Low-pressure ionization chamber, design method, and medical linear accelerator

Abstract

A low-pressure ionization chamber, a design method, and a medical linear accelerator. The low-pressure ionization chamber comprises a housing (100). A closed chamber is formed inside the housing (100), and a low-pressure environment lower than the standard atmospheric pressure is formed in the closed chamber, so that the ion recombination ratio of positive and negative charges is not greater than 0.01, thereby improving collection efficiency of positive and negative charges, and solving the technical problem in the existing technology of inaccurate monitoring results at an ultra-high dose rate due to the influence of ion recombination on a penetration-type air ionization chamber.

Description

A low-pressure ionization chamber, its design method, and a medical linear accelerator Technical Field

[0001] This invention belongs to the field of radiotherapy dose monitoring technology, specifically relating to a low-pressure ionization chamber, its design method, and a medical linear accelerator. Background Technology

[0002] Ultra-high dose rates and ultra-short pulses are the core radiation characteristics of FLASH radiotherapy, but they pose a significant obstacle to existing conventional dose detection methods (especially detectors). At the physical level, the extremely intense radiation of FLASH makes probe polarization and ion recombination effects significant and difficult to predict. The basic idea of ​​radiation dose detection is to apply ionizing radiation to the sensitive medium of the probe, convert the radiation dose signal into an electrical signal, collect the electrical signal, amplify it, and then read and analyze it. Conventional radiotherapy monitor probes typically use atmospheric pressure penetrating air ionization chambers, where the internal air pressure is essentially the same as atmospheric pressure. Under FLASH conditions, a large dose is instantaneously deposited, generating ultra-high density positive and negative charge carriers. These carriers easily encounter each other and undergo ion recombination during migration and collection, resulting in an unpredictable and significant decrease in charge collection efficiency. Simultaneously, the large amount of charge causes severe electric field distortion, further exacerbating the polarization effect. This physical process of ion migration and recombination is extremely complex and highly sensitive to the internal structure and electric field of the ionization chamber, making it difficult to predict and calculate accurately; that is, we cannot eliminate this effect through theoretical corrections. Therefore, atmospheric pressure penetrating air ionization chambers are not suitable for dose monitoring in ultra-high dose rate radiotherapy equipment.

[0003] To address the issue that atmospheric pressure penetrating air ionization chambers are unsuitable for ultra-high dose rate radiation fields, several solutions have been proposed by those skilled in the art, such as dual-beam transformers and diamond detectors. Dual-beam transformers use beam current transformers to measure beam intensity without the need for a mass layer, but their drawback is that they can only measure electron flux, not electron energy, beam shape, or spatial distribution. Therefore, the beam current transformer monitor must be coupled to the ionization chamber or other imaging detectors to obtain dose and beam profile information. Diamond detectors offer advantages such as high sensitivity, fast response, high stability, and chemical stability, but their collection efficiency is unstable under ultra-high dose rate conditions, thus preventing direct measurement of the intra-beam dose. Furthermore, the fabrication of large-area single-crystal diamonds presents challenges, limiting the application of diamond detector arrays.

[0004] Therefore, existing technologies need further development. Summary of the Invention

[0005] To address the shortcomings of existing technologies, a low-pressure ionization chamber, its design method, and a medical linear accelerator are proposed to solve the technical problem of inaccurate monitoring results at ultra-high dose rates caused by ion recombination in existing atmospheric pressure penetrating air ionization chambers. At the same time, it can solve the problem of high obstruction and strong scattering of radiotherapy electron beams in existing penetrating air ionization chambers.

[0006] In a first aspect, the present invention provides a low-pressure ionization chamber, comprising a housing, wherein a sealed chamber is formed inside the housing, and a low-pressure environment below standard atmospheric pressure is formed inside the sealed chamber.

[0007] This technical solution is further configured such that the distance d between the high-voltage electrode and the collecting electrode in the ionization chamber, the single-pulse dose D of the radiotherapy beam, the air pressure P in the sealed chamber, and the voltage U between the high-voltage electrode and the collecting electrode in the sealed chamber are... c Should meet:

[0008] Where R is the recombination ratio of positive and negative charges; k is a constant; α0 is the ion recombination coefficient under a specific gas pressure and electric field strength, α0 is a constant; ρ0 is the charge density generated under a specific intrapulse dose and gas pressure, ρ0 is a constant; p is the direct electron collection ratio; e is the elementary charge; μ +0 and μ -0 These represent the positive and negative ion mobility constants of a specific gas under a specific gas pressure and electric field strength, respectively.

[0009] This technical solution is further configured such that, depending on the single-pulse dose of the radiotherapy beam, the air pressure in the sealed chamber is 0.001 atm to 0.5 atm, the distance between the high-voltage electrode and the collecting electrode in the sealed chamber is 0.1 mm to 10 mm, and the voltage U between the high-voltage electrode and the collecting electrode in the sealed chamber is... c The voltage range is from -400V to 400V.

[0010] The technical solution is further configured such that the housing is provided with a first thin window and a second thin window for the beam to pass through, and both the first thin window and the second thin window are made of a material with a compressive strength greater than 1 atm and a high radiation transmittance.

[0011] The technical solution is further configured such that, when the beam is a low-penetration beam, the thickness of the first thin window and the second thin window is 50 μm to 100 μm.

[0012] The technical solution is further configured such that, when the beam is a high-penetration beam, the thickness of the first thin window and the second thin window is 50 μm to 800 μm.

[0013] The technical solution is further configured such that the high voltage electrode, the insulating spacer ring, and the collecting electrode are stacked in sequence, the insulating spacer ring is provided with a through hole for the beam to pass through, the collecting electrode includes multiple mutually insulated collecting parts, and the collecting parts are provided with a protective part around their periphery.

[0014] The technical solution is further configured such that the collecting part includes a central collecting part and a plurality of edge collecting parts symmetrically arranged around the central collecting part, and the protective part includes an inner protective ring and an inner protective ring extension. The inner protective ring is disposed on the side of the edge collecting part away from the central collecting part; the inner protective ring extension is connected to the inner protective ring and extends to the space between adjacent edge collecting parts.

[0015] The technical solution is further configured such that the protection part includes an outer protection ring, the inner protection ring and the outer protection ring are concentric and spaced apart, and the middle collection part pin and the edge collection part pin are located between the inner protection ring and the outer protection ring.

[0016] The technical solution is further configured such that the inner protection ring and the outer protection ring are connected by multiple connecting parts, and the multiple connecting parts divide the accommodating space between the inner protection ring and the outer protection ring into multiple independent sub-accommodating spaces, and the middle collecting part pin and the edge collecting part pin are respectively located in different sub-accommodating spaces.

[0017] The technical solution is further configured such that the protection part includes: a protection electrode, the protection electrode being connected to the inner protection ring and the outer protection ring respectively; the collecting electrode includes a second bearing layer, the collecting part being disposed on the second bearing layer; the protection electrode is located on the side of the second bearing layer away from the collecting part.

[0018] The technical solution is further configured such that the high-voltage electrode includes a first bearing layer, a first plating layer, and a second plating layer. Both the first plating layer and the second plating layer are disposed on the first bearing layer. The first plating layer is located on the side of the first bearing layer away from the insulating spacer ring. The second plating layer is located on the side of the first bearing layer closer to the insulating spacer ring.

[0019] Secondly, the present invention provides a design method for a low-pressure ionization chamber, which adjusts the air pressure of the sealed chamber, the distance between the high-pressure electrode and the collecting electrode, and the input voltage of the high-pressure electrode based on the single-pulse dose of the radiotherapy beam, until the recombination ratio of positive and negative charges is not greater than 0.01, wherein the air pressure of the sealed chamber is lower than the standard atmospheric pressure.

[0020] Thirdly, the present invention provides a medical linear accelerator comprising the aforementioned low-pressure ionization chamber.

[0021] The beneficial effects of this invention are:

[0022] By reducing the air pressure within a sealed chamber to create a low-pressure ionization chamber, ion recombination can be avoided or reduced, thereby improving the accuracy of dose rate and total dose monitoring in ultra-high dose rate radiotherapy. Simultaneously, adjusting the air pressure within the sealed chamber allows the ionization chamber to be used in both FLASH and CONV conditions, thus expanding its application range. Attached Figure Description

[0023] Figure 1 is a schematic diagram of the overall structure of the low-pressure ionization chamber in an embodiment of the present invention;

[0024] Figure 2 is a longitudinal sectional view of the low-pressure ionization chamber in an embodiment of the present invention;

[0025] Figure 3 is a schematic diagram of the housing in an embodiment of the present invention;

[0026] Figure 4 is a schematic diagram of the electrode unit in an embodiment of the present invention;

[0027] Figure 5 is a schematic diagram of the high-voltage electrode in an embodiment of the present invention;

[0028] Figure 6 is a schematic diagram of the collecting electrode and the guard electrode in an embodiment of the present invention;

[0029] Figure 7 is a schematic diagram of the partition section in an embodiment of the present invention;

[0030] Figure 8 is a graph showing the functional relationship between the signal measured in the direct ionization chamber and the relative absorbed dose in an embodiment of the present invention;

[0031] Figure 9 is a schematic diagram showing the relationship between the monitor reading and the ratio of the scintillator absorbed dose (normalized to the average value of each group) as a function of the number of measurements in an embodiment of the present invention.

[0032] Figure 10 is a schematic diagram of the pulse signal shape measured in the low-pressure ionization chamber at a high voltage of 40V under different pulse widths in an embodiment of the present invention.

[0033] Figure 11 is a schematic diagram of the pulse signal shape measured in the low-pressure ionization chamber at a high voltage of 80V under different pulse widths in an embodiment of the present invention.

[0034] Figure 12 is a schematic diagram showing the relationship between the recombination ratio R of positive and negative charges and the gas pressure P in an embodiment of the present invention;

[0035] Figure 13 is a schematic diagram showing the relationship between the recombination ratio R of positive and negative charges and the gas pressure P in an embodiment of the present invention;

[0036] Figure 14 shows the relationship between the recombination ratio R of positive and negative charges and the voltage U in an embodiment of the present invention. c Relationship diagram;

[0037] Figure 15 shows the relationship between the recombination ratio R of positive and negative charges and the voltage U in an embodiment of the present invention.c Relationship diagram;

[0038] Figure 16 is a schematic diagram showing the relationship between the recombination ratio R of positive and negative charges and the interelectrode spacing d in an embodiment of the present invention;

[0039] Figure 17 is a normalized schematic diagram of the absorbed dose and titanium film thickness in an embodiment of the present invention.

[0040] In the attached drawings: 100, housing; 110, upper housing; 111, mounting hole; 112, first thin window; 120, lower housing; 300, electrode unit; 310, high-voltage electrode; 311, first bearing layer; 312, first plating layer; 313, second plating layer; 320, insulating spacer ring; 330, collecting electrode; 331, intermediate collecting part; 332, edge collecting part; 333, inner protective ring; 334, inner protective ring extension; 335, outer protective ring; 336, second bearing layer; 337, connecting part; 338, protective electrode plate; 400, mounting bracket; 500, partition part; 501, partition; 502, second thin window; 503, partition through hole; 600, sealing component; 700, mounting part; 800, vacuum through-wall plug; 900, extraction pipe. Detailed Implementation

[0041] To enable those skilled in the art to better understand the technical solutions of the present invention, the technical solutions of the present invention will be clearly and completely described below with reference to the accompanying drawings. Based on the embodiments in this application, other similar embodiments obtained by those skilled in the art without creative effort should all fall within the scope of protection of this application. Furthermore, directional terms mentioned in the following embodiments, such as "up," "down," "left," and "right," are only for reference to the directions in the accompanying drawings; therefore, the directional terms used are for illustrative purposes and not for limiting the invention.

[0042] According to an embodiment of the present invention, a low-pressure ionization chamber is provided, as shown in Figures 1 and 2. The chamber includes a housing 100, and a sealed chamber is formed inside the housing 100. The sealed chamber forms a low-pressure environment below the standard atmospheric pressure, which promotes the recombination ratio of positive and negative charges to be no greater than 0.01, thereby improving the collection efficiency of positive and negative charges.

[0043] It should be noted that by reducing the air pressure inside the sealed chamber to form a low-pressure ionization chamber, ion recombination can be avoided or weakened, thereby improving the accuracy of radiotherapy beam dose rate monitoring. This allows the ionization chamber to be used for both conventional radiotherapy dose rate monitoring and ultra-high dose rate monitoring, thus expanding its application scope.

[0044] From a working principle perspective, the ionization chamber's operation can be divided into three stages: "radiation ionization + charge collection + acquisition and analysis." The saturation problem under FLASH conditions primarily occurs during the "charge collection" stage. The pressure on charge collection can be reduced by decreasing the amount of charge generated by radiation ionization, while simultaneously improving ion collection performance, thereby smoothing the charge collection process and increasing the collection efficiency of both positive and negative charges. (Positive and negative charge collection efficiency) Where, k s This is the ionization recombination correction factor. Ideally, η should reach 100%, i.e. Where A is the gas factor of the sealed chamber, Q0 is the total charge in the sealed chamber, and p is the proportion of electrons directly collected (the proportion of electrons directly collected without forming negative ions). However, in reality, some charge will always be canceled out by recombination. Therefore, the ionization chamber aims to achieve the second term of the formula (defined as the recombination ratio of positive and negative charges). Small enough, clinically expected to be less than 1%, i.e., R ≤ 0.01.

[0045] again Where α is the recombination rate constant, d is the distance between the high-voltage electrode and the collecting electrode in the sealed cavity, e is the elementary charge, and μ is the recombination rate constant. + and μ - U is the mobility of positive and negative ions, V is the volume of the sealed chamber, and U is the volume of the sealed chamber. c It is the voltage between the high-voltage electrode and the collecting electrode inside the sealed chamber.

[0046] Positive and negative ion mobility (μ) + and μ - R is basically inversely proportional to air pressure and directly proportional to electric field, therefore R can be further approximated as:

[0047] Where, μ +0 and μ -0 denoted as positive and negative ion mobility constants of a specific gas under a specific gas pressure and electric field strength, respectively, where P is the gas pressure of the sealed chamber.

[0048] Furthermore, according to Thomson's (1924) research, below 10¹³ hPa (standard atmospheric pressure), the recombination rate constant α is proportional to the atmospheric pressure. Simultaneously, it is also approximately proportional to the electric field E, so R can be further rewritten as:

[0049] Here, α0 is the ion recombination coefficient under a specific gas pressure and electric field strength, which is a constant.

[0050] Furthermore, since Q0 is the total charge within the sealed cavity, which is equal to the charge density ρ multiplied by the volume V, therefore... Approximately, the charge density ρ is proportional to the single-pulse dose D and gas pressure P of the radiotherapy beam, therefore Wherein, ρ0 is the charge density generated under a specific pulse dose and pressure, and is a constant.

[0051] After approximating the proportion of free electrons to a constant, and then consolidating all constants into a single constant k, we have: in, Therefore, when the single-pulse dose D of the radiotherapy beam is large, in order for R to be sufficiently small, d needs to be as small as possible, P needs to be sufficiently small, and U needs to be sufficiently large. c .

[0052] The above analysis process employed numerous approximations; therefore, the aforementioned relationships require meticulous verification. To verify the accuracy of the formulas, the inventors introduced a computer numerical calculation method for verification.

[0053] Verification 1: The relationship between the recombination ratio R of positive and negative charges and the gas pressure P

[0054] The R-P relationship was verified under three conditions. The conclusion is that R is basically proportional to the cube of P, but this relationship is not a simple proportional relationship when the recombination of positive and negative charges is more severe than R (e.g., R is much greater than 1).

[0055] 1. Under the conditions of a single-pulse dose of 82 Gy, an electrode spacing of 0.1 mm, and a voltage of -40 V, the relationship between R and gas pressure P as the gas pressure increases from 0.001 atm to 0.5 atm is shown in Figure 12. The fitting formula is R = 9.27P. 3 r 2 =1, where r is the fitting coefficient.

[0056] 2. Under the conditions of a single-pulse dose of 1.1 Gy, an inter-electrode spacing of 0.1 mm, and a voltage of -40 V, the relationship between R and gas pressure P as the gas pressure increases from 0.001 atm to 0.5 atm is shown in Figure 13. The fitting formula is R = 0.1256P. 3 r 2 =1, where r is the fitting coefficient.

[0057] Verification 2: The relationship between the recombination ratio R of positive and negative charges and voltage U c Relationship

[0058] RU was verified under two conditions. c The relationship is that R is basically consistent with U. c Inversely proportional, the index fluctuates around -1.

[0059] 1. Under conditions of a single-pulse dose of 82 Gy, an electrode spacing of 2 mm, a gas pressure of 0.005 atm, and a voltage ranging from 30 V to 400 V, the recombination ratio R and the voltage U...c The relationship is shown in Figure 14, and the fitting formula is R = 34.66U. c -1.161 r 2 =0.9979, where r is the fitting coefficient.

[0060] 2. Under the conditions of a single-pulse dose of 1.11 Gy, an electrode spacing of 2 mm, a gas pressure of 0.005 atm, and a voltage ranging from 30 V to 500 V, the recombination ratio R and the voltage U... c The relationship is shown in Figure 15, and the fitting formula is R = 0.0328U. c -0.873 r 2 =0.9961, where r is the fitting coefficient.

[0061] Verification 3: The relationship between the recombination ratio R of positive and negative charges and the interelectrode spacing d

[0062] The Rd relationship was verified under one condition, and the conclusion was that the coincidence ratio R is basically proportional to the square of d.

[0063] 1. Under the conditions of a single-pulse dose of 82 Gy, voltage of -400 V, and air pressure of 0.003 atm, the relationship between the composite ratio R and the spacing d when the spacing d ranges from 0.1 mm to 10 mm is shown in Figure 16. The fitting formula is R = 0.002668 × d. 2 r 2 =0.9996, where r is the fitting coefficient.

[0064] Therefore, based on the above verification results, the above relationship can be considered accurate.

[0065] Based on the above relationship, assuming R is fixed at a certain value, such as 0.01 (1% recombination), then if the dose rate is increased tenfold, the same recombination rate can be achieved through three methods: 1. Reducing the electrode spacing d to approximately 1 / 3.3 of its previous value; 2. Reducing the gas pressure P to approximately 1 / 2.15 of its previous value; 3. Increasing the voltage U... c Increased to 10 times the previous amount.

[0066] While reducing the electrode spacing can theoretically significantly decrease the ion recombination ratio, for extremely high single-pulse doses received by the monitor (e.g., close to 100 Gy / pulse), computer simulations at ambient pressure show that the electrode spacing needs to be kept below 100 μm to achieve a recombination ratio close to 1%. This places extremely high and difficult-to-achieve requirements on the flatness of the electrodes in large-area flat-plate ionization chambers. Furthermore, even minor temperature fluctuations, vibrations, and material aging during operation can cause significant changes in the space between the electrodes, thus significantly affecting the relationship between the collected charge and dose (nC / Gy) and resulting in poor stability of the ionization chamber. Therefore, while the electrode spacing d can be appropriately reduced for low single-pulse doses, it is not advisable to reduce the recombination rate in large-area flat-plate ionization chambers by simply adjusting the electrode spacing d alone, especially for extremely high single-pulse doses. This is because at extremely high single-pulse voltages, the required spacing d is extremely small (e.g., less than 50 micrometers), which places extremely high demands on the flatness of the electrode substrate, making engineering extremely difficult. Furthermore, the electrode substrate may undergo slight deformation under the influence of environmental temperature changes, aging, and other factors, leading to significant changes in the spacing d, thereby significantly altering the monitor's detection efficiency. By significantly increasing the voltage U... c While methods can reduce the ion recombination ratio, excessively high voltage and strong electric fields can cause collisional ionization, increasing the number of ions and electrons. This leads to a sharp increase in the output signal with increasing voltage, making the relationship between the output signal and dose complex and highly unstable. Therefore, the applicability of this method of reducing the ion recombination ratio by significantly increasing voltage is very limited. In summary, reducing the gas pressure is preferable, supplemented by selecting appropriate electrode spacing d and voltage U. c The method is the optimal solution for reducing the ion recombination ratio and is highly feasible.

[0067] It is worth noting that the choice of gas pressure needs to balance two aspects: minimizing ion recombination effect and signal strength. To minimize ion recombination, the gas pressure should be reduced as much as possible; however, if the gas pressure drops to a certain level, the current signal collected by the ionization chamber will become very small, the signal-to-noise ratio will decrease, and it will be more susceptible to noise. Furthermore, in actual treatment, the single-pulse dose range is large, requiring consideration of both ion recombination at high dose rates and signal-to-noise ratio at low dose rates. This ensures that the ionization chamber can be used for both conventional radiotherapy dose rate monitoring and ultra-high dose rate monitoring, thus expanding its application range.

[0068] In the low-pressure ionization chamber of this embodiment, the gas pressure in the sealed chamber ranges from 0.001 atm to 0.5 atm, depending on the single-pulse dose of the radiotherapy beam. The distance between the high-voltage electrode and the collecting electrode in the sealed chamber ranges from 0.1 mm to 10 mm, and the voltage U between the high-voltage electrode and the collecting electrode in the sealed chamber... c The voltage range is from -400V to 400V.

[0069] Meanwhile, the inventors verified the above numerical range, and the verification results are shown below:

[0070] Table 1:

[0071] Table 1 shows that: [The following is a possible interpretation based on the spacing d and voltage U] c And given the same single-pulse dose, simply adjusting the gas pressure P can meet the clinical expectation k. s ≤1.01; When simply adjusting the air pressure P cannot meet clinical expectations, the spacing d and voltage U can be adjusted. c And at least one of the single-pulse doses, to meet clinical expectations.

[0072] Specifically, with a spacing d of 10mm and a voltage U c At -40V and a single-pulse dose of 1.11 Gy / pulse, with a gas pressure P of 0.001 atm, k s =1.0022≤1.01; when the air pressure P is 0.01atm, k s =1.4927 > 1.01, which does not meet clinical expectations. Therefore, after adjusting the spacing d to 0.1 mm, k s =1.0000, meeting clinical expectations.

[0073] Table 2:

[0074] Table 2 shows that: [The following is a possible interpretation based on the spacing d and voltage U] c Under the premise of the same gas pressure P, a single adjustment of the single-pulse dose can meet the clinical expectation k. s ≤1.01; however, as the air pressure P increases, the evaluation index k... s The pressure gradually increases. Therefore, when using the same single-pulse dose, the higher the pressure P, the more difficult it is to guarantee that clinical expectations are met. In this case, the pressure P can be lowered to meet clinical expectations.

[0075] Specifically, with a spacing d of 1 mm and a voltage U c At 80V and a single-pulse dose of 1Gy / pulse, with a gas pressure P of 0.5atm, k s =1.9724 > 1.01, which does not meet clinical expectations. Therefore, when the air pressure P is adjusted to 0.001 atm, k s =1.0000≤1.01, which meets clinical expectations.

[0076] Table 3:

[0077] Table 3 shows that:

[0078] 1. Under the premise that the spacing d, air pressure P, and single pulse dose are the same, the single adjustment voltage U c It can meet clinical expectations. s ≤1.01, however, different single-pulse doses use the same spacing d, gas pressure P, and voltage U. c At this time, clinical expectations cannot be guaranteed to be met, and the spacing d and / or air pressure P can be reduced.

[0079] Specifically, when the spacing d is 2 mm, the gas pressure P is 0.005 atm, and the single-pulse dose is 1.11 Gy / pulse, the voltage U c When the voltage varies from -400V to 400V, all evaluation indicators meet clinical expectations; when the spacing d is 2mm, the air pressure P is 0.005atm, and the voltage U... c At -400V, with a single pulse dose of 82Gy / pulse, k s =1.0241 > 1.01, which does not meet clinical expectations. In this case, please refer to Table 4. Reducing the air pressure P (0.003 atm) and changing the spacing d from 0.1 mm to 1 mm will meet clinical expectations. s ≤1.01.

[0080] 2. Under the premise that the spacing d, air pressure P, and single-pulse dose are the same, the voltage U c When the absolute values ​​are the same, the evaluation indicators are also the same.

[0081] Table 4:

[0082] Table 4 shows that: at voltage U c Under the premise that the gas pressure P and the single-pulse dose are the same, a single adjustment interval d can meet the clinical expectation k. s ≤1.01, however, as the interval d increases to the point where the evaluation index fails to meet clinical expectations, the air pressure P and voltage U can be adjusted. c And at least one of the single-pulse doses, to meet clinical expectations.

[0083] Specifically, voltage U c At 400V, a pressure P of 0.003 atm, and a single-pulse dose of 82 Gy / pulse, the evaluation indicators met clinical expectations when the distance d varied from 0.1 mm to 1 mm; when the distance d was 5 mm, k s =1.1064 > 1.01, which does not meet clinical expectations. Therefore, the single-pulse dose is 1.11 Gy / pulse, kJ / kJ. s =1.0003≤1.01, which meets clinical expectations.

[0084] In summary, the collection efficiency of positive and negative charges involves multiple variables (voltage, gas pressure, spacing, and single-pulse dose). These variables interact with each other, and adjusting a single variable often fails to achieve the desired result. Therefore, it is necessary to comprehensively adjust multiple variables to obtain the optimal combination of variables to meet clinical expectations.

[0085] This technical solution is further configured, as shown in Figures 1 to 3, with a first thin window 112 provided on the housing 100 for the radiotherapy beam to pass through, and a sealed chamber formed inside the housing 100 for accommodating the electrode unit 300.

[0086] From a physical perspective, the interior of the housing 100 needs to form a hollow, airtight space to facilitate the operation of the electrode unit 300. Simultaneously, it needs to be matched and installed with components such as the treatment head of a medical linear accelerator via a mounting bracket 400. The working gas within the sealed chamber can be air or other gases. Specifically, the housing 100 includes a detachably connected upper housing 110 and a lower housing 120, which interlock to form a sealed chamber. Both the upper housing 110 and the lower housing 120 are provided with a first thin window 112 for the radiotherapy beam to pass through. From a material perspective, the housing 100 needs to withstand pressure differences for extended periods without significant deformation, and should not release additional gas at low pressures. From an electrical perspective, the housing 100 needs to shield against external electromagnetic interference affecting the collecting electrode signal in the electrode unit 300; therefore, the housing 100 needs to be conductive and grounded. In summary, the housing 100 must form a hollow, sealed space within which the electrode unit 300 operates. It must be able to withstand pressure differences between the inside and outside for extended periods without releasing additional gas, and it must also be conductive and grounded to provide adequate electromagnetic shielding. Therefore, the housing 100 is preferably made of steel, or other materials that meet the strength and electromagnetic shielding requirements.

[0087] In the low-pressure ionization chamber of this embodiment, please refer to Figures 1 to 3. The housing 100 is provided with a mounting hole 111, and the first thin window 112 is disposed at the mounting hole 111. The first thin window 112 and the mounting hole 111 should be airtight.

[0088] It should be noted that the first thin window 112 can be installed at the mounting hole 111 by welding. The mounting hole 111 is mainly to facilitate the passage of the radiotherapy beam, and the first thin window 112 has good penetration of the radiotherapy beam. From a physical point of view, the first thin window 112 can withstand the pressure difference and is as thin as possible. From a material point of view, the first thin window 112 needs sufficient structural strength to withstand the pressure difference for a long time without breaking, and should not release additional gas at low pressures, and should be able to connect with the shell 100 to form a sealed chamber. From an electrical point of view, for good electromagnetic shielding, the first thin window 112 also needs to be conductive and grounded together with the shell 100. In summary, the first thin window 112 is made of a material with a compressive strength greater than 1 atm and a high radiation transmittance (not less than 60%), generally titanium, beryllium, etc.

[0089] Preferably, the first thin window 112 is made of titanium film. The use of titanium film is based on two main factors: first, titanium film has sufficient strength to maintain the integrity of the structure under internal and external pressure differences; second, it can form an ultra-thin window, thereby minimizing the impact on the beam.

[0090] This technical solution is further configured such that, depending on the gas pressure of the sealed chamber, and when the beam velocity is a low-penetration beam, the thickness of the first thin window 112 is 50 μm to 100 μm. And, when the beam velocity is a high-penetration beam, the thickness of the first thin window 112 is 50 μm to 800 μm. For example, kV-level X-rays and MeV-level electron beams are low-penetration beams, while MV-level X-rays, 100 MeV-level protons, and 100 MeV-level electrons are high-penetration beams.

[0091] Specifically, the inventors verified the thickness of the first thin window 112 at an air pressure of 0.001 atm, with thicknesses of 30 μm and 50 μm respectively. The verification results are shown in Table 6.

[0092] Table 6:

[0093] Table 6 shows that at a pressure of 0.001 atm, the 50 μm first thin window maintains the structural integrity of the ionization chamber and preserves the low-pressure state. Therefore, under the same pressure conditions, a thickened first thin window also maintains the structural integrity of the ionization chamber and ensures that the low-pressure state of the ionization chamber is not compromised. Similarly, in environments with increased pressure, i.e., in smaller pressure differential environments, such as 0.5 atm, the integrity of the ionization chamber is undoubtedly maintained, ensuring that the low-pressure environment is not disrupted.

[0094] Specifically, the inventors verified the effect of first thin windows of different thicknesses on MeV-level electron beams (i.e., low-penetration beams), and the verification results are shown in Table 7:

[0095] The first and second thin windows are titanium films, the electron beam energy is 9.8 MeV, the first and second thin windows are perpendicular to the electron beam direction, and the beam spatial distribution is dotted. A water layer is placed at an SSD (fixed source-skin distance) of about 60 cm, the deposition energy at 2 cm underwater is counted, and the data is normalized and compared, as shown in Figure 17.

[0096] Table 7:

[0097] Table 7 and Figure 17 show that the absorbed dose is directly related to the thickness of the titanium film; the thicker the titanium film, the lower the received dose. At a thickness of 50 μm, the absorbed dose decreases by approximately 20% compared to a thickness of 0 μm; when the thickness exceeds 100 μm, the absorbed dose decreases by approximately 40% compared to a thickness of 0 μm. Therefore, thinner first and second thin windows are more beneficial to dose delivery efficiency. However, to maintain sufficient mechanical strength to counteract pressure differences, the inventors have preferred a window thickness of 50 μm-100 μm for the radiotherapy beam, ensuring a low-penetration environment while significantly reducing beam obstruction and scattering, thus improving the transmittance of the radiotherapy beam.

[0098] Specifically, the inventors verified the effect of first thin windows of different thicknesses on MV-level X-rays (i.e., high-penetration beams), and the verification results are shown in Tables 8 and 9:

[0099] The first thin window is a titanium film, using MV-level X-rays of 6MV or 10MV. The first thin window is perpendicular to the X-ray direction. A water layer is placed at approximately 100cm below the SSD (fixed source-skin distance). The radiation dose at 5cm underwater is counted and normalized for comparison.

[0100] Table 8:

[0101] Table 9:

[0102] Tables 8 and 9 show that the absorbed dose is directly related to the thickness of the titanium film and the energy level of the X-ray source. The lower the energy level of the X-ray source and the thicker the titanium film, the lower the received dose. For example, when the X-ray source energy level is 6 MV and the thickness is 50 μm, the absorbed dose decreases by approximately 0.3% compared to a thickness of 0 μm; when the thickness exceeds 800 μm, the absorbed dose decreases by approximately 6% compared to a thickness of 0 μm. When the X-ray source energy level is 10 MV and the thickness is 50 μm, the absorbed dose decreases by approximately 0.1% compared to a thickness of 0 μm; when the thickness exceeds 800 μm, the absorbed dose decreases by approximately 5% compared to a thickness of 0 μm. Therefore, the thinner the first and second thin windows are, the better the dose delivery efficiency. However, in order to maintain sufficient mechanical strength to counteract the pressure difference, the inventors have preferred a thin window thickness of 50um-800μm for the radiotherapy beam as a high-penetration beam. This ensures a low-pressure environment while significantly reducing the obstruction and scattering of the beam, thereby improving the transmittance of the radiotherapy beam.

[0103] In the low-pressure ionization chamber of this embodiment, as shown in Figures 1 to 3, traditional valve installations cannot guarantee the stability of the low-pressure environment within the sealed chamber. Therefore, a vacuum through-wall plug 800 is provided on the housing 100, and all wiring of the electrode unit 300 is led out through the vacuum through-wall plug 800. A vacuum extraction pipe 900 for evacuation is also provided on the housing 100. To improve airtightness, the terminals in the vacuum through-wall plug 800 are sealed using glass melting or ceramic sintering. The metal part of the vacuum through-wall plug 800 is welded to the housing 100. The airtightness between the extraction pipe 900 and the housing 100 is ensured by the following method: after the gas pressure in the sealed chamber of the ionization chamber reaches the required level, the extraction pipe 900 is vacuum-clamped and then sealed by secondary welding.

[0104] In the low-pressure ionization chamber of this embodiment, as shown in Figures 1 to 3, a mounting part 700 is provided inside the housing 100, and the mounting part 700 is detachably connected to the electrode unit 300.

[0105] In practical applications, the mounting part 700 is a protrusion protruding into the sealed cavity. In order to improve the stability of the electrode unit 300, multiple protrusions are evenly arranged around the mounting hole 111. Both the protrusion and the electrode unit 300 are provided with connection holes, and connectors are provided in the connection holes.

[0106] In the low-pressure ionization chamber of this embodiment, as shown in Figures 1 to 4, the electrode unit 300 includes a high-voltage electrode 310, an insulating ring 320, and a collecting electrode 330 stacked together. The insulating ring 320 has a through-hole for the radiotherapy beam to pass through. Both the high-voltage electrode 310 and the collecting electrode 330 are sheet electrodes. From a physical perspective, sheet electrodes need to be conductive, have sufficient structural strength, be thin enough, and have adequate radiation resistance. Therefore, the substrate of the sheet electrode is made of a high-strength, ultra-thin insulating material, including but not limited to polyimide and polyester, with a substrate thickness of 20 μm to 50 μm. In this embodiment, the substrate of the sheet electrode is fabricated by copper plating on a polyimide (PI) film. The PI film has good radiation resistance and high insulation; other materials, such as plexiglass films, do not have ideal radiation resistance under ultra-high dose rate beam irradiation. Preferably, the thickness of the insulating ring 320 is 0.5-2 mm, preferably 1 mm.

[0107] In the low-pressure ionization chamber of this embodiment, please refer to Figures 1 to 5. The high-voltage electrode 310 includes a first support layer 311 and a second plating layer 313 located on the first support layer 311 and close to the insulating ring 320.

[0108] In the low-pressure ionization chamber of this embodiment, as shown in Figures 1 to 5, the high-voltage electrode 310 further includes a first plating layer 312 located on the first bearing layer 311 and on the side away from the insulating ring 320. The area of ​​the first plating layer 312 is larger than the area of ​​the second plating layer 313.

[0109] It should be noted that the first carrier layer 311 serves as a substrate, using a PI film. Copper plating is applied to both sides of the PI film to form a first plating layer 312 and a second plating layer 313. The area of ​​the second plating layer 313 is smaller than that of the first plating layer 312 to ensure the uniformity of the electric field within the area of ​​the second plating layer 313. In use, the first plating layer 312 is grounded, and the second plating layer 313 is connected to a high voltage. The first plating layer 312 has an electromagnetic shielding function, preventing other ions from being attracted. Preferably, the thickness of the first carrier layer 311 is 250 μm, and the thicknesses of the first plating layer 312 and the second plating layer 313 are 15 μm. Of course, if the environmental electromagnetic influence is minimal, the first plating layer 312 may be omitted.

[0110] In the low-pressure ionization chamber of this embodiment, please refer to Figures 1 to 6. The collecting electrode 330 includes a plurality of mutually insulated collecting parts. The collecting part includes a central collecting part 331 and a plurality of edge collecting parts 332 symmetrically arranged around the central collecting part 331. A protective part is provided around the collecting part. The protective part includes an inner protective ring 333 and an outer protective ring 335. The inner protective ring 333 and the outer protective ring 335 are concentric and spaced apart. The pins of the central collecting part and the pins of the edge collecting parts are located in the interval between the inner protective ring 333 and the outer protective ring 335.

[0111] Specifically, the central collecting section 331 is located at the center of the collecting section, and multiple edge collecting sections 332 are symmetrically arranged around the central collecting section 331 on a plane. The central collecting section 331 is separated from the edge collecting sections 332, and adjacent edge collecting sections 332 are also separated. Preferably, the central collecting section 331 is circular, and the edge collecting sections 332 are arc-shaped. Preferably, the gaps between adjacent edge collecting sections 332 are equal.

[0112] It should be noted that by setting the intermediate collection section 331 and the edge collection section 332, the radiotherapy beam signal can be collected in sections, and the dose rate and beam symmetry and asymmetry can be monitored.

[0113] Specifically, the inner protective ring 333 and the outer protective ring 335 are connected by multiple connecting portions 337. These connecting portions 337 divide the accommodating space between the inner and outer protective rings 333 and 335 into multiple independent sub-accommodating spaces. The intermediate collecting portion pin and the edge collecting portion pin are located in different sub-accommodating spaces. The protective portion also includes an inner protective ring extension 334 extending between adjacent edge collecting portions 332, and the inner protective ring extension 334 is connected to the inner protective ring 333.

[0114] It should be noted that by separating the multiple edge collection units 332 through the protection unit, the mutual interference between the signals of each channel collection unit is avoided, and the edge distortion effect of the electric field inside the confined space is reduced, thereby improving the uniformity of the electric field and the stability of the output signal of the ionization chamber. At the same time, the outer protection ring 335 also facilitates grounding.

[0115] In the low-pressure ionization chamber of this embodiment, as shown in Figures 1 to 6, the collecting electrode 330 further includes a second support layer 336, and the collecting part is disposed on the second support layer 336 and close to the insulating spacer ring 320.

[0116] It should be noted that the second carrier layer 336 is made of PI film, and the middle collection part 331, the edge collection part 332, the inner protective ring 333, the outer protective ring 335, the inner protective ring extension part 334, and the connecting part 337 are all formed by copper plating on the PI film and surface gold immersion.

[0117] In the low-pressure ionization chamber of this embodiment, as shown in Figures 1 to 6, the protection unit further includes a protective electrode 338, which is connected to the inner protective ring 333 and the outer protective ring 335. The protective electrode 338 is located on the other side of the second support layer 336.

[0118] In the low-pressure ionization chamber of this embodiment, referring to Figures 1 to 7, the sealed chamber is divided into an upper sealed chamber and a lower sealed chamber by a partition 500. A second thin window 502 for the radiotherapy beam to pass through is provided on the partition 500. The electrode unit 300 is provided inside both the upper and lower sealed chambers. The second thin window 502 and the first thin window 112 are made of the same material. Depending on the air pressure of the sealed chamber, and when the beam velocity is a low-penetration beam, the thickness of the second thin window 502 and the first thin window 112 is 50 μm to 100 μm. And, when the beam velocity is a high-penetration beam, the thickness of the second thin window 502 and the first thin window 112 is 50 μm to 800 μm. For example, kV-level X-rays and MeV-level electron beams are low-penetration beams, while MV-level X-rays, 100 MeV-level protons, and 100 MeV-level electrons are high-penetration beams.

[0119] It should be noted that the partition section 500 includes a partition 501 and a second thin window 502 disposed on the partition 501. To facilitate the passage of the radiotherapy beam, a partition through hole 503 is provided on the partition 501. Similarly, the second thin window 502 is welded to the partition through hole 503. The second thin window 502 is made of the same material as the first thin window 112. Meanwhile, a sealing component 600 for knife-edge sealing is provided at the junction of the partition 501 with the upper housing 110 and the lower housing 120. Preferably, the sealing component 600 is a copper sealing ring.

[0120] According to the requirements for radiotherapy dose monitoring systems in the national standard (GB15213-2016), the dose linearity and repeatability of this low-pressure ionization chamber were tested. The dose linearity test results are shown in Figure 8. The test conditions for dose linearity in Figure 8 were: ionization chamber pressure P = 300 Pa and voltage U... c =-40V, spacing d=1mm, a scintillator detector was placed at a considerable distance from the rear end of the ionization chamber to measure the relative value of the absorbed dose. As shown in Figure 8, for the low-pressure ionization chamber, a linear relationship was observed between the absorbed dose and the measured signal from the direct ionization chamber, r 2 >0.999, which meets the national standard requirements.

[0121] The dose repeatability measurement results are shown in Figure 9. Figure 9 shows the relationship between the monitor reading and the ratio of the scintillator absorbed dose (normalized to the average of each group) and the number of measurements. The dashed line in Figure 9 represents ±5‰. As can be seen from Figure 9, a total of 5 groups of different macropulse widths and repetition rates were selected for dose repeatability measurement, with 10 measurements in each group. The repeatability fluctuation under the 5 conditions did not exceed 5‰, all meeting the national standard requirements.

[0122] Figures 10 and 11 show the pulse signal shapes measured in the low-pressure ionization chamber under different pulse widths. Figures 10 and 11 correspond to high-pressure U... c The pulse signals measured at 40V and 80V had pulse widths ranging from 0.5μs to 3.5μs, corresponding to single-pulse doses at the monitor ranging from approximately 12Gy / pulse to 82Gy / pulse. The waveforms show that under these conditions, the monitor signal amplitude is independent of the pulse duration (corresponding to the single-pulse dose), meaning the collection efficiency is unaffected by the single-pulse dose. This characteristic effectively ensures the consistency of monitor detection efficiency under different single-pulse doses, thereby guaranteeing the accuracy of dose and dose rate monitoring values.

[0123] According to an embodiment of the present invention, a design method for a low-pressure ionization chamber is provided. Based on the single-pulse dose of the radiotherapy beam, the air pressure of the sealed chamber, the distance between the high-pressure electrode and the collecting electrode, and the input voltage of the high-pressure electrode are adjusted until the recombination ratio of positive and negative charges is not greater than 0.01, wherein the air pressure of the sealed chamber is lower than the standard atmospheric pressure.

[0124] According to an embodiment of the present invention, a medical linear accelerator is provided, as shown in Figure 1, which includes the low-pressure ionization chamber described above. The low-pressure ionization chamber is installed in conjunction with the treatment head of the medical linear accelerator via a mounting bracket 400.

[0125] The present invention has been described in detail above. The above description is only a preferred embodiment of the present invention and should not be construed as limiting the scope of the present invention. All equivalent changes and modifications made in accordance with the scope of this application should still fall within the scope of the present invention.

Claims

1. A low-pressure ionization chamber, comprising a shell, wherein a sealed cavity is formed inside the shell, characterized in that, The sealed chamber creates a low-pressure environment below standard atmospheric pressure.

2. The low-pressure ionization chamber according to claim 1, characterized in that, The distance d between the high-voltage electrode and the collecting electrode in the ionization chamber, the single-pulse dose D of the radiotherapy beam, the gas pressure P in the sealed chamber, and the voltage U between the high-voltage electrode and the collecting electrode in the sealed chamber. c Should meet: Where R is the recombination ratio of positive and negative charges; k is a constant; α0 is the ion recombination coefficient under a specific gas pressure and electric field strength, α0 is a constant; ρ0 is the charge density generated under a specific intrapulse dose and gas pressure, ρ0 is a constant; p is the direct electron collection ratio; e is the elementary charge; μ +0 and μ -0 These represent the positive and negative ion mobility constants of a specific gas under a specific gas pressure and electric field strength, respectively.

3. A low-pressure ionization chamber according to claim 1 or 2, characterized in that, Depending on the single-pulse dose of the radiotherapy beam, the air pressure in the sealed chamber ranges from 0.001 atm to 0.5 atm, the distance between the high-voltage electrode and the collecting electrode within the sealed chamber ranges from 0.1 mm to 10 mm, and the voltage U between the high-voltage electrode and the collecting electrode within the sealed chamber... c The voltage range is from -400V to 400V.

4. A low-pressure ionization chamber according to claim 3, characterized in that, The housing is provided with a first thin window and a second thin window for the beam to pass through. Both the first thin window and the second thin window are made of a material with a compressive strength greater than 1 atm and high radiation transmittance.

5. A low-pressure ionization chamber according to claim 4, characterized in that, When the beam is a low-penetration beam, the thickness of the first thin window and the second thin window is 50 μm to 100 μm.

6. A low-pressure ionization chamber according to claim 4, characterized in that, When the beam is a high-penetration beam, the thickness of the first thin window and the second thin window is 50 μm to 800 μm.

7. A low-pressure ionization chamber according to claim 2, characterized in that, The high-voltage electrode, the insulating ring, and the collecting electrode are stacked in sequence. The insulating ring has a through hole for the beam to pass through. The collecting electrode includes multiple mutually insulated collecting parts, and a protective part is provided around the collecting part.

8. A low-pressure ionization chamber according to claim 7, characterized in that, The collecting section includes a central collecting section and a plurality of edge collecting sections symmetrically arranged around the central collecting section. The protective section includes an inner protective ring and an inner protective ring extension. The inner protective ring is disposed on the side of the edge collecting section away from the central collecting section. The inner protective ring extension is connected to the inner protective ring and extends to the space between adjacent edge collecting sections.

9. A low-pressure ionization chamber according to claim 8, characterized in that, The protection unit further includes: an outer protection ring, the inner protection ring and the outer protection ring being concentric and spaced apart, and the middle collection part pin and the edge collection part pin being located between the inner protection ring and the outer protection ring.

10. A low-pressure ionization chamber according to claim 9, characterized in that, The inner protective ring and the outer protective ring are connected by multiple connecting parts, which divide the accommodating space between the inner and outer protective rings into multiple independent sub-accommodating spaces. The middle collecting part pin and the edge collecting part pin are located in different sub-accommodating spaces.

11. A low-pressure ionization chamber according to claim 9, characterized in that, The protection part further includes: a protection electrode, which is connected to the inner protection ring and the outer protection ring respectively; the collecting electrode includes a second support layer, and the collecting part is disposed on the second support layer; the protection electrode is located on the side of the second support layer away from the collecting part.

12. A low-pressure ionization chamber according to claim 9, characterized in that, The high-voltage electrode includes a first bearing layer, a first plating layer, and a second plating layer. Both the first plating layer and the second plating layer are disposed on the first bearing layer. The first plating layer is located on the side of the first bearing layer away from the insulating spacer ring, and the second plating layer is located on the side of the first bearing layer closer to the insulating spacer ring.

13. A design method for a low-pressure ionization chamber according to any one of claims 1-12, characterized in that, Based on the single-pulse dose of the radiotherapy beam, the air pressure in the sealed chamber, the distance between the high-pressure electrode and the collecting electrode, and the input voltage of the high-pressure electrode are adjusted until the recombination ratio of positive and negative charges is no greater than 0.01, wherein the air pressure in the sealed chamber is lower than the standard atmospheric pressure.

14. A medical linear accelerator, characterized in that, It includes the low-pressure ionization chamber as described in any one of claims 1-12.

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