Ripple filter unit for use in radiotherapy, method for radiotherapy planning and delivery, and computer program product

Abstract

The ripple filter unit comprises substantially identical first and second ripple filters positioned to overlap each other in the beam, having substantially the same orientation, and movable relative to each other to dynamically change the filter characteristics, thereby varying the modulation characteristics of the ripple filters. [Selection diagram] Figure 6

Description

Technical Field

[0001] The present invention relates to a ripple filter unit for use in radiotherapy as described in the preamble of claim 1, a method for radiotherapy treatment planning using such a ripple filter unit, and a computer program product and apparatus for radiotherapy treatment planning and delivery.

Background Art

[0002] In pencil beam scanning, a beam of charged particles such as protons is continuously directed towards several spots within a patient, and the spots cover the volume to be treated. The particles deposit their energy along their path within the patient.

[0003] Each particle deposits most of its energy towards the stopping point of its path, known as the Bragg peak. The depth of the position of the Bragg peak within the patient depends on the initial energy of the particle. To cover a desired volume within the patient, different energy layers are defined such that particles of a particular energy layer deposit their energy at a particular depth within the patient. The energy layers are selected such that Bragg peaks are distributed throughout the volume to be treated.

[0004] The Bragg peaks of the different energy layers need to overlap at a particular level in order to achieve sufficient dose uniformity throughout the volume. Thus, the number of energy layers has to be sufficiently large so that the spacing between the Bragg peaks in the beam direction does not become too large. At the same time, there is a desire to limit the number of energy layers since changing from one energy layer to another takes time and increases the treatment time. Typically, changing the energy layer can take approximately 2 seconds.

[0005] It has been proposed to broaden each Bragg peak by introducing a ripple filter into the beam. A ripple filter is a slab with a pattern of finely spaced peaks and valleys. The spatial separation of the peaks and valleys is small compared to the lateral spread of protons, and therefore the filter effectively results in broadening of the Bragg peak. Typical spacing is 1-6 mm. This means that each energy layer covers a larger depth range within the patient, and thus the number of energy layers can be reduced. Bragg peaks are narrower in lower energy layers. This means that the positive effect of broadening the Bragg peak is greater in lower energy layers. During treatment delivery, clinics typically use one or two different ripple filters depending on the proximal-distal range to be covered. One ripple filter is usually manually inserted into the beam. This may remain in place throughout the treatment, or another ripple filter may be manually inserted or changed between energy layers, but this is time-consuming.

[0006] EP2241350 discloses such ripple filters for broadening the particle energy distribution of a particle beam, proposing the fabrication of a ripple filter configuration comprising two ripple filters that are detachably arranged in series in the irradiation direction and positioned at a 90° angle to each other. This results in four distinct possible options for altering the energy distribution by inserting one or the other of the filters, neither of them, or both. [Overview of the project]

[0007] The objective of this invention is to minimize the delivery time of radiotherapy while maintaining the quality of the plan.

[0008] The present invention proposes a ripple filter unit for broadening the particle energy distribution of a particle beam, comprising first and second ripple filters arranged in series in substantially the same orientation within the beam path. The first and second ripple filters are movable relative to each other such that the filter characteristics of the ripple filter unit change dynamically depending on the relative positions of the first and second ripple filters. Each of the first and second ripple filters affects the beam as the beam passes through them, and the particles in the beam are affected in different ways and lose different amounts of energy depending on the relative positions of the ripple filters. In particular, the energy of the particles decreases by different amounts depending on where they strike the ripple filter unit. At some relative positions, the particles are affected substantially similarly at all positions on the ripple filter unit, meaning that the Bragg peak is not broadened. At other relative positions, the particles are affected differently and lose different amounts of energy, which leads to broadening of the Bragg peak.

[0009] The ripple filter unit according to the present invention is dynamic and can be adapted to each energy layer to achieve optimal broadening of the Bragg peak for each energy layer. This makes it possible to change the width of the Bragg peak without having to insert or replace ripple filters during treatment. Typically, the ripple filter unit needs to be set to produce smaller broadening at higher energy layers and larger broadening at lower energy layers. By dynamically adjusting the ripple filter unit during treatment, the number of energy layers can be reduced without causing delays that would occur, for example, if static ripple filters were used and they were changed manually between energy layers. Preferably, the ripple filter unit is configured so that the first and second ripple filters can move continuously relative to each other to allow for smooth adjustment of the filter characteristics.

[0010] In a preferred embodiment, each of the first and second ripple filters has a pattern of peaks and valleys, and the first and second ripple filters are movable relative to each other such that the peaks of the first and second ripple filters are displaced relative to each other in a direction perpendicular to the peaks.

[0011] The movement is preferably a translation. One of the first and second ripple filters may be fixed, and the unfixed ripple filter may be translated in parallel with respect to the fixed ripple filter. Alternatively, both ripple filters may be translated in parallel, preferably in opposite directions.

[0012] The first and second ripple filters are preferably of the same type. In a preferred embodiment, each filter consists of uniform peaks arranged adjacent to one another, the base of each peak substantially touches the adjacent peaks on either side of it, except for the edges of the filter.

[0013] The ripple filter unit can be controlled to modulate the width of the Bragg peak for each energy layer or for each individual spot. In the latter case, there may be more than one ripple filter setting for each energy layer.

[0014] The present invention also provides a method for generating a radiotherapy plan in which the ripple filter unit described above is used for dose delivery, The steps include determining device parameters, including ripple filter settings that specify Bragg peak modulation, A step of generating a plan that includes the ripple filter settings, This includes methods.

[0015] The present invention also relates to a computer program product comprising computer-readable code means, preferably stored on a non-temporary storage medium, which causes the processor to execute the treatment planning method defined above when it is running on the processor.

[0016] The present invention also relates to a computer system comprising a processor, data memory, and program memory, wherein the program memory comprises a computer program product for a treatment planning method configured to operate on the processor to control the creation of a radiotherapy plan.

[0017] The present invention also proposes a treatment planning method and a treatment delivery method, including optimization of such ripple filter units, a computer program product for performing the method, a computer system for performing the treatment planning method, and a treatment delivery device for delivering radiotherapy to a patient according to the treatment delivery method.

[0018] The present invention also proposes a method for delivering pencil-beam scanning radiotherapy to a patient, characterized by applying the ripple filter unit described in any one of the claims into the beam and controlling the ripple filter unit to modulate the width of the Bragg peak of a spot in the energy layer or between individual spots in the energy layer.

[0019] The present invention also relates to a radiotherapy apparatus comprising a processor for controlling radiotherapy and a program memory containing a dose delivery computer program product configured to operate on the processor for controlling the radiotherapy apparatus.

[0020] The present invention will be described in more detail below, as an example, with reference to the accompanying drawings. [Brief explanation of the drawing]

[0021] [Figure 1a] This is a schematic diagram illustrating the Bragg peak. [Figure 1b] This diagram schematically shows a ripple filter related to conventional technology. [Figure 1c] This diagram schematically illustrates the wider Bragg peak that results when a ripple filter is placed within the beam. [Figure 2a]Shows a dynamic ripple filter unit according to an embodiment of the present invention at three different positions. [Figure 2b] Shows a dynamic ripple filter unit according to an embodiment of the present invention at three different positions. [Figure 2c] Shows a dynamic ripple filter unit according to an embodiment of the present invention at three different positions. [Figure 2d] It is a diagram showing different configurations of a ripple filter unit according to an embodiment of the present invention. [Figure 3a] Illustrates the resulting reduction in the number of Bragg peaks and energy layers when the dynamic ripple filter unit according to the present invention is applied. [Figure 3b] Illustrates the resulting reduction in the number of Bragg peaks and energy layers when the dynamic ripple filter unit according to the present invention is applied. [Figure 3c] Illustrates the resulting reduction in the number of Bragg peaks and energy layers when the dynamic ripple filter unit according to the present invention is applied. [Figure 4] Illustrates a situation where it may be desirable to use different settings for different spots within an energy layer. [Figure 5a] It is a flowchart of an embodiment of a treatment planning method according to an aspect of the present invention. <0​​​​​​​​​​​​​​​ Figure 1b shows a conventional ripple filter 3. The ripple filter 3 has several peaks 9, which are usually, though not necessarily, triangular in cross-section, with their bases aligned in one plane and their bases touching or nearly touching the bases of adjacent peaks. Preferably, all peaks are uniform. For clarity, only some of the peaks are labeled with reference numerals. Since all particles passing through the filter toward the patient in a given time belong to the same energy layer, it is assumed that their Bragg peaks are at approximately the same depth. However, in the situation in Figure 1b, the first particle, illustrated by the first arrow A1, passes between two peaks of the filter and, ideally, does not lose energy upon passing through the filter. In reality, of course, there is a small energy loss. The second particle, illustrated by the second arrow A2, passes near the peak of the filter's peak and loses an amount of energy close to the maximum amount of energy that can be lost in the filter 3.

[0024] As can be understood, particles with the same initial energy but passing through the filter at different positions relative to peak 9 will lose different amounts of energy and reach different depths within the patient. This means that the Bragg peaks of the energy layer will be broadened. This also means that some of the particle's energy is lost in the filter rather than being stored in the patient. The peaks do not necessarily have to be triangular, but this has been found to be a suitable shape. Also, they do not necessarily have to be arranged so that the valleys are triangular, but it is preferable that the peaks form a substantially continuous pattern on the filter.

[0025] Figure 1c illustrates the resulting broadened Bragg peak W.

[0026] Figures 2a-2c illustrate a ripple filter unit 30 according to one embodiment of the present invention, comprising a first ripple filter 3 and a second ripple filter 3'. As can be understood, the ripple filters 3, 3' can be designed in various ways, as described in relation to Figure 1b, but will have a pattern of peaks 9, 9' and valleys formed between the peaks. According to the present invention, the ripple filters 3, 3' are arranged in series such that particles moving toward the patient in the direction indicated by arrow A3 first pass through the first ripple filter 3, then the second ripple filter 3'. The two ripple filters are moved relative to each other to dynamically change the filtering characteristics of the ripple filter unit during treatment. In this specification, the ripple filters 3, 3' may be simply referred to as filters 3, 3'.

[0027] In Figure 2a, filters 3 and 3' are displaced relative to each other to produce minimal modulation by positioning the second filter 3' such that its peak coincides with the trough of the first filter 3. In this case, a particle passing through the first filter 3 on its peak passes through the second filter 3' on its trough, and vice versa, resulting in approximately the same energy loss across the entire filter.

[0028] Figure 2b shows the same ripple filter unit 30 as in Figure 2a, but with a different displacement of the second filter 3' relative to the first filter 3. In the configuration shown in Figure 2b, particles passing through the filter unit 30 at different positions will experience different degrees of attenuation depending on the total width of the peaks of filters 3 and 3' at their passing positions. This results in different energy losses for particles that had the same initial energy, causing them to reach different depths within the patient, and thus the Bragg peak is effectively broadened.

[0029] Figure 2c shows the configuration of a ripple filter unit 30 in which the peaks of the two filters coincide, so that particles passing through the filter unit can pass through corresponding positions on both filters 3, 3', i.e., the peaks of both filters, the troughs of both filters, or any position between the two extremes. This results in the maximum difference in energy loss for different particles in the energy layer, and therefore the maximum broadening of the Bragg peak.

[0030] Figure 2d shows an alternative configuration of the ripple filter unit 30' in which the two filters 3 and 3' are arranged so that their peaks, rather than their bases, face each other. Otherwise, the function of the ripple filter unit 30' is exactly the same as that of the ripple filter unit 30 in Figures 2a and 2c.

[0031] As can be understood, the displacement between the first and second ripple filters can be continuously varied between the two extremes shown in Figures 2a and 2c, respectively, thereby producing the desired modulation of particles passing through the ripple filter unit.

[0032] The dimensions of the dynamic ripple filter unit can be adapted. A suitable overall width is known to be 0.1–1.5 cm. For conventional ripple filters, the spatial separation between peaks and valleys should not be greater than the lateral width of the Bragg peak. This means that the spatial separation should typically be approximately 0.5–3 millimeters.

[0033] Figures 3a, 3b, and 3c illustrate sets of Bragg peaks resulting from the reduction in the number of energy layers by the dynamic ripple filter according to the present invention. Figure 3a shows the Bragg peaks for each energy layer using the prior art method without a ripple filter. 80% of the maximum levels are indicated by horizontal dashed lines. The Bragg peaks for the proximal peaks on the left side of the figure are considerably narrower than the Bragg peaks for the distal peaks on the right side of the figure. Therefore, lower energy layers corresponding to proximal Bragg peaks must be closer to each other than higher energy layers. A total of 32 energy layers are used, and as can be seen in the figure, the Bragg peaks overlap at levels above 80%.

[0034] Figure 3b simply shows the results when the number of energy layers used is reduced, the same distance is maintained between all energy layers, and no ripple filter is introduced. In Figure 3b, 15 energy layers are used in the same region as in Figure 3a, and the distance between Bragg peaks is larger. The 80% level is also indicated by a horizontal dashed line. As can be seen in the figure, because the overlap of Bragg peaks is reduced, the accumulated dose between Bragg peaks is well below the 80% target level.

[0035] Figure 3c shows the same number of energy layers as Figure 3b, but with the dynamic ripple filter according to the present invention applied to the beam. A smaller ripple is applied to the distal peaks, while a larger ripple is applied to the proximal peaks, thus broadening these peaks, which means they overlap at a higher level than in Figure 3a. This also means that the proximal peaks are significantly reduced by broadening, while the area under each curve remains substantially unchanged. In this figure, adjacent curves are joined at approximately the 80% level.

[0036] The present invention has so far been described in relation to how to use different filter settings for different energy layers. Figure 4 shows a situation in which it may be desirable to use different filter settings within energy layers. In this case, five different energy layers E0 to E4 are defined to cover a simplified target 41 having a cross-section C at the beam incidence angle. The target 41 has an irregular shape, and therefore, in one part of the target corresponding to a first sub-area C1 of cross-section C, the highest energy layer E0 reaches the distal end of the target. In another part of the target corresponding to a second sub-area C2, this does not extend to the patient, and the distal end is in the third energy layer E2. This means that a region of the target 43 requires a sharp Bragg peak in the highest energy layer E0, but not necessarily on the third energy layer E2. Thus, a ripple filter unit can be applied to particles in the energy layer E2 directed towards the first sub-area C1 of the target. In region 45, which is the distal end of the target in the second sub-area C2, the Bragg peak should have a sharp distal edge, and therefore the filter should be positioned as shown in Figure 2a to apply minimal modulation.

[0037] For example, in the example described in relation to Figure 4, the ripple filter unit may be configured differently for some spots within the energy layer than for other spots within the same energy layer.

[0038] When optimizing a treatment plan, the optimization of ripple filter settings in each energy layer can be considered as a single setting or as different settings within each energy layer. It should be noted that the term "optimization" is used here in the broadest possible sense. Therefore, it encompasses all methods of determining a treatment plan, including optimization using objective function gradient-based methods, or simply calculating several plans with different ripple filter settings and selecting the optimal one. The optimality of the plan is determined as a trade-off between dose measurement quality and other parameters such as delivery time. Flowcharts of the two possible methods as a whole are shown in Figures 5a and 5b.

[0039] Figure 5a illustrates how ripple filter settings are determined in a gradient-based optimization process. Thus, in step S51, the user defines an optimization problem that includes a penalty function, which in one preferred embodiment seeks to minimize the number of energy layers while maintaining the overall plan quality. Instead of directly specifying a penalty function with respect to the number of energy layers, the user can specify a penalty function that seeks to minimize the delivery time while maintaining the overall plan quality. This penalty function also has the effect of reducing the number of energy layers. Alternatively, the user can choose to use multiple ripple filter settings for each energy layer and a penalty function that maximizes the plan quality regardless of delivery time. Instead of using a penalty function, the user can specify a maximum number of allowable energy layers and / or a maximum delivery time, which can be used as a constraint during optimization. Based on the optimization problem defined in step S51, the plan is optimized in step S52. This includes determining the ripple filter settings.

[0040] Figure 5b illustrates an alternative plan optimization method that involves generating several alternative plans and selecting one of them. In the first step S53, parameter settings, including ripple filter settings, are defined, and in the second step S54, the corresponding plan is generated. In step S55, it is determined whether the generated plan is sufficiently good or whether another possible plan should be generated. If yes, the procedure returns to step S53, new parameter settings are determined, and a new plan is generated. If it is determined in step S55 that another possible plan should not be generated, the procedure proceeds to step S56, where typically one of the possible plans for delivery to the patient being treated is selected. An alternative method is to first create plans with a series of range shifter settings, then evaluate all plans, and select the best one. As mentioned above, the selection is usually based on, for example, delivery time and plan quality, and therefore, the plan with the shortest delivery time is selected from among the plans that meet the quality requirements.

[0041] The determination in step S55 may be based on different criteria. For example, a predetermined number of plans may be generated, and the plan with the fewest number of energy levels while still satisfying the quality requirements may be selected. Alternatively, steps S33 and S34 may be repeated until a plan is obtained that includes a predetermined number of energy levels and satisfies the quality requirements. In one implementation, the set of possible plans generated by the iteration of steps S53 and S54 is generated so as to satisfy the quality requirements and have a specific spacing between Bragg peaks. A suitable starting value may be based on the spacing between two distal Bragg peaks. The width of the Bragg peaks for lower energy levels may then be increased, and the plans may be recalculated, i.e., steps S53 and S54 may be repeated until the quality of the plans is no longer satisfactory. The last calculated plan, i.e., the plan with the widest spacing between Bragg peaks while satisfying the quality requirements, may be selected.

[0042] In the simplest case, the Bragg peak may be set to have the same width across all energy layers. A better overall plan can be achieved when the width of the Bragg peak is set to increase to reduce the number of energy layers, and therefore the distance between Bragg peaks decreases in lower energy layers. This reduces the delivery time because the number of energy layers is reduced, but the dose measurement quality remains acceptable because lower energy layers typically have a lower weight in the plan.

[0043] When delivering a treatment plan to a patient, the software controlling the delivery also controls the settings of the ripple filter unit to ensure that optimal smearing of the Bragg peaks in each energy layer is achieved. As illustrated in relation to Figure 4, the software may also be configured to control the settings of the ripple filter unit differently for different spots within the energy layer.

[0044] Figure 6 is an overview of a system for radiotherapy and / or treatment planning. As can be understood, such a system may be designed in any suitable manner, and the design shown in Figure 6 is merely an example. The patient 61 is positioned on a treatment couch 63. The system comprises a treatment unit having a radiation source 65 mounted on a gantry 67 for emitting radiation toward the patient positioned on the couch 63. Typically, the couch 63 and the gantry 67 are movable in several dimensions relative to each other to deliver radiation to the patient as flexibly and accurately as possible. These parts and their functions are well known to those skilled in the art. According to the present invention, the gantry also comprises a ripple filter unit (not shown in Figure 6), such as those described above. The system also comprises a computer 71 which may be used for radiotherapy planning and / or for controlling radiotherapy. As can be understood, the computer 71 may be a separate unit not connected to the imaging unit.

[0045] The computer 71 comprises a processor 73, data memory 74, and program memory 76. Preferably, there is also one or more user input means 78, 79 in the form of a keyboard, mouse, joystick, voice recognition means, or any other available user input means. The user input means may also be configured to receive data from an external memory unit.

[0046] The data memory 74 includes clinical data and / or other information used to obtain a treatment plan, including a set of clinical goals used for planning. The data memory 74 also includes device parameters that specify a penalty function set to minimize the number of energy layers. The data memory 74 also includes one or more dose maps for one or more patients used for planning according to embodiments of the present invention. The program memory 76 holds computer programs known in themselves, configured for optimizing the treatment plan. The program memory 76 also holds computer programs configured to cause the computer to perform method steps described in relation to Figure 5, and / or computer programs configured to cause the computer to control the radiotherapy of a patient, including control of ridge filter unit settings between or within energy layers.

[0047] To ensure understanding, data memory 74 and program memory 76 are illustrated and described only schematically. There may be several data memory units, each holding one or more different types of data, or one data memory holding all data in a well-structured manner, and the same applies to program memory. One or more memories may also be stored on other computers. For example, one computer may be configured to perform only one of the methods, and another computer may exist to perform the optimization.

Claims

1. A ripple filter unit (30) for broadening the particle energy distribution of a particle beam, comprising a first ripple filter (3) and a second ripple filter (3') arranged in series in substantially the same orientation within the beam path, wherein the first and second ripple filters (3, 3') are movable relative to each other such that the filter characteristics of the ripple filter unit (30) change dynamically depending on the relative positions of the first and second ripple filters, each of the first and second ripple filters having a peak-and-trough pattern, and the first and second ripple filters are the first and second ripple filters The ripple filter unit is movable relative to each other such that the peaks of the first and second ripple filters are displaced relative to each other in a direction perpendicular to the peaks, and the control means for displacing the peaks of the first and second ripple filters relative to each other to control the width of the Bragg peak for each energy layer or for each individual spot, and for minimizing the width of the Bragg peak, the peak of the second ripple filter is positioned to coincide with the trough of the first ripple filter, and for maximizing the width of the Bragg peak, the peaks of the first ripple filter and the peak of the second ripple filter are positioned to coincide, The control means further controls the ripple filter unit so that the first and second ripple filters do not move relative to each other when the particle beam passes through the first and second ripple filters, thereby providing a ripple filter unit.

2. The ripple filter unit according to claim 1, wherein either the first ripple filter (3) or the second ripple filter (3') is fixed, and the unfixed ripple filters (3', 3) are configured to move in parallel with respect to the fixed ripple filter.

3. The ripple filter unit according to claim 1, wherein the first and second ripple filters (3, 3') are composed of uniform peaks arranged adjacent to each other, and the base of each peak substantially touches the adjacent peaks on both sides of the peak, except for the edges of the filter.

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