Extrapolation ionization chamber and method for measuring the surface absorbed dose rate of β-adhesive patches

Abstract

This invention discloses a miniature extrapolation ionization chamber and method for measuring the surface absorbed dose rate of a β-adhesive patch, relating to the field of dose rate technology. The ionization chamber includes a front end and a shell. A micrometer's rotating handle is located at the rear end of the shell, and a micrometer screw extends into the shell. The front end includes an entrance window, which is fixed to the front end face of the shell by a tissue equivalent material pad. A protective ring is disposed around a collecting electrode, which is coaxially positioned inside the shell, with its rear end connected to the front end of the micrometer screw. An insulating gap is formed between the collecting electrode and the protective ring. The entrance window, collecting electrode, and protective ring together constrain the formation of a sensitive volume. The depth of the sensitive volume is changed by adjusting the distance between the collecting electrode and the entrance window using the micrometer. The ionization chamber and method provided by this invention can quickly and effectively calibrate and measure the surface absorbed dose rate of small-area β-adhesive patches.

Description

Technical Field

[0001] This invention belongs to the field of dose rate measurement technology, specifically relating to a miniature extrapolation ionization chamber and method for measuring the absorbed dose rate on the surface of a β-adhesive patch. Background Technology

[0002] Beta radionuclides are widely used in brachytherapy, particularly in beta patch applications for ophthalmic and dermatological treatments. A beta patch uses a specific dose of beta radionuclide as an external irradiation source, applied directly to the lesion. The generated beta rays produce an ionizing radiation biological effect on the lesion, achieving the therapeutic goal. To achieve good treatment quality, it is necessary to effectively treat the lesion while avoiding unnecessary radiation exposure, requiring accurate and effective estimation of the surface absorbed dose of the beta patch. Currently, the recommended instrument for calibrating beta patches is a parallel-plate ionization chamber with variable electrode spacing. This is a sensitive extrapolation ionization chamber whose volume allows for variation. By using the extrapolation method, the surface absorbed dose rate of the beta patch can be determined.

[0003] However, the detection window of the commonly used extrapolation ionization chamber is relatively large. For different sizes of applicators targeting different lesions, the applicator is sometimes insufficient to cover the entire detection window. In this case, the extrapolation ionization chamber needs to select different conversion coefficients according to the size of the applicator, and then obtain the surface absorbed dose rate of the applicator according to the different conversion coefficients. The usage steps are relatively cumbersome. Summary of the Invention

[0004] To address the shortcomings of existing technologies, the present invention aims to provide a miniature extrapolation ionization chamber and method for measuring the absorbed dose rate on the surface of a β-adhesive patch. This extrapolation ionization chamber enables rapid and effective measurement of small-area β-adhesive patches. 90 Sr+ 90 The Yβ applicator was calibrated, and the surface absorbed dose rate of the small-area β applicator was accurately measured.

[0005] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0006] A miniature extrapolation ionization chamber for measuring the surface absorbed dose rate of a β-adhesive patch comprises, from front to back: an ionization chamber front end, an ionization chamber outer shell, and a micrometer screw. The micrometer screw includes a rotating handle and a micrometer screw. The rotating handle is located at the rear end of the ionization chamber outer shell, and the micrometer screw extends into the interior of the ionization chamber outer shell.

[0007] The ionization chamber front end includes an entrance window, a tissue equivalent material pad, a collecting electrode, and a protective ring. The entrance window is disposed on the front end face of the ionization chamber shell, and the tissue equivalent material pad is disposed on the entrance window for fixing the entrance window. The collecting electrode and the protective ring are coaxially disposed inside the ionization chamber shell, and their rear ends are connected to the front end of the micrometer screw. The protective ring is disposed around the collecting electrode and is flush with both ends of the collecting electrode, thereby forming an insulating gap between the collecting electrode and the protective ring. The entrance window, collecting electrode, and protective ring together restrict and form a sensitive volume for detection. The depth of the sensitive volume can be changed by adjusting the distance between the collecting electrode and the entrance window using the micrometer screw.

[0008] The collecting electrode is connected to a ground wire and a signal wire. The ionization chamber shell is equipped with a TNC interface. A matching monitoring voltage is applied to the entrance window through the TNC interface, and the signal wire of the collecting electrode is connected to an external electrometer to realize signal collection and reading.

[0009] Furthermore, in the miniature extrapolation ionization chamber described above for measuring the surface absorbed dose rate of the β-adhesive, the incident window is made of an aluminized polyester film with a mass thickness of 1–10 mg·cm⁻¹. -2 .

[0010] Furthermore, in the micro extrapolation ionization chamber described above for measuring the surface absorbed dose rate of the β-application, the tissue equivalent material pad is made of polymethyl methacrylate and has a thickness of 1–3 mm.

[0011] Furthermore, in the miniature extrapolation ionization chamber described above for measuring the surface absorbed dose rate of the β-adhesive patch, the collecting electrode is a cylindrical structure with a diameter of 3-5 mm, made of tissue equivalent material, and its outer surface is coated with graphite to ensure the stability of the weak current measurement.

[0012] Furthermore, in the miniature extrapolation ionization chamber described above for measuring the surface absorbed dose rate of the β-application, the protective ring is used to reduce leakage current from the inflated cavity and to homogenize the parallel electric field. The protective ring is made of tissue equivalent material and has a hollow cylindrical structure. It is spaced 0.4 to 0.6 mm from the collecting electrode.

[0013] Furthermore, in the miniature extrapolation ionization chamber described above for measuring the absorbed dose rate on the surface of the β-application, the depth adjustment range of the sensitive volume is 0–10 mm.

[0014] The method for measuring the surface absorbed dose rate of a β-adhesive using the miniature extrapolation ionization chamber described above includes the following steps:

[0015] S1. Connect the ionization chamber to the electrometer, and preheat the ionization chamber and the electrometer fully under stable voltage conditions;

[0016] S2. The rate of change of the ionization chamber current with the extrapolation distance is obtained by measuring the dose rate at different extrapolation distances using the same standard radiation source. And the extrapolated zero point d0 of the ionization chamber when the current value approaches 0 is calculated;

[0017] S3. Place the front surface of the ionization chamber in close contact with the surface of the β patch to be tested, measure the ionization current multiple times under positive and negative polarization voltages, and calculate the average current value I.

[0018] S4. Calculate the temperature and pressure correction factor based on the actual temperature and pressure. The formula for calculating the temperature and pressure correction factor is as follows:

[0019]

[0020] In equation (3), T is temperature in °C; P is air pressure in kPa; K TP The humidity correction factor is set to 1 under standard test conditions to obtain the temperature and pressure correction coefficients.

[0021] S5. Based on the calculation results of steps S2-S4, the surface absorbed dose rate of the β-adhesive to be tested is obtained. The calculation formula is as follows:

[0022]

[0023] In formula (4), The tissue absorbed dose rate at 0 depth in the ionization chamber; It is the ratio of the average energy consumed to form each ion pair in dry air to the elementary charge e; ρ0 is the average mass collision capability ratio of the organization to air; ρ0 is the air density at standard temperature and standard pressure; a is the effective collection area of ​​the ionization chamber; k is the product of correction factors that vary with the depth of the ionization chamber. The extrapolated ionization chamber current is the rate of change with the extrapolation distance; where This is a constant for a given ionization chamber.

[0024] Furthermore, in the method for measuring the surface absorbed dose rate of a β-adhesive using a micro-extrapolation ionization chamber as described above, step S2 involves adjusting the extrapolation distance of the ionization chamber (i.e., the distance between the collecting electrode and the entrance window) using a micrometer screw gauge to obtain the rate of change of the ionization chamber current with respect to the extrapolation distance. And when the current value approaches 0, the extrapolation zero point d0 of the extrapolation ionization chamber is calculated.

[0025] Furthermore, in the method for measuring the surface absorbed dose rate of the β-adhesive patch using the micro-extrapolation ionization chamber as described above, the extrapolation distance in step S2 is set to 2.5, 2.0, 1.5, 1.0, or 0.5 mm.

[0026] Furthermore, in the method for measuring the surface absorbed dose rate of the β-adhesive patch using the micro-extrapolation ionization chamber as described above, step S3 specifically comprises:

[0027] After measuring a set of values ​​under a +20V polarization voltage, immediately change the polarization voltage to -20V. Once the reading stabilizes, record the ionization current value at -20V. Then, change the polarization voltage back to +20V and record the ionization current value again once the reading stabilizes. Repeat this process a total of 5 times, obtaining 3 sets of current values ​​under +20V polarization voltage and 2 sets of current values ​​under -20V polarization voltage. The formula for calculating the average current value is:

[0028]

[0029] In equation (2), This is the average value of the 1st, 3rd, and 5th current values ​​measured under a +20V polarization voltage condition; This is the average of the second and fourth current values ​​measured under a polarization voltage of -20V.

[0030] Compared with the prior art, the miniature extrapolation ionization chamber and method for measuring the absorbed dose rate on the surface of a β-application provided by the present invention have the following advantages:

[0031] This ionization chamber, while satisfying the BG principle, possesses an extremely small detection window area, enabling it to detect small areas. 90 Sr+ 90 The Yβ applicator is used for calibration and to accurately measure the surface dose distribution of a radiation source. Attached Figure Description

[0032] Figure 1 This is a schematic diagram of a miniature extrapolation ionization chamber for measuring the surface absorbed dose rate of a β-adhesive, provided in an embodiment of the present invention.

[0033] Figure 2 This is a partially enlarged schematic diagram of the front end of the ionization chamber;

[0034] In the figure: 1-Ionization chamber front end, 2-Ionization chamber shell, 3-Micrometer spiral gauge, 11-Incident window, 12-Tissue equivalent material gasket, 13-Collection electrode, 14-Protective ring, 15-Sensitive volume, 16-Insulation gap. Detailed Implementation

[0035] The present invention will now be described in further detail with reference to the accompanying drawings and specific embodiments.

[0036] Figure 1 The diagram shows a schematic of a miniature extrapolation ionization chamber for measuring the surface absorbed dose rate of a β-adhesive, provided in an embodiment of the present invention. The extrapolation ionization chamber comprises, from front to back, an ionization chamber front end 1, an ionization chamber outer shell 2, and a micrometer 3 (also called a micrometer). The ionization chamber front end 1 is located at the front end of the ionization chamber outer shell 2, and the rear end of the ionization chamber outer shell 2 is coupled to the micrometer 3. The micrometer 3 includes a rotating handle and a micrometer screw. The rotating handle is located at the rear end of the ionization chamber outer shell 2, and the micrometer screw extends into the interior of the ionization chamber outer shell 2.

[0037] As shown in the figure, the front end 1 of the ionization chamber includes an entrance window 11, a tissue equivalent material pad 12, a collecting electrode 13, and a protective ring 14. The entrance window 11 is disposed on the front end face of the ionization chamber shell 2, and the tissue equivalent material pad 12 is disposed on the entrance window 11 for fixing the entrance window 11. The collecting electrode 13 and the protective ring 14 are coaxially disposed inside the ionization chamber shell 2, and their rear ends are connected to the front end of the micrometer screw. The protective ring 14 is disposed around the collecting electrode 13 and is flush with both ends of the collecting electrode 13, thereby forming an insulating gap 16 between the collecting electrode 13 and the protective ring 14. The entrance window 11, the collecting electrode 13, and the protective ring 14 together limit the sensitive volume 15 that forms the detection area. The depth of the sensitive volume 15 is changed by adjusting the distance between the collecting electrode 13 and the entrance window 11 using a micrometer screw gauge 3. In a specific embodiment of the invention, the depth adjustment range of the sensitive volume 15 is 0-10 mm.

[0038] The collecting electrode 13 is connected to a ground wire and a signal wire. A TNC interface is provided on the ionization chamber shell 2. A matching monitoring voltage is applied to the entrance window 11 via the TNC interface, and the signal wire from the collecting electrode 13 is led out and connected to an external electrometer. After β radiation enters the sensitive volume 15, it ionizes the free air and generates positive and negative ions. The positive ions drift to the collecting electrode 13 under the influence of the electric field, generating a current signal. This current signal is led to the electrometer via the TNC interface, enabling signal collection and reading.

[0039] In one specific embodiment of the invention, the entrance window 11 of the ionization chamber is made of an aluminized polyester film with a thickness of 1–10 mg·cm⁻¹. -2 .

[0040] In one specific embodiment of the invention, the tissue equivalent material pad 12 is made of polymethyl methacrylate (PMMA) with a thickness of 1–3 mm. During measurement, the appropriate distance between the incident window 11 and the source surface can be selected by choosing tissue equivalent material pads 12 of different thicknesses.

[0041] In one specific embodiment of the invention, the collecting electrode 13 is a cylindrical structure with a diameter of 3-5 mm, made of tissue equivalent material (PMMA), and its outer surface is coated with graphite to ensure the stability of weak current measurement.

[0042] The protective ring 14 is used to reduce leakage current from the air-filled cavity and to even out the parallel electric field. It is a hollow cylindrical structure with an inner diameter 0.4 to 0.6 mm away from the outer diameter of the collecting electrode 13, and is made of tissue equivalent material (PMMA).

[0043] In one specific embodiment of the invention, the outer shell 2 of the ionization chamber is made of aluminum.

[0044] The principle of measuring the surface absorbed dose rate of a radioactive source using an extrapolation ionization chamber is explained below.

[0045] To measure the tissue absorbed dose at a specific point in the radiation field generated by the beta patch, the entrance window of the extrapolation ionization chamber is made of a low atomic number material, and the rear and side walls of the ionization chamber are thick enough to completely absorb the highest-energy beta particles present. In this case, the extrapolation ionization chamber satisfies the conditions for the Bragg-Gray equation to hold.

[0046] Under these conditions, according to the Bragg-Gray formula, the relationship between the absorbed dose in the reference material (tissue equivalent material) and the absorbed dose in the air-filled cavity is as follows:

[0047]

[0048] In the formula:

[0049] —(33.97±0.15)J / C, which is the ratio of the average energy consumed to form each ion pair in dry air to the elementary charge e;

[0050] —The ratio of the average mass impact capability of equivalent materials against air to the organizational capacity of the material. 14 C 147 Pm, 90 Sr+ 90 For β-nucleoside radioactive sources such as Y, the recommended value is 1.13;

[0051] ρ0 — air density at standard temperature and standard pressure;

[0052] a — Effective collection area of ​​the ionization chamber;

[0053] —The rate of change of current with extrapolation distance;

[0054] k back—Correction factor for the backscattering difference between the reference medium (tissue equivalent material) and the chamber collector material.

[0055] All the above parameters are constant values; only the rate of change of current with extrapolation distance is considered. Measurement is required. The extrapolation distance of the ionization chamber, i.e., the distance between the collecting electrode 13 and the entrance window 11, is adjusted using a micrometer. Commonly used extrapolation distances are 2.5, 2.0, 1.5, 1.0, and 0.5 mm. By measuring the dose rate against the same standard radiation source, the rate of change of the ionization chamber current with the extrapolation distance can be obtained. It can also calculate the extrapolated zero point d0 of the extrapolated ionization chamber when the current value approaches 0.

[0056] Based on the above measurement principle, this invention provides a method for measuring the surface absorbed dose rate of a β-adhesive using the aforementioned miniature extrapolation ionization chamber, comprising the following steps:

[0057] S1. Connect the ionization chamber to the electrometer, and allow the ionization chamber and electrometer to fully preheat under stable voltage conditions.

[0058] First, connect the ionization chamber to the electrometer, ensuring the voltage and signal terminals on the ionization chamber are correctly connected. Then, turn on the polarization power switch and select the polarization voltage to +20V bias voltage. At this point, the +20V red indicator light should illuminate. Before measurement, allow the ionization chamber and electrometer to preheat for at least 2 hours under stable voltage conditions.

[0059] S2. By measuring the dose rate at different extrapolation distances using the same standard radiation source, the rate of change of ionization chamber current with extrapolation distance is obtained. And the extrapolated zero point d0 of the ionization chamber when the current value approaches 0 is calculated.

[0060] The extrapolation distance of the ionization chamber, i.e. the distance between the collecting electrode 13 and the entrance window 11, is adjusted by a micrometer. The commonly used extrapolation distances are 2.5, 2.0, 1.5, 1.0, and 0.5 mm.

[0061] S3. Place the front surface of the ionization chamber in close contact with the surface of the β-adhesive to be tested, and measure the ionization current multiple times under both positive and negative polarization voltages, then calculate the average current value. Specifically:

[0062] To eliminate the influence of β-particle polarization, the ionization current must be measured under both positive and negative polarization voltages. After measuring one set of values ​​under a +20V polarization voltage, the polarization voltage is immediately changed to -20V. Once the reading stabilizes, the ionization current value under -20V is recorded. The polarization voltage is then changed back to +20V, and once the reading stabilizes, the ionization current value is recorded again. This process is repeated a total of 5 times, resulting in 3 sets of current values ​​under a +20V polarization voltage and 2 sets of current values ​​under a -20V polarization voltage. The current value read from the electrometer in the ionization chamber is used to calculate the average current value using formula (2). The formula is as follows:

[0063]

[0064] In equation (2), This is the average value of the 1st, 3rd, and 5th current values ​​measured under a +20V polarization voltage condition; This is the average of the second and fourth current values ​​measured under a polarization voltage of -20V.

[0065] S4. Calculate the temperature and pressure correction factors based on the actual temperature and pressure. The formula for calculating the temperature and pressure correction factors is:

[0066]

[0067] In equation (3), T is temperature in °C; P is air pressure in kPa; K TP The temperature and pressure correction factors are used to obtain the results (the humidity correction factor is taken as 1 under standard test conditions).

[0068] S5. Based on the calculation results of steps S2-S4, the surface absorbed dose rate of the β-adhesive to be tested is obtained. The calculation formula is as follows:

[0069]

[0070] In formula (4), The tissue absorbed dose rate at 0 depth in the ionization chamber; It is the ratio of the average energy consumed to form each ion pair in dry air to the elementary charge e; ρ0 is the average mass collision capability ratio of the organization to air; ρ0 is the air density at standard temperature and standard pressure; a is the effective collection area of ​​the ionization chamber; k is the product of correction factors that vary with the depth of the ionization chamber. The extrapolated ionization chamber current is the rate of change with the extrapolation distance; where For a given ionization chamber, this is a constant and can be written together as k. α .

[0071] The present invention provides a miniature extrapolation ionization chamber and a measurement method for measuring the surface absorbed dose rate of a β-application, which can quickly and effectively calibrate small-area β-applications and accurately measure the surface absorbed dose rate of small-area β-applications.

[0072] Obviously, those skilled in the art can make various modifications and variations to this invention without departing from its spirit and scope. Therefore, if these modifications and variations fall within the scope of the claims of this invention and their equivalents, this invention is also intended to include these modifications and variations.

Claims

1. A miniature extrapolation ionization chamber for measuring the absorbed dose rate on the surface of a β-adhesive patch, characterized in that, The ionization chamber comprises, from front to back, the following components: ionization chamber front end (1), ionization chamber outer shell (2), and micrometer screw (3). The micrometer screw (3) includes a rotating handle and a micrometer screw. The rotating handle is located at the rear end of the ionization chamber outer shell (2), and the micrometer screw extends into the interior of the ionization chamber outer shell (2). The front end (1) of the ionization chamber includes an entrance window (11), a tissue equivalent material pad (12), a collecting electrode (13), and a protective ring (14). The entrance window (11) is disposed on the front end face of the ionization chamber shell (2), and the tissue equivalent material pad (12) is disposed on the entrance window (11) for fixing the entrance window (11). The collecting electrode (13) and the protective ring (14) are coaxially disposed inside the ionization chamber shell (2), and their rear ends are connected to the front end of the micrometer screw. 4) The collection electrode (13) is positioned around the periphery of the collection electrode (13) and flush with both ends of the collection electrode (13), thereby forming an insulating gap (16) between the collection electrode (13) and the protective ring (14). The entrance window (11), the collection electrode (13) and the protective ring (14) together restrict the formation of a sensitive volume (15) for detection. The distance between the collection electrode (13) and the entrance window (11) is adjusted by the micrometer (3) to change the depth of the sensitive volume (15). The collecting electrode (13) is connected to a ground wire and a signal wire. The ionization chamber shell (2) is provided with a TNC interface. The matching monitoring voltage is applied to the entrance window (11) through the TNC interface, and the signal wire of the collecting electrode (13) is connected to an external electrometer to realize signal collection and reading.

2. The miniature extrapolation ionization chamber for measuring the absorbed dose rate on the surface of a β-application device according to claim 1, characterized in that, The incident window (11) is made of aluminized polyester film with a thickness of 1–10 mg·cm. -2 .

3. The miniature extrapolation ionization chamber for measuring the absorbed dose rate on the surface of a β-application device according to claim 2, characterized in that, The tissue equivalent material pad (12) is made of polymethyl methacrylate and has a thickness of 1-3 mm.

4. The miniature extrapolation ionization chamber for measuring the absorbed dose rate on the surface of a β-application device according to claim 3, characterized in that, The collecting electrode (13) is a cylindrical structure with a diameter of 3-5 mm. Its material is a tissue equivalent material, and its outer surface is coated with graphite to ensure the stability of weak current measurement.

5. The miniature extrapolation ionization chamber for measuring the surface absorbed dose rate of a β-adhesive as described in claim 4, characterized in that, The protective ring (14) is used to reduce leakage current from the air-filled cavity and to even out the parallel electric field. Its material is tissue equivalent material; its shape is a hollow cylindrical structure, and it is spaced 0.4 to 0.6 mm apart from the collecting electrode (13).

6. The miniature extrapolation ionization chamber for measuring the surface absorbed dose rate of a β-application according to any one of claims 1-5, characterized in that, The depth adjustment range of the sensitive volume (15) is 0 to 10 mm.

7. A method for measuring the surface absorbed dose rate of a β-adhesive using the miniature extrapolation ionization chamber according to any one of claims 1-6, comprising the following steps: S1. Connect the ionization chamber to the electrometer, and preheat the ionization chamber and the electrometer fully under stable voltage conditions; S2. The rate of change of the ionization chamber current with the extrapolation distance is obtained by measuring the dose rate at different extrapolation distances using the same standard radiation source. And the extrapolated zero point d0 of the ionization chamber when the current value approaches 0 is calculated; S3. Place the front surface of the ionization chamber in close contact with the surface of the β patch to be tested, measure the ionization current multiple times under positive and negative polarization voltages, and calculate the average current value I. S4. Calculate the temperature and pressure correction factor based on the actual temperature and pressure. The formula for calculating the temperature and pressure correction factor is as follows: In equation (3), T is temperature in °C; P is air pressure in kPa; K TP The humidity correction factor is set to 1 under standard test conditions to obtain the temperature and pressure correction coefficients. S5. Based on the calculation results of steps S2-S4, the surface absorbed dose rate of the β-adhesive to be tested is obtained. The calculation formula is as follows: In equation (4), The tissue absorbed dose rate at 0 depth in the ionization chamber; It is the ratio of the average energy consumed to form each ion pair in dry air to the elementary charge e; ρ0 is the ratio of the average mass collision capability of the organization with air; ρ0 is the air density at standard temperature and standard pressure. a is the effective collection area of ​​the ionization chamber; k is the product of correction factors that vary with the depth of the ionization chamber; The extrapolated ionization chamber current is the rate of change with the extrapolation distance; where This is a constant for a given ionization chamber.

8. The method for measuring the surface absorbed dose rate of a β-adhesive patch using a miniature extrapolation ionization chamber according to claim 7, characterized in that, In step S2, the extrapolation distance of the ionization chamber, i.e., the distance between the collecting electrode and the entrance window, is adjusted using a micrometer screw gauge to obtain the rate of change of the ionization chamber current with the extrapolation distance. And when the current value approaches 0, the extrapolation zero point d0 of the extrapolation ionization chamber is calculated.

9. The method for measuring the surface absorbed dose rate of a β-adhesive patch using a miniature extrapolation ionization chamber according to claim 8, characterized in that, In step S2, the extrapolation distance is set to 2.5, 2.0, 1.5, 1.0, and 0.5 mm.

10. The method for measuring the surface absorbed dose rate of a β-adhesive patch using a miniature extrapolation ionization chamber according to claim 7, characterized in that, Step S3 is as follows: After measuring a set of values ​​under a +20V polarization voltage, immediately change the polarization voltage to -20V. Once the reading stabilizes, record the ionization current value at -20V. Then, change the polarization voltage back to +20V and record the ionization current value again once the reading stabilizes. Repeat this process a total of 5 times, obtaining 3 sets of current values ​​under +20V polarization voltage and 2 sets of current values ​​under -20V polarization voltage. The formula for calculating the average current value is: In equation (2), This is the average value of the 1st, 3rd, and 5th current values ​​measured under a +20V polarization voltage condition; This is the average of the second and fourth current values ​​measured under a polarization voltage of -20V.

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