Dose distribution optimization device and method for bidirectional large-angle focusing VHEE beam

Abstract

The present invention discloses a dose distribution optimization device and method for a bidirectional large-angle focusing VHEE beam, relates to the field of electron beam transmission technology, and comprises quadrupole lenses arranged at intervals along the beam emission direction; the first n-2 quadrupole lenses expand the envelope of the beam; the size of the n-1 quadrupole lens in the lateral focusing direction can accommodate the envelope of the beam; when the beam passes through the n-th quadrupole lens, the envelope is defocused, just meeting the beam output requirement, and outputting the bidirectional large-angle focusing VHEE beam of the beam; by adjusting the magnetic field gradients of the last two quadrupole lenses, different target dose distributions are obtained. The present invention takes into account the two quadrupole lenses closest to the exit hole, and further optimizes the dose distribution effect; the use of bidirectional large-angle focusing can obtain a larger focusing angle, so that the dose before the focus is more dispersed, and the dose at the focus is more prominent.

Description

Technical Field

[0001] The present invention relates to the technical field of electron beam transmission, and more particularly to a device and method for optimizing the dose distribution of a bidirectional large-angle focused VHEE beam. Background Art

[0002] In the field of radiotherapy, electron beams are usually used to treat superficial tumors or scars, etc., and their energy is relatively low, usually 2-6MeV. The use of such low-energy electron beams is due to the limitations of accelerator performance, volume and cost, on the one hand, and also due to the unique advantages of low-energy electron beams in radiotherapy of superficial areas. Most of the dose will be deposited within 6cm of the body surface depth, and the radiation damage to deep tissues is relatively low. With the development of medical accelerator technology, it is possible to generate usable high-energy electron (HEE) beams and very high-energy electron (VHEE) beams while ensuring cost and volume. Among them, a major feature of VHEE beams (with an energy range of 50-250MeV) is that the penetration depth is greatly improved compared to low-energy electron beams, which means that it will be expected to be used to treat deep tumors near the depth of 15cm, which were more suitable for proton beams or heavy ion beams in the past, and achieve better coverage of lesions and faster treatment speed. However, the Bragg peak of the directly irradiated VHEE beam is not obvious, and has a high entrance and exit dose, which means that the radiation damage to the lesion site and normal tissue is relatively close, making it difficult to protect the non-lesion area while effectively killing the lesion. Therefore, how to concentrate the dose in the deep area as much as possible to form a narrow Bragg peak is an important goal of VHEE beam dose distribution optimization.

[0003] Using a quadrupole lens to focus the VHEE beam is a feasible method, and the penetration depth after focusing can reach about 15 cm. After that, the further improved asymmetric focusing method achieved a deeper penetration depth and significantly reduced the entrance dose, which was a significant improvement over direct irradiation and ordinary symmetric focusing in terms of dose distribution. The core idea of ​​asymmetric focusing is to utilize the characteristics of the quadrupole lens that focuses in one direction and defocuses in another direction, and through the control of multiple quadrupole lenses, make the envelope of the beam transmission system exit hole as large as possible in the focusing direction, and focus on the target area at a larger angle, thereby obtaining a better dose deposition effect.

[0004] However, the asymmetric focusing method still has some shortcomings. First, asymmetric focusing is equivalent to sacrificing the focusing ability in one direction in exchange for a larger focusing angle in another direction, which makes the utilization rate of the beam exit hole in the lateral space low, and the exit hole aperture in the defocusing direction is wasted. Secondly, in asymmetric focusing, the quadrupole lens at the exit hole plays a major role in the focusing effect. By changing its magnetic field gradient, the focusing angle can be controlled to change the penetration depth, but at the same time the beam in the defocusing direction will also be affected, resulting in changes in other dose deposition indicators except the penetration depth.

[0005] Therefore, how to provide a dose distribution optimization device and method for a bidirectional large-angle focused VHEE beam, increase the penetration depth, reduce the entrance dose, and generate a bidirectional large-angle focused VHEE beam with better dose deposition effect is an urgent problem that technical personnel in this field need to solve. Summary of the invention

[0006] In view of this, the present invention provides a dose distribution optimization device and method for a bidirectional large-angle focused VHEE beam, taking into account the two quadrupole lenses closest to the exit hole to further optimize the dose distribution effect; and verifies that its dose distribution effect has been improved in various aspects. Bidirectional large-angle focusing can obtain a larger focusing angle, making the dose before the focus more dispersed, and relatively, the dose at the focus is more prominent.

[0007] In order to achieve the above-mentioned object, the present invention adopts the following technical scheme: a dose distribution optimization device for bidirectional large-angle focusing VHEE beam, comprising: n quadrupole lenses arranged at intervals along the beam emission direction of the VHEE beam;

[0008] The first n-2 quadrupole lenses expand the envelope of the beam;

[0009] The size of the n-1th quadrupole lens in the transverse focusing direction is larger than the sizes of the other quadrupole lenses in the transverse direction, and the size of the n-1th quadrupole lens in the transverse focusing direction can accommodate the envelope of the beam;

[0010] In a certain lateral direction, when the beam passes through the n-1th quadrupole lens, the envelope starts to focus from being larger than the beam output requirement; when the beam passes through the nth quadrupole lens, the envelope is defocused, just meeting the beam output requirement, and the output beam is a bidirectional large-angle focused VHEE beam; by adjusting the magnetic field gradients of the last two quadrupole lenses, different target dose distributions can be obtained.

[0011] Preferably, n is an even number, n=6.

[0012] Preferably, the thicknesses of the quadrupole lenses are all the same.

[0013] Preferably, the magnetic field gradient value range of each quadrupole lens and the relative distance value range of two adjacent quadrupole lenses are determined according to the target beam shape.

[0014] Preferably, a method for optimizing the dose distribution of a bidirectional large-angle focused VHEE beam comprises: constructing a beam transmission system by simulation software, the beam transmission system comprising n quadrupole lenses arranged at intervals along the beam emission direction of the VHEE beam;

[0015] The first n-2 quadrupole lenses expand the envelope of the beam;

[0016] The size of the n-1th quadrupole lens in the transverse focusing direction is larger than the sizes of the other quadrupole lenses in the transverse direction, and the size of the n-1th quadrupole lens in the transverse focusing direction can accommodate the envelope of the beam;

[0017] In a certain lateral direction, when the beam passes through the n-1th quadrupole lens, the envelope starts to focus from being larger than the beam output requirement; when the beam passes through the nth quadrupole lens, the envelope is defocused, just meeting the beam output requirement, and the output beam is a bidirectional large-angle focused VHEE beam; by adjusting the magnetic field gradients of the last two quadrupole lenses, different target dose distributions can be obtained.

[0018] Preferably, the magnetic field gradient value range of each quadrupole lens and the relative distance value range of two adjacent quadrupole lenses are determined according to the target beam shape, including:

[0019] The Twiss parameters of the source are used as the initial state of the beam transport system;

[0020] The Twiss parameters of the bidirectional large-angle focused VHEE beam are used as the end state of the beam transport system;

[0021] Taking the magnetic field gradient of the quadrupole lenses and the relative distance between them as unknown variables, the transmission matrix of the beam transmission system is obtained;

[0022] The initial state, terminal state and transfer matrix constitute an optimization problem. Combined with the boundary conditions, the range of magnetic field gradient values ​​of each quadrupole lens and the range of relative distance values ​​between two adjacent quadrupole lenses are solved.

[0023] Preferably, the method further comprises: calculating weights by a matrix method to generate an extended electron beam peak of VHEE.

[0024] Preferably, n is an even number, n=6.

[0025] Preferably, the first quadrupole lens and the second quadrupole lens have the smallest lateral dimensions; the third quadrupole lens, the fourth quadrupole lens and the sixth quadrupole lens have the same lateral dimensions.

[0026] It can be known from the above technical scheme that, compared with the prior art, the present invention discloses a dose distribution optimization device and method for a bidirectional large-angle focused VHEE beam, comprising: n quadrupole lenses arranged at intervals along the beam emission direction of the VHEE beam; the first n-2 quadrupole lenses expand the envelope of the beam; the size of the n-1th quadrupole lens in the transverse focusing direction is larger than the sizes of the remaining quadrupole lenses in the transverse direction, and the size of the n-1th quadrupole lens in the transverse focusing direction can accommodate the envelope of the beam; in a certain transverse direction, when the beam passes through the n-1th quadrupole lens, the envelope starts to focus from being larger than the beam emission requirement; when the beam passes through the nth quadrupole lens, the envelope is defocused, which just meets the beam emission requirement, and a bidirectional large-angle focused VHEE beam of the output beam is output; by adjusting the magnetic field gradients of the last two quadrupole lenses, different target dose distributions are obtained. The key parameters such as penetration depth and entrance dose in the present invention are greatly improved compared with asymmetric focusing; the controllability of key indicators is improved, and more dose deposition indicators can be controlled by changing the magnetic field gradient of the last two quadrupole lenses. By increasing the size of the quadrupole lens, a bidirectional large-angle focused VHEE beam with better dose deposition effect is generated, and by controlling the last two quadrupole lenses, the dose distribution can be controlled in a more diversified way. The SOEP (Spread out Electron Peak) generated on this basis can cover lesions of different forms more accurately and quickly, and cause less damage to normal tissues. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are only embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on the provided drawings without paying creative work.

[0028] Figure 1 A schematic diagram of a dose distribution optimization device for a bidirectional large-angle focused VHEE beam provided in an embodiment of the present invention.

[0029] Figure 2 A schematic diagram comparing the central axis dose curves of no focusing, symmetric focusing, asymmetric focusing and bidirectional large-angle focusing provided in an embodiment of the present invention.

[0030] FIG3( a ) is a first schematic diagram of an envelope of bidirectional large-angle focusing in a certain lateral direction based on TOPAS provided in an embodiment of the present invention.

[0031] FIG3( b ) is a second schematic diagram of the envelope of the TOPAS-based bidirectional large-angle focusing in another lateral direction provided by an embodiment of the present invention.

[0032] FIG3( c ) is the first schematic diagram of the beam envelope of the asymmetrically focused VHEE beam near the exit hole.

[0033] FIG3( d ) is a second schematic diagram of the beam envelope of the asymmetrically focused VHEE beam near the exit hole.

[0034] Figure 4 Schematic diagram comparing the entrance percentage dose-depth curve of the bidirectional large-angle focused VHEE beam and the entrance percentage dose-depth curve of the asymmetric focusing provided in an embodiment of the present invention.

[0035] Figure 5 The penetration depth based on TOPAS provided in the embodiment of the present invention varies with the magnetic field gradient g of the sixth quadrupole lens 6 Schematic diagram of the changes.

[0036] Figure 6 The entrance percentage dose based on TOPAS provided in the embodiment of the present invention varies with the magnetic field gradient g of the fifth quadrupole lens 5 Schematic diagram for comparison of changes.

[0037] Figure 7 A schematic diagram of a SOEP curve obtained by using a matrix method based on Matlab calculations provided in an embodiment of the present invention. DETAILED DESCRIPTION

[0038] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0039] Based on asymmetric focusing, the embodiment of the present invention proposes a device and method for generating a bidirectional large-angle focused VHEE beam. Bidirectional large-angle focusing is a special symmetrical focusing method, which means that through the control of the front-end quadrupole lens, the beam envelope in both directions is expanded as much as possible at the exit hole of the beam transmission system and focused on the same point. The benefits of this are similar to asymmetric focusing. The larger focusing angle makes the dose before the focus more dispersed. Relatively speaking, the dose at the focus is more prominent, which is reflected in the lower entrance dose and narrower dose peak on the central axis dose curve.

[0040] Due to the characteristics of quadrupole lenses that they focus in one direction and defocus in another, if the envelope in both directions is to be equal to the exit aperture size and both are focused, it must mean that the aperture of the penultimate quadrupole lens must be increased. Therefore, when the defocused beam passes through the last quadrupole lens (i.e. at the exit), if it is still focused after being defocused, then it must be focused at a larger angle and envelope at the previous stage. Figure 3(a)-Figure 3(d) The comparison between asymmetric focusing and bidirectional large-angle focusing near the exit hole is shown. By increasing the size of the penultimate quadrupole lens, a bidirectional large-angle focusing VHEE beam can be generated. Figure 2 The figure shows the comparison of the central axis dose curves of no focusing, symmetrical focusing, asymmetrical focusing and bidirectional large-angle focusing. With the improvement of focusing method, the penetration depth increases and the entrance dose decreases.

[0041] In a specific embodiment of the present invention, a dose distribution optimization device for a bidirectional large-angle focused VHEE beam comprises: n quadrupole lenses arranged at intervals along a beam emission direction of the VHEE beam;

[0042] The first n-2 quadrupole lenses expand the envelope of the beam;

[0043] The size of the n-1th quadrupole lens in the transverse focusing direction is larger than the sizes of the other quadrupole lenses in the transverse direction, and the size of the n-1th quadrupole lens in the transverse focusing direction can accommodate the envelope of the beam;

[0044] In a certain lateral direction, when the beam passes through the n-1th quadrupole lens, the envelope starts to focus from being larger than the beam output requirement; when the beam passes through the nth quadrupole lens, the envelope is defocused, just meeting the beam output requirement, and the output beam is a bidirectional large-angle focused VHEE beam; by adjusting the magnetic field gradients of the last two quadrupole lenses, different target dose distributions can be obtained.

[0045] Specifically, n is an even number, n=6.

[0046] In accelerator physics, quadrupole lenses (or quadrupole magnets) are often used in pairs, and the magnetic field gradients of each pair of quadrupole magnets have opposite signs (i.e., along one direction, a pair of quadrupole magnets consists of a focusing magnet and a defocusing magnet). This pair of quadrupole magnets is called a doublet lens pair.

[0047] On the one hand, this binary lens pair improves the lateral motion stability of the system by alternating positive and negative magnetic field gradients; on the other hand, the binary lens pair is similar to an optical lens group in optics, and there are certain specific combinations of magnetic field gradient values ​​that can focus the beam in both directions after passing through, which is impossible for a single quadrupole magnet. Therefore, the number of quadrupole magnets is often selected as an even number such as 2, 4, 6, 8, and rarely an odd number (unless a triplet lens group is used).

[0048] However, when two are selected, the output freedom of too few quadrupole magnets is very low, and there is not much room for adjustment. When eight are selected, the floor space and purchase cost of the magnets must be considered. Therefore, 4 or 6 quadrupole magnets are the most ideal, but in order to output the target bidirectional large-angle focused VHEE beam, some solutions obtained when 4 are used for solution are not practical enough, such as the magnetic field gradient value of a certain magnet is too large and cannot be achieved in practice, or the relative distance between the magnets is too far and takes up too much space. Therefore, 6 quadrupole magnets were finally selected. The size of the quadrupole magnet is determined according to the envelope of the beam. If the aperture is smaller than the envelope, some particles will be wasted due to hitting the wall.

[0049] Specifically, the thicknesses of the quadrupole lenses are all the same; and the dose distribution is controlled by adjusting the magnetic field gradients of the two end quadrupole lenses of different sizes.

[0050] Specifically, the magnetic field gradient value range of each quadrupole lens and the relative distance value range of two adjacent quadrupole lenses are determined according to the target beam shape.

[0051] like Figure 1 As shown, Figure 1 A simple model of the beam transport system based on TOPAS is shown. After the 200MeV VHEE beam is incident, it passes through 6 quadrupole magnets and deposits a dose in the water phantom. The magnetic field gradients of each quadrupole magnet are g 1 -g 6 .

[0052] In a specific embodiment of the present invention, a method for optimizing the dose distribution of a bidirectional large-angle focused VHEE beam comprises: constructing a beam transmission system by simulation software, wherein the beam transmission system comprises n quadrupole lenses arranged at intervals along a beam emission direction of the VHEE beam;

[0053] The first n-2 quadrupole lenses expand the envelope of the beam;

[0054] The size of the n-1th quadrupole lens in the transverse focusing direction is larger than the sizes of the other quadrupole lenses in the transverse direction, and the size of the n-1th quadrupole lens in the transverse focusing direction can accommodate the envelope of the beam;

[0055] In a certain lateral direction, when the beam passes through the n-1th quadrupole lens, the envelope starts to focus from being larger than the beam output requirement; when the beam passes through the nth quadrupole lens, the envelope is defocused, just meeting the beam output requirement, and the output beam is a bidirectional large-angle focused VHEE beam; by adjusting the magnetic field gradients of the last two quadrupole lenses, different target dose distributions can be obtained.

[0056] In a specific embodiment of the present invention, the specific steps of bidirectional large-angle focusing VHEE beam theoretical calculation and key parameter generation are as follows:

[0057] First, the energy of the VHEE beam is set to 200 MeV. Since electrons of different energies move differently in a magnetic field, a monoenergetic beam is more convenient for simulation. Although electron beams of different energies have different penetration capabilities into tissues, the final dose distribution effect will be different at other energies, but the method is similar.

[0058] Next, the basic parameters of the quadrupole magnet are determined. For the quadrupole magnet, the number is selected to be 6, and the thickness is 18 cm. The transverse dimensions of the first two (close to the source) are 40 cm*40 cm, and the transverse dimensions of the third and fourth are 60 cm*60 cm. The size of the quadrupole magnet is further increased, and the transverse dimensions of the fifth are 80 cm*80 cm, and the transverse dimensions of the sixth are 60 cm*60 cm. The size is set in this way to accommodate the envelope of the beam as much as possible. For the large-sized fifth quadrupole magnet, some dose can be sacrificed to appropriately reduce the size.

[0059] like Figure 3(a)-Figure 3(d) As shown, it is a comparison of the beam envelope of the bidirectional large-angle focusing and asymmetric focusing VHEE beam near the exit hole based on TOPAS. Figure 3(a) and Figure 3(b) show the envelope of bidirectional large-angle focusing in two lateral directions, which are characterized by focusing in both directions at the exit hole and the envelope is as large as possible. As can be seen from Figure 3(b), the aperture of the penultimate quadrupole magnet is larger, otherwise it cannot accommodate the entire beam envelope. Figures 3(c) and 3(d) are asymmetric focusing, which only forms a large-angle focusing in the direction of Figure 3(c), while the exit hole aperture in this direction of Figure 3(d) is wasted.

[0060] The size of the fifth quadrupole magnet is the largest, also because when the beam passes through it, the envelope reaches the maximum, see Figure 3(b). As for why the beam reaches the maximum here, we must first understand the beam focused at a large angle, that is, when the beam leaves the transmission system, the envelope must be as large as possible and focused. In the direction of Figure 3(b), the fifth quadrupole magnet and the sixth quadrupole magnet form a double lens pair. In a certain lateral direction, their effect on the beam must be that the fifth is a focusing magnet and the sixth is a defocusing magnet. The sixth, as the exit, exerts a defocusing effect, but the final beam shape must be focused and have a large envelope, which can only be the shape shown in Figure 3(b): that is, the fifth is designed to have an envelope far greater than the beam output requirement and a focusing angle greater than the beam output requirement, so that when the beam passes through the sixth, even if a defocusing effect is exerted, it is still focused in the end, and the envelope also changes from far greater than the requirement to meeting the requirement.

[0061] Among them, the quadrupole lens (or quadrupole magnet) is a type of component that uses a magnetic field to interact with the electron beam through electromagnetic forces. It is composed of a magnet with two N poles and two S poles. Assume that the direction of the electron beam is z, which is called the longitudinal direction; the directions perpendicular to the direction of travel are x and y, which are called transverse directions. The electromagnetic field constructed by the quadrupole lens only acts on the electron beam in the transverse direction, and the forces in the two directions are symmetrical. The same quadrupole lens provides focusing force in the x (y) direction and defocusing force in the y (x) direction.

[0062] Therefore, to call a quadrupole lens a focusing or defocusing lens, we need to first determine the direction from which we look. For example, along the transmission direction z of the beam, there are two quadrupole lenses. If they converge first and then diverge when viewed from the x direction, then they diverge first and then converge when viewed from the y direction. For the same quadrupole lens, it can be called a focusing lens or a defocusing lens only because the direction of view is different.

[0063] In practical applications, quadrupole lenses are always intentionally arranged into a periodic structure of alternating convergence and divergence when viewed from one direction. Six quadrupole lenses can be divided into three binary lens pairs according to the period to simplify the model.

[0064] Specifically, the magnetic field gradient value range of each quadrupole lens and the relative distance value range of two adjacent quadrupole lenses are determined according to the target beam shape, including:

[0065] The Twiss parameters of the source are used as the initial state of the beam transport system;

[0066] The Twiss parameters of the bidirectional large-angle focused VHEE beam are used as the end state of the beam transport system;

[0067] Taking the magnetic field gradient of the quadrupole lenses and the relative distance between them as unknown variables, the transmission matrix of the beam transmission system is obtained;

[0068] The initial state, terminal state and transfer matrix constitute an optimization problem. Combined with the boundary conditions, the range of magnetic field gradient values ​​of each quadrupole lens and the range of relative distance values ​​between two adjacent quadrupole lenses are solved.

[0069] In a specific embodiment of the present invention, a simple model of the beam transport system is constructed in the Monte Carlo simulation software TOPAS, such as Figure 1 There are still two parameters that have not been determined, namely the magnetic field gradient of each quadrupole magnet and the relative distance between them. The determination of these parameters is carried out in two steps. First, the approximate range of values ​​is calculated according to the target beam shape, and then the values ​​are adjusted according to the target dose distribution.

[0070] Firstly, the magnetic field gradient of each quadrupole magnet and the relative distance between them are calculated according to the target beam morphology, specifically including: taking the Twiss parameters of the source as the initial state of the beam transmission system; taking the target beam morphology, that is, the Twiss parameters of the bidirectional large-angle focusing VHEE beam as the terminal state of the system; taking the magnetic field gradient of the quadrupole magnet and the relative distance between them as unknown variables, and writing the transmission matrix of the beam transmission system; the initial state, terminal state and transmission matrix constitute an optimization problem, which can be solved with certain boundary conditions.

[0071] The boundary conditions of the embodiment of the present invention can be determined according to the actual physical limitations, such as the range of values ​​of the magnetic field gradient is related to the specifications of the actual magnet, and the relative distance is restricted by the experimental site. The problem can be solved using calculation software related to accelerator design, such as MAD-X, or using Matlab code for calculation. Input the initial state, terminal state and boundary conditions, and solve the multivariate nonlinear equations. It should be noted that the solution may not converge, and fixing the values ​​of some unknown variables to reduce the number of elements may be a better choice. The embodiment of the present invention chooses to fix the relative distance of the quadrupole magnet to simplify the solution, considering that the actual quadrupole magnet moves slower and is more likely to cause offset, and the magnetic field gradient can be conveniently and accurately controlled by changing the current.

[0072] Among them, the principle of solving the magnet-related parameters in the accelerator is: the magnetic field gradient and relative distance of each quadrupole magnet are taken as unknowns and substituted into the transmission matrix. Multiplying the transmission matrices will obtain a total matrix (including unknown variables) that describes the entire transmission system. The state of the beam can be described by a column vector (that is, the vector composed of the Twiss parameters of the beam). The column vector of the initial state is multiplied on the left by the total transmission matrix, which is equal to the column vector of the final state. This constitutes an extremely large set of multivariate nonlinear equations, and the unknowns can be solved with the help of a computer.

[0073] The solution software used in the embodiment of the present invention is MAD-X developed by the European Organization for Nuclear Research CERN.

[0074] After the solution is completed, a simple model of the beam transmission system is constructed in TOPAS. The basic parameters obtained by the solution are substituted, and the magnetic field gradient is adjusted according to the simulation results to output the target bidirectional large-angle focused VHEE beam.

[0075] Specifically, different target dose distributions are obtained by adjusting the magnetic field gradients of the last two quadrupole lenses, including: in a certain lateral direction, when adjusting the magnetic field gradient of the nth quadrupole lens, the penetration depth is mainly changed; when adjusting the magnetic field gradient of the n-1th quadrupole lens, the entrance dose is mainly changed.

[0076] The dose distribution is regulated by changing the magnetic field gradient of the last two quadrupole magnets, including:

[0077] In order to obtain different target dose distributions to meet different needs, it is necessary to fine-tune the magnetic field gradient of the quadrupole magnet on the basis of the generated bidirectional large-angle focusing beam. It is most intuitive and effective to start adjusting from the nth quadrupole magnet because they are closer to the output of the system. However, due to the inherent properties of the quadrupole magnet, focusing in one direction corresponds to defocusing in another direction. Changing the magnetic field gradient of the last quadrupole magnet alone will bring about changes in two directions at the same time, that is, the focus in one direction is closer and the focus in the other direction is farther away. When reflected in the dose distribution, multiple indicators (such as penetration depth, entrance dose, etc.) will be changed at the same time, limiting the control of the dose distribution. Therefore, in order to improve the flexibility of control, the embodiment of the present invention also changes the magnetic field gradient of the n-1th quadrupole magnet. Among them, changing the magnetic field gradient value g of the nth quadrupole magnet 6 , which mainly affects the penetration depth, but slightly changes the magnetic field gradient value g of the n-1th quadrupole magnet 5 , the entrance dose can be reduced while the penetration depth remains basically unchanged. By finely adjusting the last two quadrupole magnets, multiple indicators including penetration depth and entrance dose can be controlled. Figure 5 As shown in the figure, the penetration depth varies with the magnetic field gradient g of the sixth quadrupole magnet. 6As the magnetic field gradient increases, the penetration depth becomes shallower. Figure 6 As shown, the inlet percentage dose varies with g 5 Comparison of changes in g (based on TOPAS). 6 unchanged, change g 5 The value of g can reduce the entrance dose from 13.23% to 9.23% while keeping the penetration depth basically unchanged. 6 Can't do it, because g 6 Even a small change will lead to a change in penetration depth. If more quadrupole magnets are taken into consideration, the range of control will be wider, but as the position of the quadrupole magnets gets farther and farther away from the exit hole, the control effect will become worse and the calculation difficulty will increase. Therefore, it is more appropriate to only consider the last two quadrupole magnets.

[0078] In a specific embodiment of the present invention, whether the adjustment is optimal is determined by actual needs. The adjustment range of the embodiment of the present invention is approximately plus or minus 0.5 T / m. A too large magnetic field gradient will lead to over-focusing, making the focus too far forward, and the dose concentrated in the superficial area; while a too small magnetic field gradient will lead to a too small focusing angle, the dose cannot be well focused at the focus, and the entrance dose is too large. The minimum adjustment accuracy is about 0.01 T / m, which depends on the adjustment accuracy of the actual magnet. The accuracy can be adjusted arbitrarily in the simulation.

[0079] Although adjusting the penultimate quadrupole magnet will also cause the situation of "the focus in one direction is closer and the focus in the other direction is farther away" to a certain extent. However, when the last quadrupole magnet changes the magnetic field gradient by 0.1T / m, the focus shift is about 1-2cm (see Figure 5 ); while for the penultimate quadrupole magnet, changing the same 0.1T / m magnetic field gradient has an effect on the focus that is less than 0.29cm (the smallest distance unit used in the simulation), and has a smaller effect on the focus depth.

[0080] like Figure 4 The figure shows the percentage dose-depth curve of the mid-axis of a 200MeV VHEE beam based on TOPAS in a water phantom. The penetration depth of both is controlled at around 17cm, which is suitable for deep tumors and can be further controlled by adjusting the magnetic field gradient of the quadrupole magnet. Among them, the entrance percentage dose of the bidirectional large-angle focused VHEE beam is about 9%, while that of the asymmetric focused one reaches 30%.

[0081] Specifically, it also includes: calculating weights by a matrix method to generate an extended electron beam peak of VHEE.

[0082] In a specific embodiment of the present invention, the SOEP of VHEE is generated by imitating the proton SOBP, and the weight is calculated by the matrix method, which specifically includes:

[0083] Following the method of generating the spread out Bragg peak (SOBP) of protons, the matrix method is used to calculate the generation of the spread out electron peak (SOEP). Since the dose peak of the VHEE beam is much wider than that of the proton beam, often only two VHEE beams are needed to form the SOEP. Figure 7 It can be seen that the SOEP curve is obtained based on the matrix method calculated by Matlab. Only two VHEE beams with different focal positions are used for weighted superposition to obtain a SOEP with a plateau covering 12.77cm-20.20cm. The weight of the first beam (blue dotted line) is 0.36, and the weight of the second beam (red dotted line) is 0.93. The weight values ​​have been normalized. Among them, the entrance percentage dose is only 15.94%. The entrance dose of the SOEP formed by the bidirectional large-angle focusing of the embodiment of the present invention is small, and the damage to normal tissues is also smaller.

[0084] When superimposing SOEP, different beams often have different weights. The method for calculating the weights required for each beam refers to a matrix calculation method for generating SOBP in protons. Compared with the traditional method of fitting and superimposing PDD curves and analytically solving weights, the matrix method is faster and more suitable for computers. When the number of superimposed beams is small, the effect is close to the analytical method. By calculating the SOEP weights, a high-dose plateau area in the tumor area is generated.

[0085] Specifically, several points are taken on the PDD curve of a VHEE beam. The dose values ​​of these points constitute the dose vector of the beam. The dose vectors of each beam constitute an unweighted dose matrix. The dose matrix is ​​multiplied by the weight vector to obtain the SOEP dose vector. When the unweighted dose matrix and the SOEP dose vector (the plateau values ​​are all equal) are known, the weight vector is solved according to the following formula:

[0086]

[0087] Specifically, n is an even number, n=6;

[0088] Specifically, the first quadrupole lens and the second quadrupole lens have the smallest transverse dimensions; the third quadrupole lens, the fourth quadrupole lens and the sixth quadrupole lens have the same transverse dimensions.

[0089] In this specification, each embodiment is described in a progressive manner, and each embodiment focuses on the differences from other embodiments. The same or similar parts between the embodiments can be referred to each other. For the device disclosed in the embodiment, since it corresponds to the method disclosed in the embodiment, the description is relatively simple, and the relevant parts can be referred to the method part.

[0090] The above description of the disclosed embodiments enables one skilled in the art to implement or use the present invention. Various modifications to these embodiments will be apparent to one skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the present invention. Therefore, the present invention will not be limited to the embodiments shown herein, but rather to the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A dose distribution optimization device for bidirectional large-angle focused VHEE beam, characterized in that: include: n quadrupole lenses are arranged in an interval manner along the beam emission direction of the VHEE beam; The first n-2 quadrupole lenses expand the envelope of the beam; The size of the n-1th quadrupole lens in the transverse focusing direction is larger than the sizes of the other quadrupole lenses in the transverse focusing direction, and the size of the n-1th quadrupole lens in the transverse focusing direction can accommodate the envelope of the beam; In a certain transverse direction, when the beam passes through the n-1th quadrupole lens, the envelope starts to focus from being larger than the beam output requirement; when the beam passes through the nth quadrupole lens, the envelope is defocused, just meeting the beam output requirement, and the bidirectional large-angle focused VHEE beam of the output beam is obtained; by adjusting the magnetic field gradients of the last two quadrupole lenses, different target dose distributions are obtained; Among them, changing the magnetic field gradient value of the nth quadrupole lens affects the penetration depth; changing the magnetic field gradient value of the n-1th quadrupole lens reduces the entrance dose; The weights are calculated by the matrix method to generate the extended electron beam peak of VHEE. When the extended electron beam peaks are generated by superposition, different beams have different weights. Several points are taken on the PDD curve of a VHEE beam. The dose values ​​of these points constitute the dose vector of the beam. The dose vectors of each beam constitute an unweighted dose matrix. The dose matrix is ​​multiplied by the weight vector to obtain the dose vector of the extended electron beam peak. When the unweighted dose matrix and the extended electron beam peak dose vector are known, the weight vector is solved.

2. The dose distribution optimization device for bidirectional large-angle focused VHEE beam according to claim 1, characterized in that: n is an even number, n=6.

3. The dose distribution optimization device for bidirectional large-angle focused VHEE beam according to claim 1, characterized in that: The thicknesses of the quadrupole lenses are all the same.

4. The dose distribution optimization device for bidirectional large-angle focused VHEE beam according to claim 1, characterized in that: The magnetic field gradient value range of each quadrupole lens and the relative distance value range of two adjacent quadrupole lenses are determined according to the target beam shape.

5. A method for optimizing dose distribution of bidirectional large-angle focused VHEE beams, characterized in that: include: A beam transmission system is constructed by simulation software, wherein the beam transmission system includes n quadrupole lenses arranged at intervals along the beam emission direction of the VHEE beam; The first n-2 quadrupole lenses expand the envelope of the beam; The size of the n-1th quadrupole lens in the transverse focusing direction is larger than the sizes of the other quadrupole lenses in the transverse focusing direction, and the size of the n-1th quadrupole lens in the transverse focusing direction can accommodate the envelope of the beam; In a certain transverse direction, when the beam passes through the n-1th quadrupole lens, the envelope is greater than the beam output requirement and begins to focus; when the beam passes through the nth quadrupole lens, the envelope is defocused, just meeting the beam output requirement, and the bidirectional large-angle focused VHEE beam of the output beam is output; by adjusting the magnetic field gradients of the last two quadrupole lenses, different target dose distributions are obtained; Among them, changing the magnetic field gradient value of the nth quadrupole lens affects the penetration depth; changing the magnetic field gradient value of the n-1th quadrupole lens reduces the entrance dose; The weights are calculated by the matrix method to generate the extended electron beam peak of VHEE. When the extended electron beam peaks are generated by superposition, different beams have different weights. Several points are taken on the PDD curve of a VHEE beam. The dose values ​​of these points constitute the dose vector of the beam. The dose vectors of each beam constitute an unweighted dose matrix. The dose matrix is ​​multiplied by the weight vector to obtain the dose vector of the extended electron beam peak. When the unweighted dose matrix and the extended electron beam peak dose vector are known, the weight vector is solved.

6. The method for optimizing dose distribution of bidirectional large-angle focused VHEE beam according to claim 5, characterized in that: The magnetic field gradient value range of each quadrupole lens and the relative distance value range of two adjacent quadrupole lenses are determined according to the target beam shape, including: The Twiss parameters of the source are used as the initial state of the beam transport system; The Twiss parameters of the bidirectional large-angle focused VHEE beam are used as the end state of the beam transport system; Taking the magnetic field gradient of the quadrupole lenses and the relative distance between them as unknown variables, the transmission matrix of the beam transmission system is obtained; The initial state, terminal state and transfer matrix constitute an optimization problem. Combined with the boundary conditions, the range of magnetic field gradient values ​​of each quadrupole lens and the range of relative distance values ​​between two adjacent quadrupole lenses are solved.

7. The method for optimizing dose distribution of bidirectional large-angle focused VHEE beam according to claim 5, characterized in that: n is an even number, n=6.

8. The method for optimizing dose distribution of bidirectional large-angle focused VHEE beam according to claim 7, characterized in that: The lateral sizes of the first and second quadrupole lenses are the smallest; the lateral sizes of the third, fourth and sixth quadrupole lenses are the same.

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