A low pressure ionization chamber, design method and medical linear accelerator

Abstract

The present invention relates to a low-pressure ionization chamber, a design method and a medical linear accelerator, and belongs to the technical field of radiotherapy dose monitoring. The low-pressure ionization chamber comprises a shell, a closed chamber is formed inside the shell, a low-pressure environment lower than the standard atmospheric pressure is formed inside the closed chamber, so that the ion recombination ratio of positive and negative charges is not greater than 0.01, and the collection efficiency of positive and negative charges is improved. The present invention can solve the technical problem in the prior art that the penetrating air ionization chamber is affected by ion recombination, resulting in inaccurate monitoring results at ultra-high dose rates.

Description

Technical Field

[0001] The invention belongs to the technical field of radiotherapy dose monitoring, and in particular relates to a low-pressure ionization chamber, a design method and a medical linear accelerator. Background Art

[0002] Ultra-high dose rate and ultra-short pulse are the core radiation characteristics of FLASH (flash) radiotherapy, but they are obstacles for existing conventional dose detection methods (especially detectors). At the level of physical principles, the ultra-strong radiation of FLASH makes the polarization and ion recombination effects of the probe significant and difficult to predict. The basic idea of ​​radiation dose detection is to act on the sensitive medium of the probe with ionizing radiation, convert the radiation dose signal into an electrical signal, then collect the electrical signal, and then amplify, read out and analyze it. Conventional radiotherapy monitor probes usually use a normal pressure penetrating air ionization chamber, whose internal air pressure is basically the same as the atmospheric pressure. Under FLASH conditions, a large amount of dose is deposited instantaneously, generating ultra-high density of positive and negative charge carriers. These carriers are very easy to meet and recombine during the migration and collection process, resulting in an unpredictable and significant decrease in charge collection efficiency. At the same time, a large amount of charge will cause severe electric field distortion and then aggravate the polarization effect. This ion migration and recombination process at the physical level is extremely complex and extremely sensitive to the internal structure and electric field of the ionization chamber, making it difficult to accurately predict and calculate, that is, we cannot eliminate this effect through theoretical corrections. Therefore, the atmospheric pressure penetrating air ionization chamber is not suitable for dose monitoring of ultra-high dose rate radiotherapy equipment.

[0003] In response to the problem that atmospheric pressure penetrating air ionization chambers are not suitable for ultra-high dose rate radiation fields, technicians in this field have proposed several different solutions, such as dual-beam transformers and diamond detectors. The dual-beam transformer uses a beam current transformer to measure the beam intensity without introducing a mass layer, but its disadvantage is that it can only measure the electron flux, but not the electron energy, beam shape and spatial distribution. Therefore, the beam current transformer monitor must be coupled with an ionization chamber or other imaging detector to obtain dose information and beam profile information. Diamond detectors have the advantages of high sensitivity, fast response, high stability and chemical stability. Their disadvantage is that the collection efficiency is unstable under ultra-high dose rate conditions, so the dose in the beam cannot be directly measured. At the same time, the preparation of large-area single-crystal diamond faces challenges, which limits the application of diamond detection arrays.

[0004] Therefore, the prior art needs to be further developed. Summary of the invention

[0005] In view of the various deficiencies in the prior art, a low-pressure ionization chamber, a design method and a medical linear accelerator are proposed to solve the technical problem that the atmospheric pressure penetrating air ionization chamber in the prior art is affected by ion recombination, resulting in inaccurate monitoring results under ultra-high dose rates. At the same time, it can solve the problem of high blocking and strong scattering of radiotherapy electron beams by the existing penetrating air ionization chamber.

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

[0007] The technical solution is further configured as follows: the distance d between the high-voltage electrode and the collector in the ionization chamber, the single pulse dose D of the radiotherapy beam, the air pressure P of the closed chamber, the voltage between the high-voltage electrode and the collector in the closed chamber Should meet: ;

[0008] in, is the ratio of positive and negative charged ions; , is a constant; is the ion recombination coefficient at a specific gas pressure and electric field strength, is a constant; is the charge density generated at a specific pulse dose and pressure, is a constant; is the direct collection ratio of electrons; is the elementary charge; and They represent the positive and negative ion mobility constants of a specific gas under a specific gas pressure and electric field strength.

[0009] The technical solution is further configured that, according to different single pulse doses of radiotherapy beams, the air pressure of the closed chamber is 0.001atm to 0.5atm, the distance between the high-voltage electrode and the collector in the closed chamber is 0.1mm to 10mm, and the voltage between the high-voltage electrode and the collector in the closed chamber is -400V to 400V.

[0010] The technical solution is further configured that the shell is provided with a first thin window and a second thin window for the beam to pass through, and the first thin window and the second thin window are both made of a material with a pressure bearing 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 as follows: the high-voltage pole, the insulating spacer ring and the collecting pole are stacked in sequence, the insulating spacer ring is provided with a through hole for the beam to pass through, the collecting pole includes a plurality of mutually insulated collecting parts, and a protective part is provided on the periphery of the collecting part.

[0014] The technical solution is further configured as follows: the collecting portion includes a middle collecting portion and a plurality of edge collecting portions symmetrically arranged around the middle collecting portion; the protecting portion includes an inner protecting ring and an inner protecting ring extension portion; the inner protecting ring is arranged on a side of the edge collecting portion away from the middle collecting portion; the inner protecting ring extension portion is connected to the inner protecting ring, and the inner protecting ring extension portion extends between adjacent edge collecting portions.

[0015] The technical solution is further configured as follows: 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 collecting part pins and the edge collecting part pins are located between the inner protection ring and the outer protection ring.

[0016] The technical solution is further configured such that the inner protective ring and the outer protective ring are connected via a plurality of connecting parts, and the plurality of connecting parts divide the accommodation space between the inner protective ring and the outer protective ring into a plurality of independent sub-accommodation spaces, and the middle collecting part pins and the edge collecting part pins are respectively located in different sub-accommodation spaces.

[0017] The technical solution is further configured as follows: the protection part also includes: a protection electrode plate, which is respectively connected to the inner protection ring and the outer protection ring; the collecting electrode includes a second bearing layer, and the collecting part is arranged on the second bearing layer; the protection electrode plate is located on a side of the second bearing layer away from the collecting part.

[0018] The technical solution is further configured as follows: the high-voltage pole includes a first bearing layer, a first coating layer and a second coating layer, the first coating layer and the second coating layer are both arranged on the first bearing layer, the first coating layer is located on the side of the first bearing layer away from the insulating spacer ring; the second coating layer is located on the side of the first bearing layer close to the insulating spacer ring.

[0019] In a second aspect, the present invention provides a design method for a low-pressure ionization chamber, which adjusts the air pressure of the closed chamber, the distance between the high-voltage electrode and the collecting electrode, and the input voltage of the high-voltage electrode based on the single pulse dose of the radiotherapy beam until the recombination ratio of positive and negative charged ions is no more than 0.01, wherein the air pressure of the closed chamber is lower than the standard atmospheric pressure.

[0020] In a third aspect, the present invention provides a medical linear accelerator comprising the above-mentioned low-pressure ionization chamber.

[0021] The beneficial effects of the present invention are:

[0022] By reducing the air pressure in the closed chamber to form a low-pressure ionization chamber, ion recombination can be avoided or weakened, and the monitoring accuracy of the dose rate and total dose of ultra-high dose rate radiotherapy can be improved. At the same time, by adjusting the air pressure in the closed chamber, the ionization chamber can be applied to both FLASH and CONV conditions, thereby expanding the scope of application. BRIEF DESCRIPTION OF THE DRAWINGS

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

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

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

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

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

[0028] Figure 6 Schematic diagram of a collecting electrode and a protective electrode in an embodiment of the present invention;

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

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

[0031] Fig. 9 Schematic diagram of the relationship between the ratio of the monitor reading and the scintillator absorbed dose (normalized to the average value of each group) and the number of measurements in an embodiment of the present invention;

[0032] Fig.10 1 is a schematic diagram of the shape of a pulse signal measured by a low-pressure ionization chamber when the high voltage is 40V under different pulse widths in an embodiment of the present invention;

[0033] Fig.11 1 is a schematic diagram of the pulse signal shape measured by the low-pressure ionization chamber when the high voltage is 80V under different pulse widths in an embodiment of the present invention;

[0034] Fig.12Schematic diagram of the relationship between the positive and negative charge ion recombination ratio R and the gas pressure P in an embodiment of the present invention;

[0035] Fig.13 Schematic diagram of the relationship between the positive and negative charge ion recombination ratio R and the gas pressure P in an embodiment of the present invention;

[0036] Fig.14 is the ion recombination ratio R of positive and negative charges and the voltage U in the embodiment of the present invention c Schematic diagram of the relationship between

[0037] Fig.15 is the ion recombination ratio R of positive and negative charges and the voltage U in the embodiment of the present invention c Schematic diagram of the relationship between

[0038] Fig.16 Schematic diagram of the relationship between the ion recombination ratio R of positive and negative charges and the inter-electrode distance d in an embodiment of the present invention;

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

[0040] In the accompanying drawings: 100, shell; 110, upper shell; 111, mounting hole; 112, first thin window; 120, lower shell; 300, electrode unit; 310, high voltage pole; 311, first bearing layer; 312, first coating; 313, second coating; 320, insulating spacer ring; 330, collecting pole; 331, middle 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 pole piece; 400, mounting bracket; 500, partition part; 501, partition; 502, second thin window; 503, partition through hole; 600, sealing component; 700, mounting part; 800, vacuum wall plug; 900, exhaust pipe. DETAILED DESCRIPTION

[0041] In order to enable those skilled in the art to better understand the technical solution of the present invention, the technical solution of the present invention is clearly and completely described below in conjunction with the accompanying drawings of the present invention. Based on the embodiments in this application, other similar embodiments obtained by ordinary technicians in this field without making creative work should all fall within the scope of protection of this application. In addition, the directional words mentioned in the following embodiments, such as "up", "down", "left", "right", etc., are only reference to the directions of the accompanying drawings. Therefore, the directional words used are used to illustrate rather than limit the invention.

[0042] According to an embodiment of the present invention, a low pressure ionization chamber is provided. Figure 1 and Figure 2, including a shell 100, a closed chamber is formed inside the shell 100, and a low-pressure environment lower than the standard atmospheric pressure is formed in the closed chamber, which makes the ion recombination ratio of positive and negative charges not 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 in the closed chamber to form a low-pressure ionization chamber, ion recombination can be avoided or weakened, and the monitoring accuracy of radiotherapy beam dose rate can be improved. The ionization chamber can be used for both conventional radiotherapy dose rate monitoring and ultra-high dose rate monitoring, thereby expanding the scope of application.

[0044] From the perspective of working principle, the working process of the ionization chamber can be divided into three stages: "radiation ionization + charge collection + collection analysis". Among them, the saturation problem under FLASH conditions mainly occurs in the "charge collection" stage. The pressure of charge collection can be reduced by reducing the amount of charge generated by radiation ionization, while improving the ion collection performance, thereby unblocking the charge collection process and improving the collection efficiency of positive and negative charges. Collection efficiency of positive and negative charges ,in, is the ionization recombination correction factor. Ideally, Should reach 100%, that is ,in, is the closed chamber gas factor, is the total charge in the closed chamber, is the direct electron collection ratio (the ratio of electrons that are directly collected without forming negative ions). However, in reality, some charges are always offset by recombination. Therefore, the ionization chamber pursues the second term of the formula (defined as the recombination ratio of positive and negative charges). ) is small enough, clinically expected to be less than 1%, that is, ≤0.01, .

[0045] again ,in, is the positive and negative charge recombination rate constant, is the distance between the high voltage electrode and the collector electrode in the closed chamber. is the elementary charge, and is the mobility of positive and negative ions, is the volume of the closed chamber, It is the voltage between the high voltage electrode and the collecting electrode in the closed chamber.

[0046] The mobility of positive and negative ions ( and ) is basically inversely proportional to the air pressure and directly proportional to the electric field, so It can be further approximated as:

[0047] ,in, and They represent the positive and negative ion mobility constants of a specific gas under a specific gas pressure and electric field strength, is the air pressure in the closed chamber.

[0048] Furthermore, according to Thomson (1924), below 1013 hPa (standard atmospheric pressure), the recombination rate constant of positive and negative charges is Proportional to the air pressure. At the same time, it is also approximately proportional to the electric field ,so Further rewritten as:

[0049] ,in, It is the ion recombination coefficient under a certain gas pressure and electric field strength, and is a constant.

[0050] Also, because is the total charge in the closed chamber, which is equal to the charge density Multiply by volume ,so , approximately, the charge density Proportional to the single pulse dose of the radiotherapy beam and air pressure ,therefore ,in, It is the charge density generated under a specific pulse dose and gas pressure, and is a constant.

[0051] After approximating the free electron ratio to a constant, all constants are concentrated into one constant , then ,in, , so when the single pulse dose of the radiotherapy beam When it is larger, Small enough, as small as possible , small enough , big enough .

[0052] A lot of approximations are used in the above analytical process, so the above relationship needs to be carefully verified. In order to verify the accuracy of the formula, the inventor introduced a computer numerical calculation method and verified it with numerical calculation.

[0053] Verification 1: Ion complex ratio of positive and negative charges With air pressure The relationship between

[0054] It was verified under three conditions. - The conclusion is Basically The cube of the positive and negative charges is proportional to Severe (such as When is much greater than 1), this relationship is not a simple proportional relationship.

[0055] 1. Single pulse dose 82Gy, inter-electrode spacing 0.1mm, voltage -40V, gas pressure from 0.001 atm to 0.5atm, With air pressure The relationship is as Fig.12 As shown, the fitting formula is , ,in, is the fitting coefficient.

[0056] 2. Single pulse dose 1.1 Gy, inter-electrode spacing 0.1 mm, voltage -40 V, gas pressure from 0.001 atm to 0.5 atm, With air pressure The relationship is as Fig.13 As shown, the fitting formula is , ,in, is the fitting coefficient.

[0057] Verification 2: Recombination ratio of positive and negative charges With voltage The relationship between

[0058] It was verified under two conditions. - The conclusion is consistent with Basically The index fluctuates around -1.

[0059] 1. Under the conditions of single pulse dose of 82Gy, inter-electrode spacing of 2mm, and gas pressure of 0.005 atm, when the voltage ranges from 30 V to 400 V, the composite ratio With voltage The relationship is as Fig.14 As shown, the fitting formula is , ,in, is the fitting coefficient.

[0060] 2. Under the conditions of single pulse dose of 1.11 Gy, inter-electrode spacing of 2 mm, and pressure of 0.005 atm, when the voltage ranges from 30 V to 500 V, the composite ratio With voltage The relationship is as Fig.15 As shown, the fitting formula is , ,in, is the fitting coefficient.

[0061] Verification 3: Ion complex ratio of positive and negative charges Pole spacing The relationship between

[0062] Verified under one condition - The conclusion is that Basically is proportional to the square of .

[0063] 1. Under the conditions of single pulse dose of 82 Gy, voltage of -400 V, and pressure of 0.003 atm, when the spacing d ranges from 0.1 mm to 10 mm, the composite ratio With spacing The relationship is as Fig.16 As shown, the fitting formula is , ,in, is the fitting coefficient.

[0064] It can be seen that after the above verification conclusion, it can be considered that the above relationship is accurate.

[0065] Based on the above relationship, assuming that If the dose rate is set to a certain value, such as 0.01 (1% recombination), then if the dose rate is increased by 10 times, the same recombination rate can be achieved through three solutions: 1. Reduce to about 1 / 3.3 of the previous value; 2. Reduce the air pressure Reduce to about 1 / 2.15 of the previous value; 3. Increased to 10 times the previous amount.

[0066] In principle, reducing the electrode spacing can indeed significantly reduce the ion recombination ratio. However, when the single pulse dose received by the monitor is extremely high (for example, close to 100 Gy / pulse), computer simulation numerical calculations at normal pressure show that the electrode spacing needs to be kept below 100μm to roughly make the ion recombination ratio close to 1%. At this time, extremely high and difficult-to-achieve process requirements are imposed on the electrode flatness of large-area flat-plate ionization chambers. At the same time, any slight temperature, vibration, material aging and other factors during operation can cause significant changes in the space between electrodes, thereby significantly affecting the relationship between the charge collected by the ionization chamber and the dose (nC / Gy), making the ionization chamber less stable. Therefore, the electrode spacing can be appropriately reduced when the single pulse dose is low. However, for extremely high single pulse doses, it is not suitable to adjust the electrode spacing by a single adjustment in a large-area flat-plate ionization chamber. This is because when the single pulse is very high, the required spacing The electrode substrate is extremely small (e.g. less than 50 microns). In this case, the flatness of the electrode substrate is extremely high, and the engineering difficulty is extremely high. In addition, the electrode substrate may be slightly deformed under the influence of factors such as ambient temperature changes and aging, which may lead to the spacing The value of changes significantly, thus significantly changing the detection efficiency of the monitor. The ion recombination ratio can also be reduced by increasing the voltage. However, when the voltage is too high and the electric field is too strong, collision ionization will occur, which will increase the number of ions and electrons. The output signal will rise sharply with the increase of voltage. At this time, the relationship between the output signal and the dose becomes complicated and very unstable. Therefore, the scope of application of this method of reducing the ion recombination ratio by greatly increasing the voltage is also very limited. In general, it is preferred to reduce the gas pressure and assist in selecting the appropriate electrode spacing. and voltage The method is the optimal solution to reduce the ion recombination ratio and is highly feasible.

[0067] It is worth noting that the choice of gas pressure needs to take into account both "minimizing the ion recombination effect" and "signal strength". The ion recombination effect is small, so the gas pressure needs to be reduced as much as possible; however, when 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 become low, and it will be easily affected by noise. In addition, the dose span of a single pulse in actual treatment is large, and it is necessary to consider both the ion recombination problem at high dose rates and the signal-to-noise ratio problem at low dose rates, so that the ionization chamber can be applied to both conventional radiotherapy dose rate monitoring and ultra-high dose rate monitoring, expanding the scope of application.

[0068] In the low-pressure ionization chamber of this embodiment, according to the different single pulse doses of the radiotherapy beam, the air pressure of the closed chamber is 0.001atm to 0.5atm, the distance between the high-voltage electrode and the collector in the closed chamber is 0.1mm to 10mm, and the voltage between the high-voltage electrode and the collector in the closed chamber is -400V to 400V.

[0069] At the same time, the inventors verified the above numerical range, and the verification results are as follows:

[0070] Table 1:

[0071]

[0072] From Table 1, it can be concluded that: ,Voltage And under the premise of the same single pulse dose, a single adjustment of gas pressure , can meet clinical expectations ; When a single pressure regulator , when it cannot meet clinical expectations, the spacing can be adjusted ,Voltage and at least one of a single pulse dose to meet clinical expectations.

[0073] Specifically, the spacing 10mm, voltage When the single pulse dose is 1.11Gy / pulse, the pressure When it is 0.001atm, ; Air pressure When the temperature is 0.01 atm, , does not meet clinical expectations. In this case, the spacing After adjusting to 0.1mm, , meeting clinical expectations.

[0074] Table 2:

[0075]

[0076] From Table 2, it can be concluded that: ,Voltage and air pressure Under the same premise, a single pulse dose adjustment can meet clinical expectations. However, as the air pressure When the same single pulse dose is used, the evaluation index It can be seen that when the same single pulse dose is used, the air pressure The larger the pressure, the more difficult it is to meet clinical expectations. In this case, the air pressure can be reduced. , to meet clinical expectations.

[0077] Specifically, the spacing 1mm, voltage When the single pulse dose is 1Gy / pulse and the pressure is 80V, When the temperature is 0.5 atm, , does not meet clinical expectations. At this time, the air pressure When adjusted to 0.001atm, , meeting clinical expectations.

[0078] Table 3:

[0079]

[0080] From Table 3, we can conclude that:

[0081] 1. In the spacing , air pressure As for the single pulse dose, a single regulation voltage , can meet clinical expectations , however, different single pulse doses use the same spacing , air pressure And voltage When the clinical expectation cannot be met, the spacing can be reduced. and / or air pressure .

[0082] Specifically, the spacing 2mm, air pressure When the voltage is 0.005 atm and the single pulse dose is 1.11 Gy / pulse, When the voltage changes from -400V to 400V, the evaluation indicators meet clinical expectations; when the spacing 2mm, air pressure 0.005atm, voltage When the voltage is -400V and the single pulse dose is 82Gy / pulse, , does not meet clinical expectations. In this case, please refer to Table 4 and reduce the air pressure. (0.003atm), spacing From 0.1mm to 1mm, it can meet clinical expectations .

[0083] 2. In the spacing , air pressure Assuming the same single pulse dose, the voltage When the absolute values ​​of are the same, the evaluation index is also the same.

[0084] Table 4:

[0085]

[0086] From Table 4, it can be concluded that: , air pressure And under the premise of the same single pulse dose, a single adjustment interval , can meet clinical expectations , however, with the spacing When the evaluation index cannot meet clinical expectations, the air pressure can be adjusted. ,Voltage and at least one of a single pulse dose to meet clinical expectations.

[0087] Specifically, voltage 400V, air pressure When the single pulse dose is 82Gy / pulse, the spacing When the spacing changes from 0.1 mm to 1 mm, the evaluation indicators meet clinical expectations; When it is 5mm, , which does not meet clinical expectations. At this time, the single pulse dose is 1.11 Gy / pulse. , meeting clinical expectations.

[0088] In summary, the collection efficiency of positive and negative charges involves multiple variables (voltage, gas pressure, spacing, and single pulse dose). There are interactions between multiple variables. Adjusting a single variable often cannot achieve the expected goal. Therefore, it is necessary to comprehensively adjust multiple variables to obtain the best combination of variables to meet clinical expectations.

[0089] This technical solution is further configured to refer to Figures 1 to 3 The shell 100 is provided with a first thin window 112 for the radiotherapy beam to pass through, and a closed chamber for accommodating the electrode unit 300 is formed inside the shell 100.

[0090] From a physical point of view, a hollow airtight space needs to be formed inside the shell 100 so that the electrode unit 300 can work therein, and at the same time, it is matched and installed with the treatment head and other components of the medical linear accelerator through the mounting bracket 400. The working gas in the closed chamber can be air or other gases. Specifically, the shell 100 includes a detachably connected upper shell 110 and a lower shell 120, and the upper shell 110 and the lower shell 120 are buckled to enclose a closed chamber. At the same time, the upper shell 110 and the lower shell 120 are provided with a first thin window 112 for the radiotherapy beam to pass through. From a material point of view, the shell 100 needs to bear the air pressure difference for a long time without serious deformation, and no additional gas is precipitated at low pressure. From an electrical point of view, the shell 100 can shield the influence of external electromagnetic radiation interference on the collector signal in the electrode unit 300, so the shell 100 needs to be conductive and grounded. In general, the housing 100 must be able to form a hollow and sealed space for the electrode unit 300 to work in, and it must be able to withstand the internal and external pressure difference for a long time without releasing additional gas, and it must also be conductive and grounded to provide a complete electromagnetic shielding. Therefore, the housing 100 is preferably a steel housing, or made of other materials that can meet the strength requirements and electromagnetic shielding requirements.

[0091] 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 provided at the mounting hole 111 . Air tightness should be ensured between the first thin window 112 and the mounting hole 111 .

[0092] 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 for the convenience of the radiotherapy beam to pass through. 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 the thickness is as thin as possible. From a material point of view, the first thin window 112 needs to have sufficient structural strength, can withstand the pressure difference for a long time without breaking, does not precipitate additional gas at low pressure, and can be connected to the shell 100 to form a closed chamber. From an electrical point of view, in order to achieve a good electromagnetic shielding effect, the first thin window 112 also needs to be conductive and grounded together with the shell 100. In general, the first thin window 112 is made of a material with a pressure strength greater than 1atm and a high radiation transmittance (not less than 60%), generally titanium, beryllium and other materials.

[0093] Preferably, the first thin window 112 is made of titanium film. The titanium film is used mainly for two reasons: first, the titanium film has sufficient strength to maintain structural integrity under the internal and external pressure difference; second, it can form an ultra-thin window, thereby minimizing the impact on the beam.

[0094] The technical solution is further configured such that, depending on the different gas pressures in the sealed chamber, when the shooting speed is a low-penetration beam, the thickness of the first thin window 112 is 50 μm to 100 μm. Also, when the shooting speed 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, and MV-level X-rays, 100-MeV-level protons, and 100-MeV-level electrons are high-penetration beams.

[0095] Specifically, the inventors verified the first thin window 112 with a thickness of 30 μm and 50 μm respectively when the air pressure was 0.001 atm. The verification results are shown in Table 6:

[0096] Table 6:

[0097]

[0098] It can be concluded from Table 6 that when the air pressure is 0.001 atm, the 50 μm first thin window can maintain the integrity of the ionization chamber structure and the low pressure state is not damaged. Therefore, under the same air pressure conditions, the thickened first thin window also maintains the integrity of the ionization chamber structure and ensures that the low pressure state of the ionization chamber is not damaged. Similarly, in an environment with increased air pressure, that is, in an environment with a smaller pressure difference, such as 0.5 atm, the integrity of the ionization chamber can be maintained without a doubt, ensuring that the low pressure environment is not damaged.

[0099] Specifically, the inventors verified the influence of the first thin window of different thicknesses on the MeV-level electron beam (ie, low-penetration beam), and the verification results are shown in Table 7:

[0100] 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, the beam space distribution is point-shaped, a water layer is placed at about 60 cm SSD (fixed source skin distance), the deposition energy at 2 cm underwater is counted, and normalized and compared, as shown in Fig.17 shown.

[0101] Table 7:

[0102]

[0103] From Table 7 and Fig.17 It can be concluded that the absorbed dose is directly related to the thickness of the titanium film. The thicker the titanium film is, the lower the dose it receives. When the thickness is 50um, the absorbed dose is reduced by about 20% compared to the thickness of 0um; when the thickness exceeds 100um, the absorbed dose will be reduced by about 40% compared to the thickness of 0um. Therefore, the thinner the first thin window and the second thin window are, the more conducive to the dose delivery efficiency. However, in order to maintain sufficient mechanical strength to resist the pressure difference, the inventors have selected the radiotherapy beam as a low-penetration transmission beam with a thin window thickness of 50um-100μm, which ensures a low-pressure environment while significantly reducing the blocking and scattering of the beam, thereby improving the transmittance of the radiotherapy beam.

[0104] Specifically, the inventors verified the influence of the first thin window of different thicknesses on the MV-level X-ray (i.e., high-penetration beam), and the verification results are shown in Tables 8 and 9:

[0105] The first thin window is a titanium film, and 6MV or 10MV MV-level X-rays are used. The first thin window is perpendicular to the direction of the X-rays. A water layer is placed at about 100cm of the SSD (fixed source skin distance). The radiation dose at 5cm underwater is counted and normalized for comparison.

[0106] Table 8:

[0107]

[0108] Table 9:

[0109]

[0110] From Tables 8 and 9, it can be concluded 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, the thicker the titanium film, and the lower the dose it receives. For example, when the energy level of the X-ray source is as low as 6MV, when the thickness is 50um, the absorbed dose decreases by about 0.3% compared to the thickness of 0um; when the thickness exceeds 800um, the absorbed dose decreases by about 6% compared to the thickness of 0um. When the energy level of the X-ray source is 10MV, when the thickness is 50um, the absorbed dose decreases by about 0.1% compared to the thickness of 0um; when the thickness exceeds 800um, the absorbed dose decreases by about 5% compared to the thickness of 0um. Therefore, the thinner the first thin window and the second thin window are, the more conducive to the dose delivery efficiency. However, in order to maintain sufficient mechanical strength to counteract the air pressure difference, the inventors preferably select a thin window with a thickness of 50um-800μm for the radiotherapy beam to be a high-penetration beam, which ensures a low-pressure environment while significantly reducing the blocking and scattering of the beam, thereby improving the transmittance of the radiotherapy beam.

[0111] In the low pressure ionization chamber of this embodiment, please refer to Figures 1 to 3 , the traditional installation valve cannot guarantee the stability of the low-pressure environment in the closed chamber, therefore, a vacuum wall plug 800 is provided on the shell 100, and all kinds of wiring of the electrode unit 300 are led out through the vacuum wall plug 800, and an exhaust pipe 900 for vacuuming is also provided on the shell 100. In order to improve the airtightness, the terminal in the vacuum wall plug 800 is sealed by glass melting or ceramic sintering, and the metal part of the vacuum wall plug 800 and the shell 100 are welded; the airtightness between the exhaust pipe 900 and the shell 100 is guaranteed by the following method: after the air pressure in the closed cavity of the ionization chamber reaches the requirement, the exhaust pipe 900 is vacuum clamped and sealed by secondary welding.

[0112] In the low pressure ionization chamber of this embodiment, please refer to Figures 1 to 3 The housing 100 is provided with a mounting portion 700 inside, and the mounting portion 700 is detachably connected to the electrode unit 300 .

[0113] In actual application, the mounting portion 700 is a protrusion protruding toward the closed cavity. In order to improve the stability of the electrode unit 300, a plurality of protrusions are evenly arranged around the circumference of the mounting hole 111. The protrusion and the electrode unit 300 are both provided with connecting holes, and connecting parts are provided in the connecting holes.

[0114] In the low pressure ionization chamber of this embodiment, please refer to Figures 1 to 4The electrode unit 300 includes a stacked high-voltage electrode 310, an insulating spacer ring 320 and a collecting electrode 330. The insulating spacer ring 320 is provided with a through hole for the radiotherapy beam to pass through. The high-voltage electrode 310 and the collecting electrode 330 are both sheet electrodes. From a physical point of view, the sheet electrode needs to be conductive, have sufficient structural strength, be thin enough, and have sufficient radiation resistance. Therefore, the substrate of the sheet electrode is made of high-strength ultra-thin insulating materials, including but not limited to polyimide, polyester and other materials, and the thickness of the substrate is 20μm to 50 microns. In this embodiment, the substrate of the sheet electrode is made by copper plating and gold immersion on a polyimide film (PI). The PI film has good radiation resistance and high insulation; other materials, such as organic glass film, have unsatisfactory radiation resistance under ultra-high dose rate beam irradiation. Preferably, the thickness of the insulating spacer ring 320 is 0.5-2mm, preferably 1mm.

[0115] In the low pressure ionization chamber of this embodiment, please refer to Figures 1 to 5 The high voltage electrode 310 includes a first bearing layer 311 and a second coating layer 313 located on the first bearing layer 311 and close to a side of the insulating ring 320 .

[0116] In the low pressure ionization chamber of this embodiment, please refer to Figures 1 to 5 The high voltage electrode 310 further includes a first coating layer 312 located on the first bearing layer 311 and away from the insulating ring 320 , and the area of ​​the first coating layer 312 is greater than the area of ​​the second coating layer 313 .

[0117] It should be noted that the first bearing layer 311 is used as a base lining, and a PI film is used. The two sides of the PI film are copper-plated to form a first coating 312 and a second coating 313, wherein the area of ​​the second coating 313 is smaller than the area of ​​the first coating 312 to ensure the uniformity of the electric field within the second coating 313. When in use, the first coating 312 is grounded, and the second coating 313 is connected to high voltage. The first coating 312 has an electromagnetic shielding function to prevent other ions from being attracted. Preferably, the thickness of the first bearing layer 311 is 250 μm, and the thickness of the first coating 312 and the second coating 313 is 15 μm. Of course, in the case of small environmental electromagnetic influence, the first coating 312 may not be provided.

[0118] In the low pressure ionization chamber of this embodiment, please refer to Figures 1 to 6The collecting electrode 330 includes a plurality of mutually insulated collecting parts, the collecting parts include a middle collecting part 331 and a plurality of edge collecting parts 332 symmetrically arranged around the middle collecting part 331, a protective part is arranged on the periphery of 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 middle collecting part and the pins of the edge collecting part are located in the space between the inner protective ring 333 and the outer protective ring 335.

[0119] Specifically, the middle collecting portion 331 is located in the center of the collecting portion, and the plurality of edge collecting portions 332 are symmetrically arranged around the middle collecting portion 331 on a plane, the middle collecting portion 331 is separated from the edge collecting portion 332, and at the same time, adjacent edge collecting portions 332 are separated. Preferably, the middle collecting portion 331 is circular, and the edge collecting portion 332 is arc-shaped. Preferably, the gaps between adjacent edge collecting portions 332 are equal.

[0120] It should be noted that, by providing the middle collection part 331 and the edge collection part 332 , it is possible to realize the partitioned collection of radiotherapy beam signals and to monitor the dose rate and the symmetry and asymmetry of the beam.

[0121] Specifically, the inner protective ring 333 and the outer protective ring 335 are connected by a plurality of connecting parts 337, and the plurality of connecting parts 337 divide the accommodation space between the inner protective ring 333 and the outer protective ring 335 into a plurality of independent sub-accommodation spaces, and the pins of the intermediate collecting part and the pins of the edge collecting part are respectively located in different sub-accommodation spaces. The protective part is also provided with an inner protective ring extension part 334 extending between adjacent edge collecting parts 332, and the inner protective ring extension part 334 is connected to the inner protective ring 333.

[0122] It should be noted that the multiple edge collecting parts 332 are separated by the protection part, which avoids mutual interference between the signals of each channel collecting part, reduces the edge distortion effect of the electric field inside the closed space, improves the electric field uniformity and the stability of the ionization chamber output signal, and at the same time, the outer protection ring 335 is also convenient for grounding.

[0123] In the low pressure ionization chamber of this embodiment, please refer to Figures 1 to 6 The collecting electrode 330 further includes a second bearing layer 336 , and the collecting portion is disposed on the second bearing layer 336 and is close to the insulating ring 320 .

[0124] It should be noted that the second bearing layer 336 adopts PI film, and the middle collecting part 331, the edge collecting 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 gold immersion on the surface.

[0125] In the low pressure ionization chamber of this embodiment, please refer to Figures 1 to 6 The protection part further includes a protection electrode 338 , and the protection electrode 338 is connected to the inner protection ring 333 and the outer protection ring 335 . Meanwhile, the protection electrode 338 is located on the other side of the second bearing layer 336 .

[0126] In the low pressure ionization chamber of this embodiment, please refer 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 arranged on the partition 500. The electrode unit 300 is arranged inside the upper sealed chamber and the lower sealed chamber. The material requirements of the second thin window 502 and the first thin window 112 are the same. According to the different air pressures of the sealed chambers, and when the shooting speed is a low-penetration beam, the thickness of the second thin window 502 and the first thin window 112 is 50μm to 100μm. And, when the shooting speed is a high-penetration beam, the thickness of the second thin window 502 and the first thin window 112 is 50μm to 800μm. For example, kV-level X-rays and MeV-level electron beams are low-penetration beams, and MV-level X-rays, 100-MeV-level protons, and 100-MeV-level electrons are high-penetration beams.

[0127] It should be noted that the partition part 500 includes a partition 501 and a second thin window 502 disposed on the partition 501. In order 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 at the partition through hole 503. The material requirements of the second thin window 502 and the first thin window 112 are the same. At the same time, a sealing component 600 used for knife edge sealing is provided at the junction of the partition 501 and the upper shell 110 and the lower shell 120. Preferably, the sealing component 600 is a copper sealing ring.

[0128] According to the requirements of the national standard (GB15213-2016) for radiotherapy dose monitoring system, the dose linearity and repeatability of the low-pressure ionization chamber were tested. The dose linearity test results are as follows: Figure 8 As shown, Figure 8 The test condition for dose linearity is the ionization chamber pressure P =300 Pa, voltage U c =-40 V, spacing d =1 mm, and a scintillator detector is placed farther away from the rear end of the ionization chamber to measure the relative value of the absorbed dose. Figure 8 It can be seen that for the low-pressure ionization chamber, a linear relationship is observed between the absorbed dose and the measured direct ionization chamber signal. , meeting national standard requirements.

[0129] The dose repeatability measurement results are as follows Fig. 9 As shown, Fig. 9 is the relationship between the ratio of the monitor reading and the scintillator absorbed dose (normalized to the average value of each group) and the number of measurements, Fig. 9 The dashed line represents ±5‰. Fig. 9 It can be seen that a total of 5 groups of different macro pulse widths and repetition rates were selected for dose repeatability measurement, and each group was measured 10 times. The repeatability fluctuation under the 5 conditions did not exceed 5‰, which met the national standard requirements.

[0130] Under different pulse widths, the pulse signal shape measured by the low-pressure ionization chamber is as follows: Fig.10 and Fig.11 shown. Fig.10 and Fig.11 Corresponding to high pressure U c The pulse signals measured when set to 40 V and 80 V, the pulse width ranges from 0.5 μs to 3.5 μs, and the single pulse dose at the monitor ranges from about 12 Gy / pulse to 82 Gy / pulse. The waveform diagram shows that under these conditions, the monitor signal amplitude is independent of the pulse time width (corresponding to the single pulse dose), that is, the collection efficiency is not affected by the single pulse dose. This feature can effectively ensure the consistency of the monitor detection efficiency under different single pulse doses, and then ensure the accuracy of the dose and dose rate monitoring values.

[0131] 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 closed chamber, the distance between the high-voltage electrode and the collecting electrode, and the input voltage of the high-voltage electrode are adjusted until the recombination ratio of positive and negative charged ions is no more than 0.01, wherein the air pressure of the closed chamber is lower than the standard atmospheric pressure.

[0132] According to an embodiment of the present invention, a medical linear accelerator is provided. Figure 1 , including the above-mentioned low-pressure ionization chamber, which is matched and installed with the treatment head of the medical linear accelerator through a mounting bracket 400.

[0133] The present invention has been described in detail above. The above description is only a preferred embodiment of the present invention and should not limit the scope of implementation of the present invention. That is, all equivalent changes and modifications made within 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 closed chamber is formed inside the shell, characterized in that: A low pressure environment lower than the standard atmospheric pressure is formed in the sealed chamber; The distance d between the high-voltage electrode and the collector in the ionization chamber, the single pulse dose D of the radiotherapy beam, the air pressure P in the closed chamber, and the voltage between the high-voltage electrode and the collector in the closed chamber Should meet: ; in, is the ratio of positive and negative charged ions; , is a constant; is the ion recombination coefficient at a specific gas pressure and electric field strength, is a constant; is the charge density generated at a specific pulse dose and pressure, is a constant; is the direct collection ratio of electrons; is the elementary charge; and They represent the positive and negative ion mobility constants of a specific gas under a specific gas pressure and electric field strength respectively; the gas pressure in the closed chamber is 0.001atm to 0.5atm.

2. A low pressure ionization chamber according to claim 1, characterized in that: Depending on the single pulse dose of the radiotherapy beam, the distance between the high-voltage electrode and the collector in the closed chamber is 0.1mm to 10mm, and the voltage between the high-voltage electrode and the collector in the closed chamber is -400V to 400V.

3. A low pressure ionization chamber according to claim 2, characterized in that: The shell is provided with a first thin window and a second thin window for the beam to pass through, and the first thin window and the second thin window are both made of a material with a pressure bearing strength greater than 1 atm and a high radiation transmittance.

4. A low pressure ionization chamber according to claim 3, 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.

5. A low pressure ionization chamber according to claim 3, 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.

6. A low pressure ionization chamber according to claim 1, characterized in that: The high voltage pole, the insulating spacer ring and the collecting pole are stacked in sequence, the insulating spacer ring is provided with a through hole for the beam to pass through, the collecting pole includes a plurality of mutually insulated collecting parts, and a protective part is provided on the periphery of the collecting part.

7. A low pressure ionization chamber according to claim 6, characterized in that: The collecting portion includes a middle collecting portion and a plurality of edge collecting portions symmetrically arranged around the middle collecting portion, the protecting portion includes an inner protecting ring and an inner protecting ring extension portion, the inner protecting ring is arranged on a side of the edge collecting portion away from the middle collecting portion; the inner protecting ring extension portion is connected to the inner protecting ring, and the inner protecting ring extension portion extends between adjacent edge collecting portions.

8. A low pressure ionization chamber according to claim 7, characterized in that: The protection part further includes: an outer protection ring, the inner protection ring and the outer protection ring are concentric and spaced apart, and the middle collecting part pins and the edge collecting part pins are located between the inner protection ring and the outer protection ring.

9. A low pressure ionization chamber according to claim 8, characterized in that: The inner protective ring and the outer protective ring are connected by a plurality of connecting parts, and the plurality of connecting parts divide the accommodation space between the inner protective ring and the outer protective ring into a plurality of independent sub-accommodation spaces, and the middle collecting part pins and the edge collecting part pins are respectively located in different sub-accommodation spaces.

10. A low pressure ionization chamber according to claim 8, characterized in that: The protection part also includes: a protection electrode, which is respectively connected to the inner protection ring and the outer protection ring; the collecting electrode includes a second bearing layer, and the collecting part is arranged on the second bearing layer; the protection electrode is located on a side of the second bearing layer away from the collecting part.

11. A low pressure ionization chamber according to claim 8, characterized in that: The high-voltage pole includes a first bearing layer, a first coating layer and a second coating layer. The first coating layer and the second coating layer are both arranged on the first bearing layer. The first coating layer is located on a side of the first bearing layer away from the insulating spacer ring; the second coating layer is located on a side of the first bearing layer close to the insulating spacer ring.

12. A method for designing a low-pressure ionization chamber according to any one of claims 1 to 11, characterized in that: Based on the single pulse dose of the radiotherapy beam, the air pressure of the closed chamber, the distance between the high voltage electrode and the collecting electrode, and the input voltage of the high voltage electrode are adjusted until the recombination ratio of positive and negative charged ions is no more than 0.01, wherein the air pressure of the closed chamber is lower than the standard atmospheric pressure.

13. A medical linear accelerator, characterized in that: A low-pressure ionization chamber comprising any one of claims 1-11.

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