CN119075203B

CN119075203B - Dose distribution optimization device and method for bidirectional large-angle focusing VHEE beam

Info

Publication number: CN119075203B
Application number: CN202411197234.XA
Authority: CN
Inventors: 黄永盛; 莫灿; 毕远杰
Original assignee: Sun Yat Sen University
Current assignee: Sun Yat Sen University
Priority date: 2024-08-29
Filing date: 2024-08-29
Publication date: 2025-05-16
Anticipated expiration: 2044-08-29
Also published as: CN119075203A

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

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

Technical Field

The invention relates to the technical field of electron beam transmission, in particular to a dose distribution optimizing device and method for a bidirectional large-angle focusing VHEE beam.

Background

In the field of radiation therapy, electron beams are commonly used for the treatment of superficial tumors or scars, etc., with low energies, typically 2-6MeV. The use of such low energy electron beams is limited by the performance, volume and cost of the accelerator on the one hand, and on the other hand, has unique advantages in radiation treatment of superficial areas due to the low energy electron beams, and has low radiation damage to deep tissues because most of the dose is deposited within a body surface depth of 6 cm. With the development of medical accelerator technology, it is possible to generate usable high-energy Electron (HEE, high Energy Electron) beams and ultra-high-energy Electron (VHEE, very High Energy Electron) beams while ensuring cost, volume, and the like. Among them, a large feature of VHEE beam (energy range up to 50-250 MeV) is the greatly improved penetration depth compared to low energy electron beam, which means that it will be expected to be used for treating deep tumors around 15cm depth, which was more suitable for proton beam or heavy ion beam, and achieve better coverage of lesions and faster treatment speed. However, the bragg peak of the directly irradiated VHEE beam is not apparent, with higher entrance and exit doses, meaning that the focal site and normal tissue are subject to relatively close radiation damage, which makes it difficult to protect non-focal areas in the event of effective killing. Therefore, how to concentrate the dose as far as possible in the deep region to form a narrow bragg peak is an important goal of VHEE beam dose distribution optimization.

Focusing the VHEE beam using quadrupole lenses is a viable method, and the depth of penetration after focusing can reach around 15 cm. After that, the further improved asymmetric focusing method achieves deeper penetration depth and significantly reduces the entrance dose, with a better improvement compared to direct irradiation and ordinary symmetric focusing dose distribution effects. The core idea of asymmetric focusing is that the envelope at the output hole of the beam transmission system is as large as possible in the focusing direction by utilizing the characteristic that the quadrupole lenses focus in one direction and defocus in one direction, and the envelope is focused on the target area at a larger angle by controlling a plurality of quadrupole lenses, so that a better dose deposition effect is obtained.

However, there are still some disadvantages to the way in which asymmetric focusing occurs. First, asymmetric focusing is equivalent to sacrificing the focusing ability in one direction in exchange for a larger focusing angle in the other direction, which results in lower utilization of the beam exit aperture in lateral space and wasted exit aperture in the defocused direction. Secondly, in asymmetric focusing, the focusing effect is mainly achieved by a quadrupole lens at the exit aperture, and by changing the magnetic field gradient, the focusing angle can be controlled to change the penetration depth, but the beam current in the defocusing direction can be influenced, so that other dose deposition indexes except the penetration depth are changed.

Therefore, how to provide a dose distribution optimizing device and method for bi-directional large-angle focused VHEE beam, which increases penetration depth, reduces entrance dose, and generates bi-directional large-angle focused VHEE beam with better dose deposition effect is a problem to be solved by those skilled in the art.

Disclosure of Invention

In view of the above, the invention provides a dose distribution optimizing device and method for bi-directionally focusing VHEE beam at a large angle, which take all 2 quadrupole lenses nearest to the exit aperture into consideration to further optimize the dose distribution effect, and verify that the dose distribution effect is improved to a certain extent in all aspects. The use of two-way large-angle focusing can achieve a larger focusing angle, so that the dose in front of the focus is more dispersed, and the dose of the focus is more prominent relatively.

In order to achieve the aim, the invention adopts the following technical scheme that the dosage distribution optimizing device for the two-way large-angle focusing VHEE beam comprises n quadrupole lenses which are arranged at intervals along the beam emission direction of the VHEE beam;

The front n-2 quadrupole lenses expand the envelope of the beam;

the dimension of the n-1 quadrupole lens in the transverse focusing direction is larger than the dimension of each other quadrupole lens in the transverse direction, and the dimension of the n-1 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-1 th quadrupole lens, the envelope starts focusing from a position larger than the beam-outlet requirement, when the beam passes through the n-th quadrupole lens, the envelope is defocused, the beam-outlet requirement is just met, the VHEE beam is focused in a bidirectional large angle of the output beam, and different target dose distribution is obtained by adjusting the magnetic field gradient of the last two quadrupole lenses.

Preferably, n is an even number, n=6.

Preferably, the quadrupole lenses are all the same thickness.

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

Preferably, the dose distribution optimization method of the bidirectional large-angle focusing VHEE beam comprises the steps of constructing a beam transmission system by simulation software, wherein the beam transmission system comprises n quadrupole lenses which are arranged at intervals along the beam emission direction of the VHEE beam;

The front n-2 quadrupole lenses expand the envelope of the beam;

Preferably, determining the magnetic field gradient value range of each quadrupole lens and the relative distance value range of two adjacent quadrupole lenses according to the target beam shape includes:

Taking Twiss parameters of a source as initial states of a beam transmission system;

The Twiss parameter of the bi-directional large-angle focused VHEE beam is taken as the end state of the beam-streaming system;

Taking the magnetic field gradient of the quadrupole lens and the relative distance between the magnetic field gradient and the relative distance as unknown variables to obtain a transmission matrix of the beam transmission system;

The initial state, the end state and the transmission matrix form an optimization problem, and a boundary condition is added to solve and obtain a magnetic field gradient value range of each quadrupole lens and a relative distance value range of two adjacent quadrupole lenses.

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

Preferably, n is an even number, n=6.

Preferably, the lateral dimensions of the 1 st and 2 nd quadrupole lenses are the smallest, and the lateral dimensions of the 3 rd, 4 th and 6 th quadrupole lenses are the same.

Compared with the prior art, the invention discloses a dose distribution optimizing device and method for focusing a VHEE beam in a two-way large-angle mode, wherein the dose distribution optimizing device and method comprise n quadrupole lenses which are arranged at intervals along the beam emission direction of the VHEE beam, the front n-2 quadrupole lenses expand the envelope of the beam, the size of the n-1 quadrupole lens in the transverse focusing direction is larger than that of the other quadrupole lenses in the transverse focusing direction, the size of the n-1 quadrupole lens in the transverse focusing direction can accommodate the envelope of the beam, when the beam passes through the n-1 quadrupole lens in a certain transverse direction, the envelope starts focusing from being larger than the beam-out requirement, when the beam passes through the n-th quadrupole lens, the envelope is defocused, the beam-out requirement is just met, the two-way large-angle focused VHEE beam of the beam is output, and different target dose distributions are obtained by adjusting the magnetic field gradients of the last two quadrupole lenses. The invention has the advantages that the key parameters such as penetration depth, entrance dose and the like are greatly improved compared with asymmetric focusing, the controllability of key indexes is improved, and more dose deposition indexes can be controlled by changing the magnetic field gradient of the last two quadrupole lenses. By increasing the size of the quadrupole lenses, a bi-directional large-angle focused VHEE beam with better dose deposition is generated, and by controlling the last two quadrupole lenses, the dose distribution is controlled more. SOEP (Spread out Electron Peak, expanding electron peaks) generated on the basis of the method can cover focuses of different forms more accurately and rapidly, and has less damage to normal tissues.

Drawings

In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings that are required to be used in the embodiments or the description of the prior art will be briefly described below, and it is obvious that the drawings in the following description are only embodiments of the present invention, and that other drawings can be obtained according to the provided drawings without inventive effort for a person skilled in the art.

Fig. 1 is a schematic diagram of a dose distribution optimizing apparatus for bi-directional large-angle focused VHEE beam according to an embodiment of the present invention.

FIG. 2 is a graph showing the contrast of axial dose curves for unfocused, symmetrical focused, asymmetrical focused and bi-directional large angle focused according to an embodiment of the present invention.

Fig. 3 (a) is a schematic diagram of a first envelope of a TOPAS-based bidirectional wide-angle focusing in a transverse direction according to an embodiment of the present invention.

Fig. 3 (b) is a second schematic view of an envelope of a TOPAS-based bidirectional wide-angle focusing in another lateral direction according to an embodiment of the present invention.

Fig. 3 (c) is a first schematic diagram of the beam envelope of an asymmetrically focused VHEE beam near the exit aperture.

Fig. 3 (d) is a second schematic diagram of the beam envelope of an asymmetrically focused VHEE beam near the exit aperture.

FIG. 4 is a graph showing the contrast of the inlet percentage dose-depth curve of a bi-directional large angle focused VHEE beam with the asymmetric focused inlet percentage dose-depth curve provided by the embodiment of the present invention.

Fig. 5 is a schematic diagram showing a change of penetration depth based on TOPAS according to a magnetic field gradient g ₆ of a sixth quadrupole lens according to an embodiment of the present invention.

Fig. 6 is a graph showing the variation of the entrance percent dose based on TOPAS with the magnetic field gradient g ₅ of the fifth quadrupole lens according to an embodiment of the present invention.

Fig. 7 is a schematic diagram of SOEP curves obtained by matrix method based on Matlab calculation according to an embodiment of the present invention.

Detailed Description

The following description of the embodiments of the present invention will be made clearly and completely with reference to the accompanying drawings, in which it is apparent that the embodiments described are only some embodiments of the present invention, but not all embodiments. All other embodiments, which can be made by those skilled in the art based on the embodiments of the invention without making any inventive effort, are intended to be within the scope of the invention.

The embodiment of the invention provides a device and a method for generating a bidirectional large-angle focusing VHEE beam on the basis of asymmetric focusing, wherein the bidirectional large-angle focusing is a special symmetrical focusing mode, and the device and the method refer to expanding beam envelopes in two directions as much as possible at an outlet hole of a beam transmission system through the control of a front-end quadrupole lens and focusing the beam envelopes on the same point. This has the advantage of being similar to asymmetric focusing, with a larger angle of focus, making the dose before the focus more diffuse, and, in contrast, the dose at the focus more pronounced, as reflected by a lower entrance dose and a narrower dose peak on the axial dose curve.

Because the quadrupole lens focuses in one direction and focuses in another direction, if the envelope of the two directions is equal to the size of the exit aperture, and both the two directions are focused, the aperture of the last-last quadrupole lens must be enlarged. Thus, when a defocused beam passes through the last quadrupole lens (i.e. at the exit aperture), the upper level is necessarily focused at a greater angle and envelope, and fig. 3 (a) -3 (d) show the asymmetric focusing versus the bi-directional high angle focusing near the exit aperture. By increasing the size of the penultimate block quadrupole lens, a bi-directional large angle focused VHEE beam can be generated. The mid-axis dose curve contrast for unfocused, symmetrical focused, asymmetrical focused and bi-directional large angle focused is shown in fig. 2. As the focusing method improves, the penetration depth increases and the entrance dose decreases.

In one embodiment of the invention, the dosage distribution optimizing device of the two-way large-angle focusing VHEE beam comprises n quadrupole lenses which are arranged at intervals along the beam emission direction of the VHEE beam;

The front n-2 quadrupole lenses expand the envelope of the beam;

Specifically, n is an even number, and n=6.

In accelerator physics, quadrupole lenses (or quadrupoles) are often used in pairs, with the magnetic field gradients of each pair of quadrupoles being of opposite sign (i.e., in one direction, a pair of quadrupoles consisting of a focusing magnet and a defocusing magnet), the pair of quadrupoles being referred to as a binary lens pair (Doublet).

On the one hand, the binary lens pair improves the transverse motion stability of the system by alternating positive and negative magnetic field gradients, and on the other hand, similar to an optical lens group, the binary lens pair has certain specific magnetic field gradient value combinations, and can focus a beam in two directions after passing through, which cannot be achieved by a single quadrupole magnet. Therefore, the number of quadrupole magnets is often selected to be an even number of 2, 4, 6, 8, etc., and an odd number is rarely present (unless a ternary lens group Triplet is used).

However, when 2 magnets are selected, the output degrees of freedom of the quadrupole magnets are low, and there is not much adjustment space. When 8 magnets are selected, the occupation area and the purchase cost of the magnets are considered. Therefore, 4 or 6 quadrupolar magnets are most ideal, but when 4 are used for solving in order to output the two-way large-angle focused VHEE beam of the target, the obtained solutions are not practical enough, for example, the magnetic field gradient value of a certain magnet is too large to be achieved in practice, or the relative distance between the magnets is too far, and the space is too occupied. Thus, 6 quadrupole magnets were finally selected for use. Regarding the size of the quadrupole magnet, it is determined according to the envelope of the beam current, and if the aperture is smaller than the envelope, some particles are wasted due to collision with the wall.

The dosage distribution is controlled by adjusting the magnetic field gradient of the two terminal quadrupole lenses with different sizes.

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

As shown in fig. 1, fig. 1 illustrates a simplified model of a TOPAS-based beam-streaming system. The VHEE beam of 200MeV was incident and passed through 6 quadrupole magnets to deposit the dose in the water phantom. The magnetic field gradient of each quadrupole magnet is g ₁-g₆.

In one embodiment of the invention, the method for optimizing the dose distribution of the two-way large-angle focused VHEE beam comprises the steps of constructing a beam transmission system by simulation software, wherein the beam transmission system comprises n quadrupole lenses which are arranged at intervals along the beam emission direction of the VHEE beam;

The front n-2 quadrupole lenses expand the envelope of the beam;

In one embodiment of the invention, the specific steps of the two-way large-angle focusing VHEE beam theory calculation and key parameter generation are as follows:

Firstly, the energy of the VHEE beam is 200MeV, and the single energy beam is more convenient to simulate due to the fact that electrons with different energies move in a magnetic field differently. Although the penetration of the tissue by the electron beam at different energies is different, the resulting dose distribution effect will differ to some extent at other energies, but the method is similar.

The basic parameters of the quadrupole magnet are determined as follows. For four-pole magnets, 6 magnets were selected, each 18cm thick. The first two (near the source) are 40cm in lateral dimension, the third and fourth are 60cm in lateral dimension, the fourth is further enlarged, the fifth is 80cm in lateral dimension, the sixth is 60cm in lateral dimension, the dimensions are set to accommodate the envelope of the lower beam as much as possible, and for the fifth quadrupole of large size, the partial dose can be sacrificed to reduce the size appropriately.

As shown in fig. 3 (a) -3 (d), there is a contrast of beam envelope of the TOPAS-based bi-directional large angle focused and asymmetrically focused VHEE beam near the exit aperture. Fig. 3 (a) and 3 (b) show envelopes focused in two lateral directions at a two-way large angle, which is characterized in that both directions at the exit hole are focused and the envelopes are as large as possible. It can be seen from fig. 3 (b) that the aperture of the penultimate quadrupole magnet is larger, otherwise the full beam envelope cannot be accommodated. Fig. 3 (c) and 3 (d) are asymmetrically focused, which creates a large angle focus in only one direction of fig. 3 (c), while the exit aperture in this direction of fig. 3 (d) is wasted.

The 5 th quadrupole magnet is the largest in size, again because the envelope is the largest when the beam passes through it, see fig. 3 (b). As regards the reason why the beam current is at its maximum here, it is understood that a beam focused at a large angle, i.e. when the beam current leaves the transmission system, the envelope is as large as possible and focused. In the direction of fig. 3 (b), the 5 th quadrupole magnet and the 6 th quadrupole magnet form a binary lens pair, and in a certain transverse direction, the effect of the binary lens pair on the beam current is necessarily that the 5 th focusing magnet and the 6 th defocusing magnet. The 6 th one as the exit gives a defocusing effect, but the final beam profile is focused and has a large envelope, which can only be the profile as in fig. 3 (b), i.e. the profile is designed to be much larger than the beam-out requirement at 5 th one, and the focusing angle is larger than the beam-out requirement, so that the beam flow is focused at 6 th one, even if the defocusing effect is applied, and the envelope is much larger than the requirement to meet the requirement.

The quadrupole lens (or quadrupole magnet) is a component for electromagnetic force interaction of electron beams through magnetic fields, and consists of magnets with two N poles and two S poles sharing four poles. The electron beam is assumed to travel in the direction z, which is referred to as the longitudinal direction, and in the directions x and y perpendicular to the direction of travel, which are referred to as the transverse directions. The electromagnetic field of the quadrupole lens structure only acts on the force of the electron beam in the transverse direction, and the forces in just two directions are symmetrical, the same quadrupole lens provides focusing force in the x (y) direction and defocusing force in the y (x) direction.

Therefore, a quadrupole lens is called a focusing or defocusing lens, and it is necessary to determine from which direction to look first. For example, along the beam propagation direction z, 2 quadrupole lenses are distributed, which, viewed in the x-direction, are first-focusing and then-focusing, and, viewed in the y-direction, are then-focusing and then-focusing. The same quadrupole lens, which is only different in viewing direction, may be referred to as a focusing lens or a defocusing lens.

In practical use, the quadrupole lenses are always deliberately arranged in a periodic structure of alternating poly-diffusion, seen in one direction. 6 quadrupole lenses can be divided into 3 binary lens pairs according to the period to simplify the model.

Specifically, determining the magnetic field gradient value range of each quadrupole lens and the relative distance value range of two adjacent quadrupole lenses according to the target beam shape comprises:

In one embodiment of the present invention, the simple model construction of the beam transport system is accomplished in the monte carlo simulation software TOPAS, as shown in fig. 1. There are still two parameters that are not determined, namely the magnetic field gradient of each quadrupole magnet and the relative distance between them. The parameters are determined in two steps, the approximate value range is calculated according to the target beam shape, and then the value is adjusted according to the target dose distribution.

The method comprises the steps of firstly calculating the magnetic field gradient of each quadrupole magnet and the relative distance between the magnetic field gradient of each quadrupole magnet and the magnetic field gradient according to the target beam shape, and specifically comprises the steps of taking Twiss parameters of a source as an initial state of a beam transmission system, taking Twiss parameters of a target beam shape, namely a bidirectional large-angle focusing VHEE beam, as an end state of the system, taking the magnetic field gradient of the quadrupole magnet and the relative distance between the magnetic field gradient of the quadrupole magnet and the relative distance as unknown variables, writing a transmission matrix of the beam transmission system, and solving the initial state, the end state and the transmission matrix to form an optimization problem and adding a certain boundary condition.

The boundary conditions of the embodiment of the invention can be determined according to actual physical limitations, such as the range of the magnetic field gradient values is related to the specification of an actual magnet, and the relative distance is limited by an experimental field. Solving the problem may use accelerator design related computing software, such as MAD-X, or Matlab code. And inputting the initial state, the end state and the boundary condition, and solving a multi-element nonlinear equation set. It should be noted that the solution may not converge, and that it may be a better option to fix the values of some unknown variables to reduce the number of elements. The embodiment of the invention selects the relative distance of the fixed quadrupole magnets to simplify the solution, and considers that the actual quadrupole magnets move more slowly and are easier to shift, and the magnetic field gradient can be conveniently and accurately controlled by changing the current.

The principle of solving the magnet related parameters in the accelerator is that the magnetic field gradient and the relative distance of each quadrupole magnet are substituted into a transmission matrix, the transmission matrix is multiplied to obtain a matrix (containing unknown variables) for describing the whole transmission system, the state of the beam can be described by a column vector (namely a vector formed by Twiss parameters of the beam), the column vector in the initial state is multiplied by the total transmission matrix to be equal to the column vector in the final state, so that an extremely huge multi-element nonlinear equation system is formed, and the unknown can be solved by means of a computer.

The solution software used in the embodiments of the present invention was MAD-X developed by the European Nuclear research center CERN.

After the solution is completed, a simple model of the beam transmission system is built in the TOPAS, the basic parameters obtained by the solution are substituted, the magnetic field gradient is adjusted according to the simulation result, and the bidirectional large-angle focused VHEE beam of the target can be output.

Specifically, different target dose distributions are obtained by adjusting the magnetic field gradients of the last two quadrupole lenses, wherein the method comprises the steps of mainly changing the penetration depth when the magnetic field gradient of the nth quadrupole lens is adjusted in a certain transverse direction, and mainly changing the entrance dose when the magnetic field gradient of the nth-1 quadrupole lens is adjusted.

The regulation and control of the dose distribution are carried out by changing the magnetic field gradient of the last two quadrupole magnets, which comprises the following steps:

To obtain different target dose distributions to meet different requirements, it is necessary to fine tune the magnetic field gradient of the quadrupole magnets on the basis that a bi-directional large angle focused beam has been generated. The adjustment from the nth quadrupole magnet is most intuitive and efficient because they are closer to the output of the system. However, due to the inherent nature of the quadrupole magnets, focusing in one direction corresponds to defocusing in the other direction, and independently changing the magnetic field gradient of the last quadrupole magnet can simultaneously bring about two-direction changes, namely, the focus in one direction is close, the focus in the other direction is far away and reflected on the dose distribution, and then a plurality of indexes (such as penetration depth, entrance dose and the like) can be simultaneously changed, so that the regulation and control of the dose distribution are limited. Therefore, in order to improve the flexibility of regulation, the magnetic field gradient of the n-1 quadrupole magnet is also changed in the embodiment of the invention. Wherein, changing the magnetic field gradient value g ₆ of the nth quadrupole magnet mainly affects the penetration depth, while slightly changing the magnetic field gradient value g ₅ of the nth-1 quadrupole magnet can reduce the entrance dose under the condition that the penetration depth is basically unchanged. The last two quadrupolar magnets can be finely adjusted to regulate and control a plurality of indexes including penetration depth, entrance dose and the like. As shown in fig. 5, the penetration depth varies with the magnetic field gradient g ₆ of the sixth quadrupole magnet (TOPAS based). As the magnetic field gradient increases, the penetration depth becomes shallower. As shown in fig. 6, the percent dose at entry is compared as a function of g ₅ (TOPAS based). Maintaining g ₆ unchanged and changing the value of g ₅ allows the inlet dose to be reduced from 13.23% to 9.23% while maintaining the penetration depth substantially unchanged. This is not possible with only the change of g ₆, since small variations in g ₆ will result in a change in penetration depth. Considering more quadrupole magnets, the adjusting and controlling range is wider, but as the position of the quadrupole magnets is farther from the outlet hole, the adjusting and controlling effect is also worsened, and the calculating difficulty is also improved, so that only the last two quadrupole magnets are considered to be suitable.

In one embodiment of the present invention, the adjustment is best determined by the actual requirements, and the adjustment range of the embodiment of the present invention is approximately plus or minus 0.5T/m, and an excessive magnetic field gradient will cause excessive focusing, too much focusing to focus on the superficial area, while an excessive dose will cause too small focusing angle, too little dose will not focus on the focus well, and the entrance dose is too large. The minimum adjustment accuracy is about 0.01T/m, which is dependent on the actual magnet adjustment accuracy, and can be arbitrarily adjusted in the simulation.

Although the penultimate quadrupole magnet is adjusted, the situation that the focus in one direction is close to the focus in the other direction is far away is also caused to some extent. However, the last quadrupole magnet changed the magnetic field gradient by 0.1T/m, the shift to the focus was approximately 1-2cm (see FIG. 5), while the penultimate quadrupole magnet changed the same magnetic field gradient by 0.1T/m, had less effect on the focus than 0.29cm (i.e., the minimum distance unit used in the simulation), and had less effect on the depth of focus.

As shown in fig. 4, there is a percentage dose-depth profile of a VHEE beam of 200MeV in water phantom based on TOPAS. The penetration depth of the two is controlled to be about 17cm, and the method is suitable for deep tumors and can be further controlled by adjusting the magnetic field gradient of the quadrupole magnet. Wherein the entrance percent dose of a bi-directional large angle focused VHEE beam is about 9% compared to 30% for asymmetric focusing.

Specifically, the method further comprises the step of calculating weights through a matrix method to generate an extended electron beam peak of the VHEE.

In a specific embodiment of the present invention, SOEP of generating VHEE in analogy to proton SOBP, weights are calculated by matrix method, specifically including:

The generation of extended electrons Shu Feng (Spread out Electron Peak, SOEP) is calculated by matrix method, which imitates the generation method of extended bragg peaks (Spread outBragg Peak, SOBP) of protons, because the dosage peak of VHEE beam is much wider than that of proton beam, only two VHEE beams are needed to form SOEP. Fig. 7 shows a SOEP curve obtained by matrix method based on Matlab calculation. Only two VHEE beams with different focal positions are used for weighted superposition, so as to obtain SOEP with the plateau coverage of 12.77cm-20.20 cm. The first beam (blue dotted line) has a weight of 0.36, the second beam (red dotted line) has a weight of 0.93, and the weight value is normalized. Wherein the inlet percent dose is only 15.94%. The bidirectional large-angle focusing of the embodiment of the invention forms SOEP with smaller inlet dose and less damage to normal tissues.

In the superposition generation SOEP, the different beams often have different weights, and the method of 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 superposing PDD curves and resolving weights, the matrix method is faster and is more suitable for a computer, and the effect is similar to that of the resolving method when the number of superposed beams is small. By calculating SOEP weights, a high dose plateau of the tumor region is generated.

Specifically, a plurality of points are taken from the PDD curve of a VHEE beam, the dose values of the points form the dose vector of the beam, the dose vector of each beam forms an unweighted dose matrix, and the dose vector is multiplied by the weight vector to obtain the dose vector SOEP. Knowing the unweighted dose matrix and SOEP dose vectors (all equal in magnitude), the weight vector is solved according to:

specifically, n is an even number, n=6;

Specifically, the transverse dimensions of the 1 st quadrupole lens and the 2 nd quadrupole lens are the smallest, and the transverse dimensions of the 3 rd quadrupole lens, the 4 th quadrupole lens and the 6 th quadrupole lens are the same.

In the present specification, each embodiment is described in a progressive manner, and each embodiment is mainly described in a different point from other embodiments, and identical and similar parts between the embodiments are all enough to refer 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 points refer to the description of the method section.

The previous description of the disclosed embodiments is provided to enable any person skilled in the art to make or use the present invention. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the generic principles defined herein may be applied to other embodiments without departing from the spirit or scope of the invention. Thus, the present invention is not intended to be limited to the embodiments shown herein but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. The dose distribution optimizing device for the bidirectional large-angle focusing VHEE beam is characterized by comprising n quadrupole lenses which are arranged at intervals along the beam emission direction of the VHEE beam;

The front n-2 quadrupole lenses expand the envelope of the beam;

the size of the n-1 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-1 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-1 quadrupole lens, the envelope begins to focus from a position larger than the beam outlet requirement; when the beam passes through the nth quadrupole lens, the envelope is defocused, the beam outlet requirement is just met, and the VHEE beam is focused in a bidirectional large angle of the output beam;

changing the magnetic field gradient value of the nth quadrupole lens to influence the penetration depth, and changing the magnetic field gradient value of the nth-1 quadrupole lens to reduce the entrance dose;

calculating weights by a matrix method, generating an extended electron beam peak of the VHEE, wherein when the extended electron beam peak is generated by superposition, different beams have different weights;

Taking several points on the PDD curve of a VHEE beam, the dose values of these points form the dose vector of the beam, the dose vector of each beam forms an unweighted dose matrix, multiplying the dose matrix by the weight vector to obtain the dose vector of the peak of the extended electron beam, and solving the weight vector under the condition that the unweighted dose matrix and the dose vector of the extended electron Shu Feng are known.

2. The dose distribution optimizing apparatus of a bi-directional large angle focused VHEE beam according to claim 1, wherein n is an even number, n=6.

3. The dose distribution optimizing apparatus of a bi-directional large angle focused VHEE beam according to claim 1, wherein the quadrupole lenses are all the same thickness.

4. The device for optimizing the dose distribution of a bi-directional high-angle focused VHEE beam according to claim 1, wherein 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. The method for optimizing the dose distribution of the bidirectional large-angle focused VHEE beam is characterized by comprising the steps of constructing a beam transmission system through simulation software, wherein the beam transmission system comprises n quadrupole lenses which are arranged at intervals along the beam emission direction of the VHEE beam;

The front n-2 quadrupole lenses expand the envelope of the beam;

In a certain transverse direction, when the beam passes through the n-1 quadrupole lens, the envelope is larger than the beam-out requirement and focusing is started; when the beam passes through the nth quadrupole lens, the envelope is defocused, the beam outlet requirement is just met, and the VHEE beam is focused in a bidirectional large angle of the output beam;

6. The method of optimizing dose distribution of a bi-directional high angle focused VHEE beam according to claim 5, wherein determining the magnetic field gradient value range of each quadrupole lens and the relative distance value range of two adjacent quadrupole lenses according to the target beam shape comprises:

7. The method of optimizing dose distribution of a bi-directional high angle focused VHEE beam according to claim 5, wherein n is an even number, n=6.

8. The method of optimizing the dose distribution of a bi-directional high angle focused VHEE beam according to claim 7, wherein the lateral dimensions of the 1 st and 2 nd quadrupole lenses are the same as each other, and the lateral dimensions of the 3 rd, 4 th and 6 th quadrupole lenses are the same as each other.