JP2012042415A - Dose distribution measurement apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To highly accurately measure a distribution of dose of radiation beams.SOLUTION: A holder 108 holds a plurality of parallel plate ionization chambers 104 in a stacked state. The holder 108 has a spacer 120 for forming a space between the parallel plate ionization chambers 104. The parallel plate ionization chamber 104 has an ionization chamber case 201 internally forming an ionization chamber cavity 202 and an ionization chamber electrode 203 formed on an inner surface of the ionization chamber case 201 in contact with the ionization chamber cavity 202. The ionization chamber electrode 203 has a signal electrode 203a to which a signal is inputted, and a high-voltage electrode 203b to which a high potential is inputted, and the plurality of parallel plate ionization chambers and the holder are disposed in water 103.

Description

  The present invention relates to a dose distribution measuring apparatus that measures a dose distribution of a radiation beam used for, for example, radiotherapy of cancer.

  In general, in radiation therapy for cancer, in order to confirm the energy and shape of radiation beams such as X-rays, electron beams, and particle beams used for treatment, before irradiating a patient with a beam, in a water phantom that simulates the human body Measure the dose distribution. In addition, in order to adjust the radiation irradiation device such as an accelerator and to confirm the beam energy distribution and shape which are different for each patient, the dose distribution is routinely measured as a quality control of the radiation beam.

  For example, as described in Patent Document 1, a conventional dose distribution measuring apparatus includes a film holder that supports a plurality of acrylic resin plates stacked and holds a radiation detection film between the plates. I have. Then, the film holder is attached in a hollow spherical container filled with distilled water or the like so that the radiation dose distribution can be measured at a plurality of cross-sectional locations. With this structure, it is possible to acquire dose distribution data in three dimensions with a single measurement.

Utility Model Registration No. 3096924 (Steps 0011 to 0016)

  The conventional dose distribution measuring apparatus is different from water that is a human tissue because the peripheral member of the radiation detection film that is a sensor is made of acrylic resin instead of water. Therefore, measurement accuracy is reduced in dose distribution measurement for cancer treatment that requires measurement in water. In particular, in particle beam therapy using proton beam, carbon beam, etc., the physical characteristics with respect to radiation are indistinguishable with water and acrylic resin.

  The present invention has been made to solve the above-described problems, and an object thereof is to perform radiation beam dose distribution measurement with high accuracy.

  A holder for holding a plurality of parallel plate ionization chambers in a laminated form, the holder has a spacer that forms a space between each of the parallel plate ionization chambers, and the parallel plate ionization chamber has an ionization chamber cavity inside thereof. An ionization chamber case to be formed and an ionization chamber electrode formed on the inner surface of the ionization chamber case in contact with the ionization chamber cavity, the ionization chamber electrode being a signal electrode to which a signal is input and a high voltage electrode to which a high voltage potential is input Have The plurality of parallel plate ionization chambers and holders are arranged in a water phantom.

  Since the dose distribution measuring apparatus according to the present invention interposes water between parallel plate ionization chambers when placed in a water phantom, the radiation absorption characteristic of the apparatus as a whole is very close to water, that is, human tissue. The dose distribution measurement of the radiation beam can be performed with high accuracy.

It is a figure which shows the dose distribution measuring apparatus by Embodiment 1 of this invention. It is a perspective view which shows schematic structure of the parallel plate ionization chamber of FIG. It is a figure which shows the ionization chamber case by the side of the signal electrode of the parallel plate type ionization chamber of FIG. It is a figure which shows the ionization chamber case by the side of the high voltage electrode of the parallel plate type ionization chamber of FIG. It is sectional drawing of the parallel plate type ionization chamber cut | disconnected by AA of FIG.3 and FIG.4. It is sectional drawing of the parallel plate type ionization chamber cut | disconnected by BB of FIG.3 and FIG.4. It is a figure explaining the principal part and calibration method of the dose distribution measuring apparatus by Embodiment 2 of this invention.

Embodiment 1 FIG.
FIG. 1 is a diagram showing a dose distribution measuring apparatus according to Embodiment 1 of the present invention. Water 103 is stored inside the water tank 102 to form a “water phantom” equivalent to a human tissue. The dose distribution measuring apparatus 1 includes a parallel plate ionization chamber 104 (abbreviated as ionization chamber 104 as appropriate), a support rod 110 that supports the parallel plate ionization chamber 104, and a support plate 111 to which the support rod 110 is fixed. And a driving device 112 for driving the parallel plate ionization chamber 104 and a ball screw 113 that is rotated by the driving device 112 and moves on the support plate 111. The drive device 112 is installed in the drive device installation part 114 of the water tank 102. The parallel plate ion chamber 104 includes an ion chamber case 201, an ion chamber cavity 202, and an ion chamber electrode 203. The ionization chamber cavity 202 is formed by an ionization chamber case 201. In FIG. 1, the parallel plate ionization chamber 104 shows a cross section, and the holder 108 has a bowl shape (U-shaped lateral shape) when viewed from the traveling direction of the radiation beam 101.

  Before irradiating the patient, a plurality of parallel plate ionization chambers 104, which are irradiated with a therapeutic radiation beam 101 such as an X-ray, an electron beam, and a particle beam and positioned in the water, are used. By measuring the dose distribution in the water phantom, it is possible to confirm whether the energy and beam profile of the radiation beam 101 are appropriate. Next, the outline of the dose distribution measuring apparatus 1 and the significance of the present invention will be described.

  Each parallel plate ionization chamber 104 is held in a laminated form by a holder 108 provided with a spacer 120 so that water 103 can be interposed between the ionization chambers 104. The spacer 120 forms a space between the ionization chambers 104. The parallel plate ion chamber 104 mainly comprises an ion chamber case 201 having a side wall, an ion chamber electrode 203 formed on the inner surface of the ion chamber, and an ion chamber cavity 202. A DC voltage is generated between the two ion chamber electrodes 203. Applied. The ionization chamber cavity 202 is filled with gas, but is usually filled with air. When the radiation beam 101 is incident and energy is applied to the air between the two opposing electrodes 203, a part of the air is ionized into electrons and ions by the ionizing action of the radiation. The ionized electrons and ions flow in opposite directions due to the electric field applied between the two opposing electrodes 203 and are observed as a current. Since this current value is proportional to the energy applied to the air by radiation per unit time, the current value that is the ionization chamber output of the parallel plate ionization chamber 104 or the charge amount that is the integral value of the current is measured. Can know the dose rate or dose. By arranging a plurality of parallel plate ionization chambers 104 at predetermined intervals and measuring them simultaneously, the dose distribution in the water phantom can be measured in a short time.

  If the plurality of parallel plate ionization chambers 104 are stacked in the same direction as the traveling direction of the radiation beam 101, the dose distribution in the depth direction of the beam 101, that is, the deep dose distribution is measured. The deep dose distribution greatly depends on the energy of the radiation beam 101, the thickness and shape of the scatterers and filters of the particle beam therapy apparatus, and is important information for quality control of the beam 101 in cancer radiotherapy. Therefore, ensuring the accuracy of the deep dose distribution measurement is essential for improving the quality of treatment.

  Deep dose distribution refers to the dose distribution in water that is equivalent to human tissue. However, if there is a substance other than water in the water, it becomes an inhomogeneous medium, so that the radiation absorption characteristic is different from that in the case of water, and an accurate dose distribution in water cannot be obtained. Therefore, when a large number of ionization chambers are arranged and high-speed measurement of the dose distribution is performed by simultaneous measurement, it is necessary to reduce the non-uniformity of the medium as much as possible and bring it close to water equivalent to ensure measurement accuracy.

  Therefore, in the dose distribution measuring apparatus 1 according to the present invention, water 103 is interposed between the parallel plate ionization chamber 104 and the parallel plate ionization chamber 104 as shown in FIG. That is, most of the space between the parallel plate ionization chamber 104 and the parallel plate ionization chamber 104 was water. The ionization chamber case 201 is made of a synthetic resin that has a physical property with respect to radiation close to that of water, and the wall thickness through which the radiation beam 101 passes is as thin as possible. In addition, the synthetic resin whose physical property with respect to radiation is close to water refers to a resin having a density of about 1 and an effective atomic number of about 7, for example, acrylic, polystyrene, polyethylene, and the like. Further, the ionization chamber electrode 203 formed on the inner surface of the ionization chamber case 201 is made of graphite, and is much closer to the characteristics of water than a metal electrode used in a normal ionization chamber, and the electrode thickness is very high. Thin, for example a thin film. The density of the air in the ionization chamber cavity 202 is about 1/1000 compared to water, and the influence on radiation is about 0.1% of water, which is negligible. With such a structure, most of the water travels on the path along which the radiation beam 101 travels. Therefore, the speed can be increased by simultaneous measurement using a number of ionization chambers, and the measurement accuracy of the dose distribution can be ensured.

  In the following, a configuration for making most of the path along which the radiation beam 101 travels water and reducing (thinning) structures other than water will be described. FIG. 2 is a perspective view showing a schematic configuration of one parallel plate ionization chamber 104. FIG. 3 is a diagram showing an ionization chamber case on the signal electrode side of the parallel plate ionization chamber, and FIG. 4 is a diagram showing an ionization chamber case on the high voltage electrode side of the parallel plate ionization chamber. The outer wall of the parallel plate ionization chamber 104 is made up of ionization chamber cases 201a and 201b. The outer periphery of the parallel plate ionization chamber 104 is sealed with a resin or O-ring (not shown) to prevent water from entering the ionization chamber. . Inside the ionization chamber cases 201a and 201b is an ionization chamber cavity 202. On the inner surface of the ionization chamber case 201a, among the ionization chamber electrodes 203 of the ionization chamber, a signal electrode 203a to which a signal is input and a signal lead pattern 204a that is a lead portion are formed thinly. On the inner surface of the ionization chamber case 201b, among the ionization chamber electrodes 203 of the ionization chamber, a high voltage electrode 203b to which a high voltage potential is inputted and a high voltage lead wire pattern 204b which is a lead portion are formed thinly.

  The electrodes 203a and 203b and the lead wire patterns 204a and 204b are formed, for example, by thinly applying graphite or conductive carbon paste on the inner wall surfaces of the cases 201a and 201b. The lead wire patterns 204a and 204b are connected to lead wires 205a and 205b for taking out the signal wires and the high voltage wires to the outside of the ionization chamber. Here, the signal lead wire 205a and the high-voltage side lead wire 205b are provided at different positions on the projection surface when the surfaces on which the electrodes 203a and 203b are arranged are projected. By adopting such a configuration, the entire ionization chamber can be made thin while securing a space necessary for taking out the lead wires 205a and 205b to the outside. In addition, the signal line close to the ground potential and the high-voltage line on which high voltage is applied can be separated from each other, ensuring an insulation distance, improving reliability against dielectric breakdown, and reducing noise by reducing leakage current. Can be realized.

In FIG. 3, a guard electrode 207, a lead wire guard pattern 208, and a connection portion 214 are formed around the signal electrode 203 a, the signal lead wire pattern 204 a, and the connection portion 215. Further, the signal line lead pattern 204 a is electrically connected to the signal line lead wire 205 a via the connection portion 215. The lead wire guard pattern 208 is electrically connected to the connection portion signal shield 210 a via the connection portion 214. The signal line shield 210a is grounded (not shown). In FIG. 3, the signal line shield 210a is described as being arranged in parallel with the signal lead wire 205a. However, in order to further enhance the shielding performance, the signal lead wire 205a is a core wire, and the signal line shield 210a is a peripheral portion. The form of a coaxial cable is desirable. By providing the guard electrode 207 and grounding in this way, the lead wire guard pattern 208 and the guard electrode 207 are maintained at the same potential. Since the guard electrode 207 and the guard pattern 208 for the lead wire that are the guard portions surround almost all of the periphery of the signal electrode 203a and the signal lead wire pattern 204a, the inner surface of the ionization chamber case 201a that forms the ionization chamber cavity 202 ( Most of the leakage current flowing on the surface of the inner surface in contact with the ionization chamber cavity 202 flows into the guard portion and can block the inflow to the signal electrode 203a. Thereby, noise caused by leakage current mixed in the signal line (signal electrode 203a, lead wire pattern 204a) through which the signal of the parallel plate ionization chamber 104 flows can be suppressed low, and the measurement accuracy can be improved.

  In FIG. 4, the high-voltage electrode 203b and the high-voltage lead wire pattern 204b are electrically connected to the high-voltage lead wire 205b through the connection portion 217. The high-voltage shield 210b is grounded (not shown), and the high-voltage electrode 203b and the high-voltage lead pattern 204b, which are high-voltage portions, are protected to ensure insulation safety. In FIG. 4, the arrangement is shown in parallel with the high-voltage lead wire 205b. However, as in the case of the signal wire in FIG. 3, the high-voltage lead wire 205b is a core wire and the high-voltage shield 210b is arranged to further enhance the shielding performance. The form of a coaxial cable as the peripheral portion is desirable. Further, the guard portions 207 and 208 in FIG. 3 extend to the connection portion 211a and are electrically connected to the connection portion 211b and the high-voltage shield 210b in FIG.

  5 is a cross-sectional view of the parallel plate ionization chamber cut along AA in FIGS. 3 and 4, and FIG. 6 is a cross-sectional view of the parallel plate ionization chamber cut along BB in FIGS. is there. In FIG. 5, the high-voltage lead wire 205b and the high-pressure shield 210b are passed through a tube 212 formed inside the ionization chamber case 201b. With such a structure, insulation can be ensured and the entire ionization chamber 104 can be made thin. Further, the connecting portions 211a and 211b in FIG. 6 are electrically connected by a connecting body 218 such as a metal part or a conductive paste. As a result, the signal line (signal lead wire 205a) and the high voltage line (high voltage lead wire 205b) are grounded by the shield wires (signal shield 210a and high voltage shield 210b) connected to the guard portion. Since it is enclosed, it is possible to improve measurement accuracy and ensure safety.

  The parallel plate ion chamber 104 is placed in water as shown in FIG. Since the walls on the path along which the radiation beam 101 travels in the cases 201a and 201b are formed as thin as possible, deflection due to water pressure may occur and the volume of the sensitive part may change, which may affect the measurement accuracy. . In order to avoid this, the pressure of a gas such as air enclosed in the ionization chamber case 201 may be increased to the same level as the water pressure at the depth of the ionization chamber cavity 202. Thereby, since the volume fluctuation | variation of the sensitive part of an ionization chamber can be suppressed, measurement accuracy can be ensured. Also, as shown in FIGS. 2 to 4, a support 206 is provided in the inside of the ionization chamber cavity 202, and the inner surfaces of the ionization chamber cases 201a and 201b are pressed to each other so as to suppress the deflection as much as possible. Thereby, since the volume fluctuation | variation of the sensitive part of an ionization chamber can be suppressed, measurement accuracy can be ensured.

  With the structure described above, the dose distribution measuring apparatus 1 of the first embodiment uses water as a major part of the path along which the radiation beam 101 travels, and the radiation beam 101 in the cases 201a and 201b, which is a part other than water. The walls on the traveling path, the ionization chamber electrode 203, and the ionization chamber cavity 202 can be made as thin as possible, and at the same time, the measurement accuracy can be improved and the insulation and safety can be ensured.

  Further, the signal electrode 203a and the high voltage electrode 203b have dimensions smaller than the upper and lower surfaces of the ionization chamber cavity 202 (wall surfaces on the path on which the radiation beam 101 travels in the cases 201a and 201b). Since the space between the opposing electrode surfaces becomes a sensitive part of the ionization chamber 104, the sensitive part becomes smaller than the entire ionization chamber cavity 202. By adopting such a configuration, the output of the ionization chamber 104 is not affected by the dose distribution distortion caused by the radiation beam being scattered on the side wall portions of the ionization chamber cases 201a and 201b. Can measure.

  As shown in FIG. 1, a structure integrated by a plurality of parallel plate ionization chambers 104 and a holder 108 is connected to a moving stage such as a support rod 110 and a support plate 111, and the moving stage is connected via a ball screw 113. The driving device 112 is connected. Thereby, the dose distribution of the arbitrary positions in the water tank can be measured. In addition, by making the moving step of the driving device 112 smaller than the installation interval of the ionization chamber 104, it is possible to increase the substantial spatial resolution. Even in the case where there is no configuration connected to the moving stage and the driving device 112, the dose distribution measuring device 1 can make the radiation absorption characteristic of the device as a whole very close to that of water, that is, human tissue. The dose distribution measurement of the beam can be performed with high accuracy.

  As described above, the dose distribution measuring apparatus 1 according to the first embodiment includes the holders 108 that hold a plurality of parallel plate ionization chambers 104 in a stacked shape, and the holders 108 are spaced between the parallel plate ionization chambers 104. The parallel plate ionization chamber 104 includes an ionization chamber case 201 in which an ionization chamber cavity 202 is formed, and an ionization chamber formed on the inner surface of the ionization chamber case 201 in contact with the ionization chamber cavity 202. Since the electrode 203 has a signal electrode 203a to which a signal is input and a high voltage electrode 203b to which a high voltage potential is input, the ionization chamber electrode 203 is disposed between parallel plate ionization chambers when placed in a water phantom. Water can be interposed, and the radiation absorption characteristic of the entire device can be made very close to that of water, that is, human tissue. It can be carried out in.

Embodiment 2. FIG.
FIG. 7 is a diagram for explaining the structure and the calibration method of the main part of the dose distribution measuring apparatus according to the second embodiment of the present invention. First, the structure will be described. Since the configuration other than the parallel plate ionization chamber 104 and the holder 108 is the same as that of the first embodiment, the description thereof is omitted. In Embodiment 2 of the present invention, among the plurality of parallel plate ionization chambers 104a, 104b, 104c, 104d, 104e, 104i, 104j, and 104k, the parallel plate ionization is located upstream in the traveling direction of the radiation beam 101. The portion of the box 104a is detachable. That is, the parallel plate ion chamber 104a is detachably held by the holder 108. The parallel plate ion chambers 104a, 104b, 104c, 104d, 104e, 104i, 104j, and 104k are arranged at a cell pitch L that is a predetermined interval. The reason for doing this is as follows.

  In general, a dose distribution measuring apparatus having a large number of detector cells (corresponding to the parallel plate ionization chamber 104 of the present invention) requires calibration work for correcting sensitivity variations and aging variations in each cell. In the second embodiment, the calibration work of the parallel plate ionization chamber 104 can be performed in a short time, and the relative calibration work of the multi-cell detector can be greatly saved.

Next, a calibration method for the dose distribution measuring apparatus will be described. FIG. 7 shows an example of the calibration work. FIG. 7A is a diagram for explaining the first measurement, and FIG. 7B is a diagram for explaining the second measurement. The radiation beam 101 is a substantially stationary beam that does not change with time. At this time, if the system is physically the same, the air dose field generated by the radiation beam 101 does not change with time. In the calibration work, the radiation beam 101 is irradiated a plurality of times, and the measurement is performed by moving the position of the ionization chamber to a predetermined position in each irradiation. First, in the first measurement, a certain measured value is obtained in each parallel plate ion chamber (detector cell) 104. The first measurement results of the parallel plate ion chambers 104a, 104b, 104c, 104d, 104e, 104i, 104j, and 104k, which are detector cells, are respectively M1a, M1b, M1c, M1d, M1e, M1i, M1j, and M1k. To do.

  Next, only the parallel plate ionization chamber 104a is removed from the holder 108, the detector position is shifted to the upstream side of the beam 101 by the same distance as the cell pitch L, and the second measurement is performed. At this time, the position of the first parallel plate ion chamber 104a and the second parallel plate ion chamber 104b are in the same position. Similarly, for the detector cell on the downstream side of the parallel plate ionization chamber 104b, the second position is the same as the position of the previous upstream detector cell in the first time. There is no other detector cell upstream of the parallel plate ionization chamber 104b, and since the spatial dose field generated by the radiation beam 101 does not change with time, the dose received by the first parallel plate ionization chamber 104a. The dose received by the second parallel plate ionization chamber 104b is the same.

If the measurement results of the parallel plate ion chambers 104b, 104c, 104d, 104e, 104i, 104j, and 104k by the second irradiation are M2b, M2c, M2d, M2e, M2i, M2j, and M2k, respectively, M1a and M2b are completely Since it is the result measured in the same dose field, the relative sensitivity k (ba) of 104b with respect to the parallel plate ion chamber 104a is expressed as in equation (1).
k (ba) = M2b / M1a (1)

The other detector cells can be considered in the same manner. For example, the relative sensitivity k (cb) of 104c with respect to the parallel plate ionization chamber 104b is expressed as shown in Expression (2).
k (cb) = M2c / M1b (2)

  Thus, since the relative sensitivity of each detector cell next to each other can be obtained, the relative sensitivity of all the detector cells can be obtained by sequentially performing this for all the detector cells. Since the calibration constant of each detector cell is the reciprocal of the relative sensitivity obtained as described above, all cells can be calibrated. For example, it is assumed that the parallel plate ion chamber 104a has been calibrated by another method. The detector cell 104b may be calibrated to be 1 / k (ba) times. The detector cell 104c may be calibrated to be 1 / (k (cb) × k (ba)) times. Similarly, the detector cell 104j, which is another detector cell, may be calibrated to be 1 / K times. Here, K is obtained by multiplying all the relative sensitivities k up to the detector cell 104j.

  As described above, the dose distribution measuring apparatus 1 according to Embodiment 2 has a structure in which the parallel plate ionization chamber 104a is detachable, so that the detector position is shifted by the same distance as the cell pitch L and the parallel plate ionization chamber. All detector cells can be calibrated in a short period of time by simply performing a number of measurements less than 104, for example, twice, and the relative calibration work of the multi-cell detector can be greatly saved. Note that the radiation beam 101 irradiation, the number of times the detector is driven, and the number of measurements are not limited to two as described above, and the accuracy and reliability of the calibration data can be improved by increasing the number of times.

DESCRIPTION OF SYMBOLS 1 ... Dose distribution measuring device, 104, 104a, 104b, 104c, 104d, 104e, 104i, 104j, 104k ... Parallel plate ionization chamber, 108 ... Holder, 112 ... Driving device, 120, 120a, 120b, 120c, 120d, 120e, 120i, 120j ... spacer, 201, 201a, 201b ... ionization chamber case, 202 ... ionization chamber cavity, 203 ... ionization chamber electrode, 203a ... signal electrode, 203b ... high voltage electrode, 204 ... lead wire pattern, 204a ... signal lead Wire pattern, 204b ... High voltage lead wire pattern, 205a ... Signal lead wire, 205b ... High voltage lead wire, 206 ... Post, 207 ... Guard electrode, 208 ... Lead wire guard pattern, 210a ... Signal shield, 210b ... High voltage Shield for 212, pipe, L ... cell pitch.

Claims (12)

A dose distribution measuring device having a plurality of parallel plate ionization chambers arranged in a water phantom,
A holder for holding a plurality of the parallel plate ionization chambers in a laminated form,
The holder has a spacer that forms a space between each of the parallel plate ionization chambers, and the parallel plate ionization chamber is in contact with an ionization chamber case that forms an ionization chamber cavity therein and the ionization chamber cavity. An ionization chamber electrode formed on the inner surface of the ionization chamber case;
The ionization chamber electrode has a signal electrode to which a signal is input and a high voltage electrode to which a high voltage potential is input. The ionization chamber case is made of synthetic resin,
The dose distribution measuring apparatus according to claim 1, wherein the ionization chamber electrode is a graphite thin film. A signal lead wire pattern is connected to the signal electrode, a high voltage lead wire pattern is connected to the high voltage electrode, and the signal lead wire pattern and the high voltage lead wire pattern include the surface on which the signal electrode is disposed and the high voltage The dose distribution measuring device according to claim 1 or 2, wherein the projection surface is provided at different positions on the projection surface when the surfaces on which the electrodes are arranged are projected in an overlapping manner.   4. The dose distribution measuring apparatus according to claim 3, wherein a guard electrode is disposed around the signal electrode, and a guard pattern is disposed around the signal lead pattern. A signal shield is arranged around the signal lead wire connected to the signal lead wire pattern, and a high voltage shield is arranged around the high voltage lead wire connected to the high voltage lead wire pattern,
The dose distribution measuring apparatus according to claim 3 or 4, wherein the signal shield is electrically connected to the high voltage shield. The signal shield and the signal lead are arranged in a first tube formed in the ionization chamber case,
6. The dose distribution measuring apparatus according to claim 5, wherein the high-voltage shield and the high-voltage lead are arranged in a second tube formed in the ionization chamber case.   The dose distribution measuring apparatus according to any one of claims 1 to 6, wherein the ionization chamber electrode is smaller than an inner surface of the ionization chamber case in which the ionization chamber electrode is formed.   The pressure of the gas sealed in the ionization chamber cavity is approximately the same as the water pressure at the water depth of the ionization chamber cavity when the ionization chamber cavity is disposed in the water phantom. The dose distribution measuring device according to item.   The dose distribution measuring apparatus according to any one of claims 1 to 8, further comprising a column that presses two inner surfaces of the ionization chamber case on which the ionization chamber electrode is formed.   The dose distribution measuring apparatus according to any one of claims 1 to 9, further comprising a driving device for moving a plurality of parallel plate ionization chambers held by the holder.   11. The dose distribution measuring apparatus according to claim 1, wherein one of the parallel plate ionization chambers is detachably held on the holder.   The dose distribution measuring apparatus according to claim 11, wherein the parallel plate ionization chambers are arranged at predetermined intervals.

Images