JP7272798B2 - Dose calculation method, dose calculation program - Google Patents

Description

Application of Article 30, Paragraph 2 of the Patent Act 2018 Spring Annual Meeting of the Atomic Energy Society of Japan Date March 26, 2018

The present invention relates to, for example, a dose calculation method and a dose calculation program for radiation facilities.

Conventionally, in radiation irradiation rooms where radiation generators such as electronic linacs (linear accelerators) are installed, in order to suppress the leakage of radiation outside the controlled area and to keep the effective dose at the boundary of the controlled area below the value stipulated by law, concrete and a thick shielding wall made of iron.

In Japan, it is common to use the simple calculation formula described in Non-Patent Document 1 to evaluate doses at the boundaries of controlled areas of radiation facilities. In the case of more complicated shielding, the effective dose can be calculated by a three-dimensional model of the object and radiation transport calculation by the Monte Carlo method.

In addition to the above, non-patent documents 2 to 4 summarize simple calculation formulas for dose evaluation of radiation facilities. In particular, in Non-Patent Document 2, a cone of X-rays is directed to the maze of the irradiation room, and after the X-rays pass through the wall of the maze, they are scattered once by the inner wall of the maze, and the entrance and exit of the irradiation room A simple calculation formula for the effective dose of X-rays reaching . is described below with reference to FIG.

where _DwT is the dose at the entrance E of the irradiation chamber, W is the irradiation dose, _Um is the directional utilization rate, _Bpr is the transmittance of the maze wall M, _αp is the reflectance at wall P, and _Ap is the reflectance at wall P. _dp is the distance from the radiation source S to the center of the wall P, and d'' is the distance from the center of the wall P to the entrance E of the irradiation chamber.

"Radiation Facility Shield Calculation Practical Manual 2015", Nuclear Safety Technology Center, Published by Radiation Hazard Prevention Law Publications Editorial Committee, March 2015 International Atomic Energy Agency, “Radiation protection in the design of radiotherapy facilities. Safety Reports Series No. 47”, Vienna (Austria): International Atomic Energy Agency, 2006, ISBN-10: 92-0-100505-9 National Council on Radiation Protection and Measurements, “Structural shielding design and evaluation for megavoltage x- and gamma-ray radiotherapy facilities. NCRP Report No. 151.” Bethesda, MD (USA): National Council on Radiation Protection and Measurements, 2005, ISBN -13: 978-0-929600-87-1 McGinley PH, “Shielding techniques for radiation oncology facilities.” Madison, WI (USA): Medical Physics, 2002, ISBN-13: 978-1930524071

By the way, with the spread of seismic isolation structures in recent years, there is an increasing need for dose evaluation of the seismic isolation layer directly below the irradiation room.

In the conventional simple calculation method described above, X-rays pass through the floor and are reflected in the space with a low ceiling inside the seismic isolation layer, such as the X-rays irradiated toward the floor with the seismic isolation layer. It is difficult to accurately evaluate the passing evaluation points.

In addition, the dose calculation by the conventional Monte Carlo method requires more calculation time than the simple calculation formula, and requires several days to a week depending on the calculation system.

The present invention has been made in view of the above, and an object thereof is to provide a dose calculation method and a dose calculation program capable of accurately calculating the dose in an outdoor space such as under the floor of a radiation irradiation room.

In order to solve the above-described problems and achieve the object, the dose calculation method according to the present invention provides an outdoor dose calculation method outside a radiation shielding shield provided around a radiation irradiation room having a radiation source for irradiating radiation. A method of calculating the dose in space, assuming that at least part of the radiation emitted from the radiation source and transmitted through the shield scatters within the shield and reaches a predetermined evaluation point set in the outdoor space. Then, the value of the contribution that the radiation emitted from the source scatters inside the shield and reaches the evaluation point, and the value of the contribution of the radiation emitted from the source after passing through the shield in the outdoor space outside the shield It is characterized in that the dose at the evaluation point is obtained by summing the value of the contribution that reaches the evaluation point through multiple reflection.

In addition, another dose calculation method according to the present invention is based on the above-described invention, assuming that the reflectance of multiple reflection of radiation in the outdoor space is a constant value regardless of the attenuation of the energy, and multiple reflection in the outdoor space is performed. It is characterized by obtaining the value of the contribution that reaches the evaluation point by

Further, another dose calculation method according to the present invention is the energy of radiation emitted from the radiation source, the material of the shield, and the material of the object facing the surface of the shield across the outdoor space , the dose rate due to the radiation emitted from the source that passes through the shield and then reflects off the surface of the object and travels toward the evaluation point, and the Calculate the reflectance of multiple reflections of radiation in the outdoor space based on the dose rate due to the radiation heading toward the evaluation point, and use the obtained reflectance to determine the value of the contribution that reaches the evaluation point after multiple reflections in the outdoor space. is characterized by asking for

A dose calculation program according to the present invention causes a computer to execute the dose calculation method described above.

According to the dose calculation method according to the present invention, a method for calculating the dose of an outdoor space outside a radiation shielding shield provided around a radiation irradiation room having a radiation source for irradiating radiation, Assuming that at least part of the radiation emitted from the source and transmitted through the shield scatters within the shield and reaches a predetermined evaluation point set in the outdoor space, the radiation emitted from the source is shielded. The contribution value that scatters inside the body and reaches the evaluation point, and the contribution value that reaches the evaluation point after the radiation emitted from the source passes through the shield and is multiple-reflected in the outdoor space outside the shield. are summed up to determine the dose at the evaluation point, the effect is that the dose in an outdoor space such as under the floor of a radiation irradiation room can be calculated quickly and accurately without using the Monte Carlo method.

Further, according to another dose calculation method according to the present invention, the reflectance of the multiple reflection of radiation in the outdoor space is assumed to be a constant value regardless of the attenuation of the energy, and the evaluation point is obtained by multiple reflection in the outdoor space. Since the value of the contribution reaching to is obtained, there is an effect that the dose in the outdoor space can be calculated quickly.

Further, according to another dose calculation method according to the present invention, the energy of the radiation emitted from the radiation source, the material of the shield, the material of the object facing the surface of the shield across the outdoor space, and the radiation After the radiation emitted from the source passes through the shield, it reflects off the surface of the object and travels to the evaluation point. Based on the dose rate due to the directed radiation, the reflectance of the multiple reflection of the radiation in the outdoor space is obtained, and using the obtained reflectance, the value of the contribution of the multiple reflection in the outdoor space to reach the evaluation point is obtained. , the dose of radiation including multiple reflections in the path of interest can be calculated quickly and accurately.

Further, according to the dose calculation program according to the present invention, since the computer executes the dose calculation method described above, the dose of the outdoor space can be calculated quickly and accurately without using the Monte Carlo method.

FIG. 1 is an explanatory diagram of an embodiment of a dose calculation method and a dose calculation program according to the present invention. FIG. 2 is a flow chart showing an embodiment of a dose calculation method and a dose calculation program according to the present invention. FIG. 3 is a comparison diagram of the calculation result by the Monte Carlo method and the calculation result by the present invention. FIG. 4 is a diagram showing an example of a reflectance calculation flow chart. FIG. 5 is an explanatory diagram of a conventional dose evaluation method.

BEST MODE FOR CARRYING OUT THE INVENTION An embodiment of a dose calculation method and a dose calculation program according to the present invention will be described below in detail with reference to the drawings. In addition, this invention is not limited by this embodiment.

(Dose calculation method)
First, an embodiment of a dose calculation method according to the present invention will be described.

As shown in FIG. 1, a radiation source S is installed in a radiation irradiation room, and X-rays (radiation) are emitted from the radiation source S in the form of a beam (X-ray beam). Assume that the floor of the radiation irradiation room is composed of a shield a for shielding radiation, and a space b (outdoor space) is arranged immediately below the shield a. In this embodiment, a medical linac room is assumed as the radiation irradiation room, and a seismic isolation layer is assumed as the space b.

Regarding the X-rays irradiated in the form of a beam from the radiation source S, the X-rays (path I) that scatter within the shield a and reach the evaluation point E in the space b, pass through the shield a and undergo multiple reflections in the space b. X-rays (path II) that are raised and finally reflected by the surface d of the floor (or wall on the far side) e below the space b and reach the evaluation point E (path II), after passing through the shield a, in the space b After undergoing multiple reflections at , the X-rays are finally scattered on the surface c of the shield a and reach the evaluation point E (path III). FIG. 1 shows the paths of X-rays.

The effective dose rate D _S (Sv/h) of the X-ray path I is assumed to be scattering on the wall surface c of the boundary between the shield a and the space b, and is calculated by the following formula (2). Here, W is the maximum dose rate (Gy/h) at a position 1 m away from the X-ray source S, B is the transmittance of the shield a, and F is the conversion factor (Sv/ Gy), U is the directional utilization rate, A _C is the irradiation area on the surface c of the X-ray cone, α _S is the angle at the reflection point on the surface c (the intersection of the central axis of the X-ray cone and the surface c) The reflectance at θ _S , d _C0 is the distance from the source S to the reflection point on surface c, and d _C is the distance from the reflection point on surface c to the evaluation point E.

The effective dose rate D _F (Sv/h) of X-ray path II is considered as follows. After passing through the shield a, part of the X-rays repeat multiple reflections so as to go back and forth between the shield surface c and the floor surface d. Also, when reflected by the floor surface d, a portion of the light travels toward the evaluation point E with a reflectance _αF . Letting the reflectance of multiple reflection be a constant value r, treat it as a sum of infinite geometric series with a common ratio ^r2 , and calculate the effective dose rate _DF of X-ray path II by the following equation (3). Here, A _F is the irradiation area of the X-ray cone on the floor surface d, and α _F is the angle θ _F at the reflection point on the floor surface d (the intersection of the central axis of the X-ray cone and the floor surface d). _dF0 is the distance from the radiation source S to the reflection point on the floor surface d, and _dF is the distance from the reflection point on the floor surface d to the evaluation point E.

The effective dose rate D _C (Sv/h) of the X-ray path III is that the X-rays transmitted through the shield a and reflected from the floor surface d to the surface c with a reflectance r cause multiple reflections in the space b. Considering that, using the formula for the sum of infinite geometric series in the same way as for path II, it can be calculated by the following formula (4). Here, α _C is the reflectance at the angle θ _C at the reflection point on the surface c (the intersection of the central axis of the X-ray cone and the surface c).

The effective dose rate D (Sv/h) at the evaluation point E in the space b is a summary of the above three equations (equations (2) to (4)). In particular, when the irradiation area at a position 1 m directly below the radiation source S is A _IC , A _IC = _AC /d _C0 ² = _AF /d _F0 ² , so it can be expressed by the following equation (5). can.

When the evaluation point E is at the middle height of the space b, the reflectance is α _F =α _C . Also, the distance from the reflection point to the evaluation point E can be approximated to the distance d from the central axis of the X-ray cone to the evaluation point E (d≈d _C ≈d _F ).

If the evaluation point E is in the range directly reached by the X-ray cone in the space b or in the vicinity thereof, the effective dose rate obtained by the method of the present invention will be underestimated, so care must be taken. This is because the transmission line of simple X-rays becomes dominant.

A well-known method can be used to calculate the transmittance B. As an example, it can be determined from the X-ray energy, the material (concrete, iron, lead, etc.) and the thickness of the shield by the method described in Non-Patent Document 1 above.

A well-known technique can be used to calculate the reflectances α _S , α _F , and α _C . As an example, it can be obtained from the X-ray energy and reflection angle according to the semi-empirical formula of Chilton and Huddleston.

The reflectance r of multiple reflection can be calculated, for example, by the method described later, but is not limited to the method described later. As for the reflectance r, it is desirable that the value corresponding to the accelerator energy and the material of the surface c and the floor surface d be stored in a database so that the necessary value can be obtained at the time of calculation.

Therefore, in actual calculations, X-ray irradiation conditions (maximum dose rate at a position 1 m away from the radiation source, irradiation area, directional utilization rate), facility layout, shielding rate of shield a, position of evaluation point E , the reflectance r of multiple reflection in the space b, the reflectance corresponding to the reflection angle on the surface c, and the floor surface d. The conversion factor F is a constant and 1 is used.

(Dose calculation program)
Next, an embodiment of a dose calculation program according to the present invention will be described. A dose calculation program according to the present embodiment causes a computer to execute the linear calculation method described above.

FIG. 2 is an example of a flow chart of a dose calculation program according to this embodiment. This calculation program calculates the effective dose at any point in the space b.

First, X-ray irradiation conditions (maximum dose rate at a position 1 m away from the radiation source S, irradiation area, directional utilization rate, accelerator energy), structure of shield a (material, thickness), position of evaluation point E is input as a parameter (step S1). Transmittance or reflectance may be used as input parameters.

The transmittance B of the shield a is obtained from the accelerator energy and the structure of the shield a (step S4). According to the position of the evaluation point E, the distance from the reflection point to the evaluation point E and the angle of reflection are obtained (step S2), and the reflectances α _S , α _F , and α _C corresponding to the energy and the angle of reflection are obtained (step S3 ). From a database prepared in advance, the reflectance r of multiple reflection corresponding to the accelerator energy and the material of the surface c and the floor surface d is obtained (step S5). Using the above values, the effective dose at evaluation point E is calculated (step S6).

According to the present embodiment, assuming that at least some of the X-rays irradiated from the radiation source S and transmitted through the shield a are scattered within the shield a and reach the evaluation point E, the radiation source S The value of the contribution that the X-rays emitted from from are scattered within the shield a and reach the evaluation point E, and the outside of the shield a after the X-rays emitted from the source S pass through the shield a The effective dose at the evaluation point E is obtained by summing the value of the contribution that reaches the evaluation point E after multiple reflections in the space b. Therefore, the effective dose of the seismic isolation layer immediately below the medical linac room, for which there was no suitable simple calculation formula, can be calculated quickly and accurately without using the Monte Carlo method.

Also, when there is multiple reflection of X-rays between layers such as the space b, assuming that the reflectance r of the multiple reflection is a constant value regardless of the attenuation of the energy, the idea of the sum of infinite geometric series is used. , and the value of the contribution reaching the evaluation point E after multiple reflections in the space b is obtained, so the effective dose in the space b can be calculated quickly.

It should be noted that the above-described embodiment can be applied to any facility where X-rays are irradiated in the form of a beam, and is not limited to a medical linac. Further, the irradiation direction of the X-rays from the radiation source is not limited to the floor direction, and can be applied, for example, when there is a structure similar to the shield in the ceiling direction or the side wall direction of the irradiation room.

(Dose calculator)
Next, an embodiment of a dose calculation device according to the present invention will be described.

A dose calculation apparatus according to the present embodiment is an apparatus that embodies the dose calculation method described above, and includes, for example, an input section, a storage section, a calculation section, and an output section. This dose calculation device can be composed of, for example, a computer having a CPU, hardware such as a memory, a display, a keyboard, etc., and software such as a computer program executed using these hardware.

The input unit is for inputting the input parameters in step S1, and can be configured by, for example, a keyboard. The storage unit can be composed of a database or the like that stores calculation data such as values corresponding to the accelerator energy and the material of the surface c and the floor surface d.

The calculation unit executes the calculations in steps S2 to S6 described above, and can be composed of, for example, a computer and calculation software. The output unit outputs the result of arithmetic processing by the arithmetic unit, and can be configured by, for example, a display or a printer.

According to the dose calculation device configured in this way, the effective dose of the seismic isolation layer immediately below the medical linac room, for which there was no suitable simple calculation formula, can be calculated quickly and accurately without using the Monte Carlo method. .

(Verification of effects of the present invention)
Next, calculation results performed to verify the effects of the present invention will be described.

In this verification, the effect of the present invention was verified by comparing the calculation result (example) by the dose calculation method described above and the calculation result (comparative example) by the Monte Carlo method. The calculation conditions are that the X-rays are irradiated in a cone from a position 2.295 m away from the shield a, and the irradiation conditions are W = 360 Gy/h, A _IC = 0.16 m ² , and U = 1. bottom. The shield a and the floor e are made of ordinary concrete, the thickness of the shield a is 1.8 m, and the transmittance is obtained by the method described in Non-Patent Document 1 above, and B=1.18×10 ^{−42.1/ 180} . The height of the space b was set to 1.0 m, and the evaluation point E was positioned at the middle height of the space b, 3 to 9 m in the horizontal direction from the center axis of the X-ray cone. Also, the reflectance r of multiple reflection in the space b was set to 0.4. MCNP5 was used as the Monte Carlo calculation code of the comparative example, and points at horizontal distances of 1 m from the center axis of the cone used were evaluated.

FIG. 3 shows the comparison results. As shown in this figure, the maximum difference between the results of this example and the comparative example was 19%. Therefore, according to the present invention, it is possible to calculate with relatively high accuracy the effective dose of the seismic isolation layer immediately below the medical linac room, for which there was no suitable simple calculation formula in the past.

(Calculation method of reflectance of multiple reflection)
Next, an example of a method for calculating the reflectance of multiple reflection will be described.

In the above embodiment, the X-ray causes multiple reflection in the space b, and the effective dose can be easily calculated by treating the reflectance (multiple scattering rate) at that time as a constant value regardless of the energy attenuation. As shown, the reflectance of multiple reflections is required depending on the energy of the X-rays and the surrounding material. Therefore, the reflectance is calculated as follows.

First, the effective dose rates D _F and D _C when the evaluation point E is at the middle height of the space b are defined as D _F ′ and D _C ′, respectively. Since the reflectance is α _F =α _C and the distance from the reflection point to the evaluation point E is d _F =d _F , the following equation (6) holds when taking the ratio. It can be seen that _C ' and D _F ' should be obtained.

FIG. 4 is an example of a flow chart of a calculation program for obtaining the reflectance r of multiple reflection. As shown in this figure, first, the accelerator energy and the materials of the shield a and the floor e are input as parameters (step S11). The effective dose _DS is calculated at the evaluation point E located at the middle height of the space b by Monte Carlo calculation ignoring X-ray interactions on the floor surface d and the floor e (step S13). An effective dose D _S +D _C ' is calculated by Monte Carlo calculation for determining the effective dose of only X-rays reaching the evaluation point E from the shield a side with respect to the evaluation point E located at the same position (step S12). Further, the effective dose D _F ' is calculated by Monte Carlo calculation for determining the effective dose of only X-rays reaching the evaluation point E from the floor e side (step S15). D C ' is obtained from D _S and D _S +D _{C '} (step S14), and reflectance r is obtained from D _F ' and D _C ' (step S16).

According to the calculation method of the reflectance r described above, it is possible to calculate the reflectance r of multiple reflections in the space b after passing through the shield a by simply changing the calculation method of the effective dose in the program from one irradiation condition. can be done. Effective dose calculation of X-rays including multiple reflections in the path of interest becomes possible.

As described above, according to the dose calculation method according to the present invention, the dose of the outdoor space outside the radiation shielding shield provided around the radiation irradiation room having the radiation source for irradiating radiation is calculated. In the method, assuming that at least part of the radiation emitted from the radiation source and transmitted through the shield is scattered within the shield and reaches a predetermined evaluation point set in the outdoor space, The value of the contribution of the emitted radiation scattering within the shield and reaching the evaluation point, and the evaluation point after the radiation emitted from the source passes through the shield and is multiple-reflected in the outdoor space outside the shield. Since the dose at the evaluation point is obtained by summing the contribution value reaching the , the dose in the outdoor space such as the floor of the radiation irradiation room can be calculated quickly and accurately without using the Monte Carlo method.

Further, according to another dose calculation method according to the present invention, the reflectance of the multiple reflection of radiation in the outdoor space is assumed to be a constant value regardless of the attenuation of the energy, and the evaluation point is obtained by multiple reflection in the outdoor space. By determining the value of the contribution reaching

Further, according to another dose calculation method according to the present invention, the energy of the radiation emitted from the radiation source, the material of the shield, the material of the object facing the surface of the shield across the outdoor space, and the radiation After the radiation emitted from the source passes through the shield, it reflects off the surface of the object and travels to the evaluation point. Based on the dose rate due to the directed radiation, the reflectance of the multiple reflection of the radiation in the outdoor space is obtained, and using the obtained reflectance, the value of the contribution of the multiple reflection in the outdoor space to reach the evaluation point is obtained. , the dose of radiation including multiple reflections in the path of interest can be calculated quickly and accurately.

Further, according to the dose calculation program according to the present invention, since the computer executes the dose calculation method described above, the dose in the outdoor space can be calculated quickly and accurately without using the Monte Carlo method.

As described above, the dose calculation method and dose calculation program according to the present invention are useful for calculating the radiation dose due to photon beams in an outdoor space such as a seismic isolation layer immediately below a radiation irradiation room such as a medical linac room. , which is particularly suitable for calculating the effective dose quickly and accurately.

E evaluation point S radiation source a shield b space c surface d surface e floor (or wall)

Claims (3)

Calculate the dose of an outdoor space formed between a radiation shield provided around a radiation irradiation room having a radiation source and a structure provided outside the shield. a method,
Assuming that at least part of the radiation emitted from the radiation source and transmitted through the shield is scattered within the shield and reaches a predetermined evaluation point set in the outdoor space,
obtaining a first dose due to the radiation emitted from the radiation source scattering within the shield and reaching the evaluation point; and after the radiation emitted from the radiation source passes through the shield, obtaining a second dose due to multiple reflection in the outdoor space outside the shield and reaching the evaluation point from the surface of the structure on the outdoor space side ; and radiation emitted from the radiation source. A step of obtaining a third dose by reaching the evaluation point from the outdoor space side surface of the shield through multiple reflection in the outdoor space outside the shield after passing through the shield; , adding the first dose, the second dose, and the third dose to determine the dose at the evaluation point;
The first dose is a maximum dose rate at a position in the radiation irradiation chamber separated from the radiation source by a predetermined distance in the irradiation direction of radiation, a transmittance of the shield, a directional utilization factor, and the shield. a radiation irradiation area on the surface on the outdoor space side of and a reflectance with respect to the evaluation point at a first reflection point that is the intersection of the axis of the irradiation direction of the radiation and the surface of the shield on the outdoor space side; Based on the distance from the radiation source to the first reflection point and the distance from the first reflection point to the evaluation point,
The second dose is the reflectance of multiple reflection of radiation in the outdoor space, the maximum dose rate, the transmittance, the directional utilization factor, and the radiation on the outdoor space side surface of the structure. and the reflectance with respect to the evaluation point at the second reflection point, which is the intersection of the axis of the irradiation direction of the radiation and the surface of the structure on the outdoor space side, and the second reflection from the radiation source obtained based on the distance to the point and the distance from the second reflection point to the evaluation point ,
The third dose is the reflectance of multiple reflection of radiation in the outdoor space, the maximum dose rate, the transmittance, the directional utilization factor, and the radiation on the outdoor space side surface of the structure. and the reflectance with respect to the evaluation point at the third reflection point, which is the intersection of the axis of the irradiation direction of the radiation and the surface of the shield on the outdoor space side, and the second reflection from the radiation source A dose calculation method , wherein the dose is calculated based on a distance to a point and a distance from the third reflection point to the evaluation point . 2. The dose calculation method according to claim 1, wherein the reflectance of multiple reflection of radiation in the outdoor space is assumed to be a constant value regardless of energy attenuation, and the second dose is calculated. A dose calculation program for causing a computer to execute the dose calculation method according to claim 1 or 2.

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