ∀j). In this way, a compact ridge filter can be obtained.

[0083] Ridge filter (11) and particle accelerator equipped with ridge filter (11) As shown in Figure 3(c), the ridge filter (11) is formed by a plurality of energy degrading units (11.i) arranged in a row on a support base (11b). The position of each energy degrading unit (11.i) corresponds to the position of the corresponding spot (Si), and its orientation is parallel to the corresponding beam axis (Xi).

[0084] In Figures 1(a), 1(b), 3(a), and 3(e), all beam axes (Xi) are shown parallel to each other. This is not exactly correct, as all beam axes (Xi) start from the same point at the center of the scanning electromagnet in the nozzle and deviate to scan all spots (Si) characterizing the treatment volume (V). This is shown in Figures 3(b), 3(d), and 3(f), where the deflection angles are 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 volume (V) and the size of the treatment volume (V). Figure 3(b) shows a diagram of the irradiation system corresponding to the diagram in Figure 3(a), with the beam axes deviated from parallel to account for the beam scanning effect. In Figure 3(b), the aperture angles are exaggerated. Figure 3(c) shows the corresponding ridge filters (11) with pins protruding from the support base (11b) and deviated from parallel to each other. Figure 3(d) shows two energy degrading units formed by two corresponding orifices in the support base (11b). Here again, the angles of the openings have been exaggerated for clarity.

[0085] In reality, the aperture angle is generally within ±5°, preferably within ±3°, and more preferably within ±1°, of the irradiation axis (X). For this reason, although not strictly accurate, representing the beam axes (Xi) parallel to each other in the drawings is a simplified representation of acceptable reality.

[0086] Figures 3(e) and 3(f) show similar cubic elements of the treatment volume (V) including several partial volumes (Vi). The partial volumes (V1, V4, and V5) are shown in a cross-sectional side view indicating the location of the corresponding cells (Cij, i=1, 4, and 5, j=1~4). In Figure 3(e), the partial volumes (Vi) extend (approximately) parallel to each other and (approximately) parallel to the irradiation axis (X), and in Figure 3(f), the partial volumes extend along the diverging beam axis (Xi) (again, the aperture angles are exaggerated for clarity).

[0087] The longest degrading subunit (11.i1) measured along the corresponding beam axis (Xi) absorbs more energy from the beam (100.i) than shorter degrading subunits. Therefore, the longest degrading subunit (11.i1) determines the shortest cell water equivalent thickness (di1) that defines the location of the Bragg peak closest to the patient's skin (3s). As the length (Lij) of the degrading subunit (11.ij) decreases, the corresponding cell water equivalent thickness (dij) increases up to the shortest degrading subunit (11.iN) of the shortest length (LiN) that determines the cell water equivalent thickness (diN) of the Bragg peak furthest from the patient's skin (3s). The superposition of all Bragg peaks forms the SOBP that must be followed in the treatment plan (see Figures 4(a)–4(e) and 5(a)–5(e)). The length (Lij) of each degrading subunit (11.ij) can be easily determined as Lij = Wij / Wu = (W0 - dij) / Wu, as discussed above (see Figure 2(b)).

[0088] The area (Aij) of each degrading subunit (11.ij) must be sized to produce the number of charged particles necessary to impart a predetermined dose (Dij) to the corresponding cell at the water equivalent thickness (dij) of that cell. Equation (1) is used to determine the value of the area (Aij) of the degrading subunit (11.ij). TIFF2023074477000010.tif11170

[0089] In equation (1), the normalized beam weight (ωij) is equal to the normalized value of the integral of the beam fluence (F(y,z)) over a dimensionable area (Aij). Since the area (Aij) defines the boundary, and the integral of the numerator in equation (1) is calculated along the boundary, the area (Aij) can be determined for each degrading subunit (11.ij). As discussed above, the preferred arrangement of the individual degrading subunits (11.ij) is to assemble them coaxially so as to form the corresponding energy degrading unit (11.i) (see Figures 4(a) to 4(e) and Figures 5(a) to 5(e)). In this series arrangement, the area (Aij) of the first degrading subunit (11.ij) has an annular geometric shape that forms an annular step around the degrading subunit (11.i(j+1)) coaxially inscribed within the first degrading subunit. In the parallel arrangement shown in Figures 4(f) to 4(j), 5(h) to 5(j), and 7(e) and 7(g), the area (Aij) of the degrading subunit (11.ij) is the area at the bottom of each degrading subunit.

[0090] In the case of a dense array of spots (Si), the spots (Si) are separated from each other by a distance (ds) of no more than 1.8 times, preferably no more than 1.5 times, the standard deviation (σ) of the beam (100.i) fluence (Fi(y,z)), and the fluence of the beam passing through the base area (Abi) is approximated to be constant over all values ​​of the plane (Y,Z)j defining the boundary inscribed in the volume (V). This configuration greatly simplifies solving the integral in the numerator of equation (1).

[0091] When the arrangement of spots (Si) is relatively sparse, the spots are separated from each other by a distance (ds) greater than 1.2 times the standard deviation (σ) of the beam (100.i) fluence (Fi(y,z)) (i.e., ds > 1.2σ), preferably greater than 1.5 times, and the fluence (Fi(y,z)) of the beam (100.i) passing through the base area (Abi) is Gaussian-distributed. It is approximated as TIFF2023074477000011.tif12170, where (yi,zi) is the coordinate of the position of the maximum value (Ai) of the fluence of the spot (Si) in the (Y,Z) plane, and in the case of a circular spot, σ y =σ z = σ. Solving the integral in the numerator of equation (1) is not as straightforward as when the arrangement of spots (Si) is dense (i.e., ds < 1.8σ or < 1.5σ), but it can at least be solved numerically. The spots are circular, and σ y =σ z If σ = σ, then solving equation (1) becomes easy.

[0092] Conclusion The method proposed herein for designing and dimensioning a ridge filter (11) for single-layer PBS painting of a treatment volume (V) is simple, reliable, and repeatable. The design of a cavity-shaped energy degrading unit (11.i) is more robust to manufacturing tolerances than a pin formed by concentric degrading subunits (11.ij), because in the case of a pin, the central degrading subunit (11.i1) is the longest and thinnest within the entire energy degrading unit (11.i), making the accurate manufacture of the pin more complex. Non-concentric configurations, such as stacking, are also possible and can mitigate this problem. Using a material with a high water equivalent thickness (Wu) per unit length of the subunit for the longest length (Lij) degrading subunit (11.ij) is also a solution to mitigate the challenge of accurately manufacturing elongated pins.

[0093] Starting from a treatment plan, the arrangement of spots (Si) is defined as described above, and the treatment volume (V) is divided into partial volumes (Vi) (one per spot), and the partial volumes (Vi) may be divided into N cells (Cij) as appropriate. The dose (Dij) to be delivered to each cell (Cij) is determined based on the treatment plan.

[0094] The ridge filter is designed to have the same number of energy degrading units (11.i) as there are spots (Si). Each energy degrading unit (11.i) is formed by N degrading subunits (11.ij) having length (Lij) and area (Aij).

[0095] The length (Lij) of each degrading subunit (11.ij) is calculated as the ratio of the water equivalent thickness (Wij) of the desired subunit to the water equivalent thickness (Wu) of the subunit per unit length (i.e., Lij = Wij / Wu). The water equivalent thickness of the subunit 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 support block (11b) supporting all energy degrading units (11.i).

[0096] The area (Aij) of each degrading subunit (11.ij) is obtained by calculating the area (Aij) over which the integral in the numerator of equation (1) is calculated. This operation can be performed numerically.

[0097] Each energy degrading unit (11.i) is positioned on a support block (11b) so as to extend coaxially along its respective beam axis (Xi). The ridge filters can be manufactured by machining the blocks, attaching individual pins to the support block (11b), or by three-dimensional printing techniques. The ridge filters (11) thus manufactured can be installed between the outlet of the charged particle accelerator and the treatment volume (V) such that each sub-volume (Vi) is coaxial with its corresponding beam axis (Xi). Irradiation can then be initiated by single-layer PBS painting.

[0098] The ridge filter (11) designed by the method of the present invention can cover the entire treatment volume with a single painting layer, which significantly reduces the scanning time required to impart dose (Dij) to each slice (Tj). Therefore, it is particularly suitable for treatment plans that include FLASH irradiation of at least a portion of the treatment volume (V) where dose (Dij) needs to be imparted to cells (Cij) at an ultra-high dose rate (HDR) by PBS. [Explanation of symbols]

[0099] 3s skin 11 Ridge Filter 11.i Energy Degrading Unit 11.ij Degrading Subunit 100.i Beam Base area (Abi) of the Abi degrading unit (11.i) Aij Degrading Subunit 11.ij Base Area Cross-sectional area of ​​the energy degrading unit (11.i) at the depth of the degrading subunit (11.ij) Axij Thickness of the support block along the Bi irradiation axis (X) Cij cell d0 Minimum water equivalent thickness d1 Farthest water equivalent thickness Water equivalent thickness of a DIJ cell Dij dose F(y,z) beam fluence Length of the degrading subunit (11.ij) along the Lij beam axis (Xi) Lsij = Bi-Lij. Energy degrading unit in the form of a cavity. Si Spot TJ Slice V Treatment area Vi partial volume W0 Maximum Beam Range Water equivalent thickness of subunits per unit length of Wu X irradiation axis Li beam axis X, Y, Z coordinate system (Y,Z) A plane perpendicular to the irradiation axis (X) (Y,Z)j Plane upstream of slice Tj ωij Weights of beam 100.i in slice Tj

Claims

1. 1. A method for designing a ridge filter for a charged particle accelerator, preferably a proton accelerator, for applying a specific dose (Dij) to a specific location within a treatment volume (V) of tissue containing tumor cells (3t) by a pencil beam scan (PBS) spot by spot (Si) according to a predetermined treatment plan (TP) in a single painting layer that defines the entire treatment volume (V), using a beam (100.i) of accelerated particles, said beam (100.i) extending along a corresponding beam axis (Xi) that is substantially parallel to an irradiation axis (X) and deviates from parallelism with said irradiation axis (X) by an angle comprised within ±5°, preferably within ±3°, said tissue being characterized by a maximum beam range (W0) defined as the water equivalent distance at which said beam ceases to propagate through said tissue, said method comprising: - defining a boundary inscribed in the treatment volume (V) by defining an area (Aj) over planes (Y,Z)j upstream of N slices (Tj=T1 to TN) of thickness (dxj), the planes (Y,Z)j being perpendicular to the irradiation axis (X), and the shortest (d0) and longest water equivalent thicknesses (d1) relative to the patient's skin being 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 partial volumes (Vi), each of which extends parallel to the corresponding beam axis (Xi) from the patient's skin to the corresponding farthest water equivalent thickness (d1), and wherein a projection of the array of partial volumes (Vi) onto a plane (Y, Z) perpendicular to the illumination axis (X) defines an array of spots (Si) covering the entire area of ​​the projection of the volume (V) onto the plane (Y, Z), - for each slice (Tj) of the N slices (T1 to TN) contained in a subvolume (Vi), defining a cell (Cij) defined as a portion of the subvolume (Vi) contained in the corresponding slice (Tj); for each cell (Cij) of the given partial volume (Vi), determining the water equivalent thickness (dij) of the cell from the skin (3s) to the geometric center of the cell (Cij) and determining the beam weight (ωij) required to deliver the specific dose (Dij) to the cell (Cij) according to the TP, the beam weight (ωij) being proportional to the number of charged particles in the water equivalent thickness (dij) of the cell; designing the ridge filter (11) with a set of energy degrading units (11.i), each energy degrading unit (11.ij) configured to reduce the initial energy (E0) of a corresponding beam of charged particles (100.i) of a beam diameter (D100.i) coaxial with the corresponding beam axis (Xi) and partial volume (Vi) to a reduced energy (Eij) such that the specific dose (Dij) is imparted to the corresponding cell (Cij) contained within the partial volume (Vi) at the water equivalent thickness (dij) of the cell according to the TP, the energy degrading unit (11.i) of a given partial volume (Vi) being: for each cell (Cij) of said partial volume (Vi), dimensions are defined for a degrading subunit (11.ij) having a generalized cylindrical geometry with a base of area (Aij) perpendicular to said corresponding beam axis (Xi) and a generatrix of length (Lij) parallel to said corresponding beam axis (Xi), said degrading subunit being made of a material having a subunit water equivalent thickness (Wu) per unit length along said corresponding beam axis (Xi), said length (Lij) being determined so that said degrading subunit (11.i) has a subunit water equivalent thickness (Wij = Wu × Lij) equal to the product of said subunit water equivalent thickness (Wu) per unit length and said length (Lij), and the sum of said subunit water equivalent thickness (Wij) and the cell water equivalent thickness (dij) is equal to said maximum beam range (W0) (i.e. W0 = Wij + dij); The area (Aij) of the degrading subunit (11.ij) is calculated by the normalized beam weight (ωij / Σ ｊ ω ｉｊ ) is determined by equating the ratio of the integral of the fluence (F(y,z)) over the base area (Aij) of said sub-unit to the same integral over the base area (Abi) of said degrading unit (11.i), where the fluence F(y,z) is the number of charges per unit area of ​​the beam (100.i) at the beam position (y,z), and the base area (Abi) is equal to the sum of the areas of the subunits (Aij), i.e., Abi = Σ ｊ Aij) combining the N degrading subunits (11.ij) to obtain the energy degrading unit (11.i) designed to degrade the energy of the beam (100.i) so as to impart the required dose (Dij) to the subvolume (Vi); Designed as,step, designing said energy degrading units (11.i) corresponding to all remaining subvolumes (Vi) as defined above; wherein the expression "water equivalent thickness" (=WET) is defined as the thickness of water that causes the same energy degradation of the particle beam as a given thickness of one or more materials traversed by said particle beam.

2. 2. The method of claim 1, wherein the specific dose (Dij) is delivered according to the treatment plan to at least the selected specific location within the tissue volume (V) at a very high dose delivery rate (HDR), where HDR is defined as a dose delivery rate of HDR ≥ 1 Gy / s.

3. 3. The method according to claim 1 or 2, characterized in that the spots (Si) of the array of spots 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 one single spot (i.e. ds≦1.8σ), preferably less than or equal to 1.5σ, and the fluence (F(y,z)) of the beam (100.i) passing through the base area (Abi) is approximated to be constant over all values ​​of the plane (Y,Z) j that defines the boundary inscribed in the volume (V).

4. 3. The method according to claim 1 or 2, wherein the spots (Si) of the array of spots are separated from each other by a distance (ds) greater than 1.2 times (i.e. ds>1.2σ), preferably greater than 1.5 times, the standard deviation (σ) of the fluence (Fi(y,z)) of the beam (100.i) at a single spot, and the fluence (Fi(y,z)) of the beam (100.i) passing through the base area (Abi) follows a Gaussian distribution where (yi, zi) are the coordinates in the (Y, Z) plane of the position of the fluence maximum (Ai) of the spot (Si), and for a circular spot, σ ｙ = σ ｚ = σ.

5. 3. The method according to claim 1 or 2, the energy degrading units (11.i) are in the form of orifices arranged in a row according to the arrangement of the spots (Si) in a support base (11b) of thickness (Bi) measured along the beam axis (Xi), each orifice extending from an opening in the surface of the support base (11b) and penetrating to a given depth measured along the corresponding beam axis (Xi); Each energy degrading unit (11.i) formed by one or more degrading sub-units (11.ij, 11.i3, 11.i2, 11.i1) in the form of orifices having a generalized cylindrical geometry of cross-sectional area (Ai) and extending from the opening of the support block (11b) along the corresponding beam axis (Xi) over a length (Lsij) such that Lij = Bi - Lsij, said degrading subunits (11.ij, 11.i3, 11.i2, 11.i1) are arranged within said base area (Abi).

6. 6. The method according to claim 5, wherein the energy degrading unit (11.i) comprises at least two sub-units (11.ij) arranged in the bottom area (Abi) in one of the following configurations: ・In a series structure, the degrading sub-units are aligned along the corresponding beam axis (Xi) in order of decreasing length (Lsij), preferably coaxially, and with the orifice with the longest length (Lsi3) located in a central position, the base area (Aij) of a given degrading subunit (11.ij) is equal to the difference in cross-sectional area (Axij-Axi(j+1)) between the cross-sectional area (Axij) of the given degrading unit (11.ij) and the cross-sectional area (Axi(j+1)) of the degrading unit circumscribed within the given degrading unit, In a parallel structure, the degrading subunits are arranged side by side within the base area (Abi) with either no space between two degrading subunits or with a space between two adjacent degrading subunits; - In a mixed structure of both parallel and serial, three or more degrading subunits (11.ij) are arranged both in serial and parallel, one or more structures are formed by two or more degrading subunits aligned in series along the corresponding beam axis (Xi), and optionally one or more individual degrading subunits are arranged side by side within the bottom surface area (Abi).

7. 3. The method according to claim 1 or 2, the energy degrading units (11.i) are in the form of pins arranged in a row according to the arrangement of the spots (Si) and supported on a support base (11b) of thickness (Bi) measured along the beam axis (Xi), each pin extending from the support base along the corresponding beam axis (Xi); Each energy degrading unit (11.i) formed by one or more degrading sub-units (11.ij, 11.i3, 11.i2, 11.i1) having a generalized cylindrical geometry of cross-sectional area (Aij) and extending from said support base along said corresponding beam axis (Xi) over a length (Lsij) such that Lij=Bi-Lsij, said degrading subunits (11.ij, 11.i3, 11.i2, 11.i1) are arranged within said base area (Abi).

8. 8. The method according to claim 7, wherein the energy degrading unit (11.i) comprises at least two sub-units (11.ij) arranged in the bottom area (Abi) in one of the following configurations: ・In a series structure, the degrading sub-units are aligned along the corresponding beam axis (Xi) in order of decreasing length (Lsij), preferably coaxially, and with the pin having the longest length (Lsi1) located in a central position, the base area (Aij) of a given degrading sub-unit (11.ij) is equal to the difference in cross-sectional area (Axij-Axi(j-1)) between the cross-sectional area (Axij) of the given degrading unit (11.ij) and the cross-sectional area (Axi(j-1)) of the degrading unit circumscribed within the given degrading unit, - In a mixed structure of both parallel and serial, three or more degrading subunits (11.ij) are arranged both in serial and parallel, one or more structures are formed by two or more degrading subunits aligned in series along the corresponding beam axis (Xi), and optionally one or more individual degrading subunits are arranged side by side within the bottom surface area (Abi).

9. 6. The method of claim 5, wherein at least a first degrading subunit (11.11) of a first energy degrading unit (11.1) is made of a first material different from a second material of a second degrading subunit (11.ij) of the first energy degrading unit (11.1) or the second energy degrading unit (11.2), the first material having a value of water equivalent thickness (Wu) of the subunit per unit length different from that of the second material so as to change, preferably decrease, the value of the length (L11 = W11 / Wu) of the first degrading subunit (11.11) compared to the length of the corresponding first energy subunit (11.11) made from the second material.

10. 10. The method of claim 9, wherein the length (L11) of the first degrading subunit (11.11) is within ±20% of the length (Lij) of the second degrading subunit, and preferably the lengths (Lij) of all the degrading subunits (11.ij) of an energy degrading unit (11.i) have the same length (Lij) within a variation of ±20% of the average length (Lm,ij) (i.e., Lij = Lm,ij ±20%∀j).

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