JP2003225315A - Radiation generator and radiation energy detector - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To solve the problem that an output radiation dose and a dose distribution in an irradiation field can be monitored in a real time during treatment in a medical linac but there is no means for monitoring radiation energy exerting effect on the dose distribution in a depth direction in a real time during treatment. <P>SOLUTION: The dose distributions in the irradiation fields formed on a beam axis and in the peripheral part of the beam axis are changed when radiation energy is changed. This change is utilized to monitor the ratio of doses by the electrode (8) arranged on the beam axis and the dose distribution monitoring electrodes (5a, 5b, 5c and 5d) arranged in the periphery of the electrode (8) so as to be symmetric with respect to the beam axis and the present value of radiation energy is calculated using the relation between a dose ratio predetermined by measurement and the change of radiation energy. <P>COPYRIGHT: (C)2003,JPO

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a radiation generator such as a medical linac, and more particularly to a radiation generator and radiation energy capable of detecting the energy distribution of output radiation and controlling the radiation dose to the human body. It concerns a detector.

[0002]

2. Description of the Related Art When radiation treatment is performed using a radiation generator such as a medical linac, the radiation dose given (absorbed) to the human body is the output radiation dose from the medical linac, the radiation beam (hereinafter referred to as "beam"). Beam) and the energy of the radiation in a plane perpendicular to the beam axis of the beam. Usually, the output radiation dose is a reference point (a distance from the radiation source on the beam axis that is at a specified human tissue depth (= rated treatment distance)) It means the absorbed dose given to the tissue and is the standard for evaluating the output of the radiation generator.

However, since the radiation output from the medical linac spreads radially, in order to accurately evaluate the output of the radiation generator, not only the radiation dose at one point on the beam axis but also the radiation dose It is also necessary to understand the distribution of radiation dose within the irradiated area (irradiation field). Further, when the radiation applied to the human body passes through the inside of the human body, the radiation is gradually attenuated while giving the radiation to the human body, so that the dose distribution can be made in the depth direction. Therefore, in order to estimate the radiation dose given to the human body, strictly speaking, it is necessary to consider the dose distribution in the depth direction. The dose distribution in the depth direction is determined by the energy of radiation.

The radiation dose absorbed by the human body cannot be measured directly during treatment. As an alternative to this direct measurement, the medical linac is equipped with a dose monitor that monitors the radiation dose in real time. The dose monitor consists of a parallel plate ionization chamber. FIG. 7 shows a schematic configuration thereof. In the figure, the high voltage electrodes 1 and 3 and the output radiation dose detecting electrode 2 form one parallel plate ionization chamber.

The output radiation dose detecting electrode 2 is sandwiched between the high voltage electrodes 1 and 3 in order to correct the positional displacement of the detecting electrode 2. Further, with such a configuration, the effective volume of the ionization chamber can be increased without increasing the electrode interval, and the detection signal can be increased. Although the effective volume is increased even if the electrode interval is widened, the electric field strength between the electrodes is weakened, so that the rate of recombination of ionized ions before they are collected by the electrodes is increased and the detection performance is deteriorated.

The procedure for monitoring the output radiation dose using this dose monitor will be described below. Prior to treatment, place a standard dosimeter (a dosimeter that serves as a standard for each radiation treatment facility) attached with a tissue equivalent substance such as water (a substance whose radiation characteristics are equivalent to human tissue) at the above rated treatment distance. , Measure the output radiation dose and distribution. This measurement estimates in advance the radiation dose and distribution absorbed by the human body that cannot be measured during actual radiation treatment.

On the other hand, the radiation dose and distribution at the linac output port are also measured by a dose monitor incorporated in the head portion of the linac. The radiation dose detected by this dose monitor is different from the radiation dose at the rated treatment distance measured by the standard dosimeter. However, both doses show a proportional relationship within the limits of dose rate and energy. The distribution also shows the same tendency. Therefore, by calibrating the radiation dose measured by the dose monitor to show the radiation dose at the rated treatment distance, it becomes possible to measure the output radiation dose by the dose monitor.

In FIG. 3, reference numerals 5a, 5b, 5c and 5d denote radiation dose distribution monitoring electrodes formed on the insulating sheet 4. Such electrodes are arranged symmetrically with respect to the beam axis. This electrode is for detecting the distribution of the radiation dose and controlling the distribution. Electrode 5
If a and 5c are used to detect the radiation dose distribution before and after, the electrode 5
b and 5d correspond to the left and right radiation dose distribution detection. Prior to treatment, the beam trajectory is adjusted so that the dose distribution in the irradiation field does not deviate from the beam axis while measuring the dose distribution using a measuring device such as the standard dosimeter.
Under normal conditions, the dose monitor electrodes 5a, 5b, 5c, 5
The signals detected at d are equal.

During treatment, the electrodes 5a, 5c are somehow caused.
Alternatively, if there is a difference between the detection signals of the electrodes 5b and 5d, it is possible to take safety measures such as stopping the generation of radiation as an abnormal dose distribution in the irradiation field. Further, usually, a medical linac has a function of displacing a beam trajectory back and forth and left and right by an electromagnet called a steering magnet to change a dose distribution back and forth and left and right. Therefore, by constructing a system that keeps the dose distribution in the irradiation field constant by feeding back abnormal detection of the dose distribution in the irradiation field, controlling the beam trajectory, and maintaining the symmetry of the beam around the beam axis. You can

[0010]

As described above, the dose distribution in the irradiation field is monitored in real time during treatment by a dose monitor and is made constant by adjusting the beam orbit with a steering magnet. Is possible. However, the uniformity of the dose distribution here means the symmetry with respect to the beam axis, and does not mean a uniform distribution in the irradiation field in the strict sense. Also,
No consideration was given to the dose distribution in the depth direction. And the energy of radiation that affects the dose distribution in the depth direction was not monitored in real time during treatment. As a result, there has been a problem that the radiation dose given to the human body cannot be accurately estimated. The present invention has been made to solve such a problem, and an object of the present invention is to provide a radiation generator and a radiation energy detector capable of monitoring radiation energy during treatment, which was not possible with conventional apparatuses. And

[0011]

According to a first aspect of the present invention, there is provided a radiation generator which detects a radiation dose at a central portion of a radiation beam and a radiation dose at a peripheral portion of the radiation beam. A second radiation detector for detecting, and a beam control means for controlling the energy of the radiation beam based on a comparison between the output from the first radiation detector and the output from the second radiation detector Is.

Further, in the radiation generating apparatus according to claim 2 of the present invention, the first radiation detector is arranged on the beam axis of the radiation beam, and the second radiation detector is in the peripheral portion of the radiation beam. A plurality of them are arranged in.

Further, in the radiation generator according to the third aspect of the present invention, the plurality of second radiation detectors are arranged at positions symmetrical with respect to the beam axis.

According to a fourth aspect of the present invention, in the radiation generating apparatus, the first or second radiation detector is a parallel plate composed of a high voltage electrode and a detection electrode arranged to face the high voltage electrode. It has the structure of an ionization chamber.

Further, in the radiation generating apparatus according to claim 5 of the present invention, the high voltage electrode of the first radiation detector and the high voltage electrode of the second radiation detector are formed by a single metal flat plate, The detection electrode of the first radiation detector and the detection electrode of the second radiation detector are formed by separate metal parts formed on one insulating flat plate.

According to a sixth aspect of the present invention, there is provided a radiation energy detector which detects a radiation dose in a central portion of the radiation beam and a radiation dose detector in a peripheral portion of the radiation beam. 2 radiation detectors, and the first
Comparing means for comparing the relative magnitudes of the output from the radiation detector and the output from the second radiation detector,
In some cases, the energy of the radiation beam is detected based on the comparison result.

[0017]

BEST MODE FOR CARRYING OUT THE INVENTION A radiation generating apparatus according to the present invention has a group of dose distribution monitoring electrodes arranged symmetrically with respect to the beam axis on and around the beam axis in a dose monitor section, These electrodes allow the dose distribution during treatment to be monitored, and have a function of calculating the present value of energy from the change in the dose distribution in the plane perpendicular to the detected beam axis.

As described above, the beam trajectory is adjusted so that the dose distribution is uniform in the irradiation field while measuring the dose distribution using a measuring instrument such as a standard dosimeter prior to treatment, but the beam trajectory changes. In that case, the dose distribution will have a slope. As a result, the signals detected by the two electrodes symmetrically arranged on the beam axis are different from each other. On the other hand, when the energy of the radiation fluctuates, there is a difference in dose between the beam center axis and its surroundings, and the higher the energy, the higher the dose on the beam center axis.

As a result, a difference occurs in the signals detected between the electrodes on the central axis of the beam and the electrodes arranged around the electrodes. If the relationship between the signal difference and the energy change is determined in advance by measurement, the energy can be known during the treatment by the signal difference.

Further, in the radiation generator according to the present invention, if the current value of the calculated energy is different from the energy at the time when the dose distribution is measured in advance by using a measuring instrument such as a standard dosimeter, the energy is accelerated in the accelerating tube. It has a function of automatically changing the beam energy to be generated and correcting the energy of radiation.

Embodiment 1. An embodiment of the present invention will be described below with reference to FIG. In the figure, 6 is a radiation generating part of the linac body, and 7 is a dose monitor incorporated in the linac head part. Dose monitor
An electrode 8 on the beam axis as an electrode for monitoring the dose distribution,
Electrodes 5 arranged around it symmetrically with respect to the beam axis
a, 5b, 5c, 5d, and FIG. 1 shows this state in an enlarged manner.

The detection signals of the electrodes for monitoring the dose distribution are D8 (signal from the electrode 8 on the beam axis) and D5, respectively.
a, D5b, D5c, D5d (signals from the electrodes 5a, 5b, 5c, 5d symmetrically arranged with respect to the beam axis), the dose distribution is measured using a measuring instrument such as a standard dosimeter prior to treatment. However, since the beam trajectories and energies are adjusted so that the dose distribution in the irradiation field becomes uniform, in such a normal state, the respective detection signals become equal as follows. D8 = D5a = D5b = D5c = D5d In this normal state, the dose distribution in the depth direction is measured using a measuring instrument such as the standard dosimeter. An example of this dose distribution in the depth direction is shown in FIG. In the figure, the horizontal axis shows the distribution of radiation dose, and the vertical axis shows the depth from the surface of the irradiated body. Generally, the absorption amount of radiation becomes maximum at a certain depth from the surface and then attenuates. In the treatment plan, the dose given to the affected part in the body is calculated based on this dose distribution.

If there is a difference between the detection signals D5a, D5c or D5b, D5d of the peripheral electrodes, it means that the dose distribution which has been adjusted uniformly has a slope. An example of this state is shown in FIG. Such a signal difference occurs when the symmetry of the beam is broken as shown in FIG. In such a case, it has a function of controlling the steering magnetic field to change the beam trajectory so as to eliminate the difference between these detection signals. In FIG. 1, 14 is a steering magnet, and 10 is a control unit for detecting the gradient of the dose distribution and feeding it back to the steering magnetic field. Due to the function of the steering magnetic field controller, the detection signals D5a, D5c and D5b, D5d of the surrounding electrodes become equal,
The following relationship holds if the dose distribution does not have a directional gradient. D5a = D5b = D5c = D5d

Generally, the dose distribution of the braking X-ray generated when the electron beam collides with a metal such as a target used in a medical linac, the directivity toward the front becomes stronger as the energy of the electron beam becomes higher. There is a tendency.
Then, the dose at the center of the beam is attenuated by a conical metal called a flattening filter to make the dose distribution uniform in the irradiation field plane (FIG. 4A). However, for example, when the energy of the electron beam becomes higher than in the normal state, the directivity toward the front becomes stronger, so that the dose distribution becomes higher near the beam axis (FIG. 4B). on the other hand,
When the energy of the electron beam is lower than in the normal state, the directivity to the front is weakened, so that the dose distribution becomes lower near the beam axis (FIG. 4 (c)).

Therefore, if there is a difference between the detection signals D8 and D5a (= D5b = D5c = D5d) of the beam axis electrode and the surrounding electrodes, it can be determined that the radiation energy has a difference. Then, if the relationship between the change in energy and the ratio of detection signals (= D5a / D8) is determined in advance by measurement, the energy can be known from the value of D5a / D8. E = E0 + f (D5a / D8) E: Radiation energy during treatment E0: Radiation energy at normal time (pre-measured before treatment) f (X): Function of D5a / D8 and energy change, determined by actual measurement

FIG. 5 is a graph showing an outline of the function f (X). When D5a / D8 = 1 (when the detection signals of the beam axis electrode and the peripheral electrode are equal), f (1) = 0
And E = E0. Therefore, the radiation energy during treatment is the same as the normal radiation energy measured before treatment. When D5a / D8 <1 (when the detection signal of the surrounding electrode is lower than the detection signal of the beam axis electrode)
, F (1) <0 and E <E0. Therefore, the radiation energy during treatment is lower than the normal radiation energy measured before treatment. When D5a / D8> 1 (when the detection signal of the surrounding electrode is higher than the detection signal of the beam axis electrode), f (1)> 0 and E> E0. Therefore, the radiation energy during treatment is higher than the normal radiation energy measured before treatment. A unit for calculating energy from the detection signal of the dose distribution monitoring electrode based on this relationship is a block 9 shown in FIG.

Embodiment 2. Next, a second embodiment will be described with reference to FIG. In FIG. 2, 11 is a klystron which is a microwave amplifying tube for forming an accelerating electric field in an accelerating tube (also called a resonance cavity) for accelerating an electron beam, and 12 is a voltage applied to the klystron (specifically, a cathode electrode of the klystron). It is a klystron power supply that supplies the voltage applied to the klystron. The microwave amplified by the klystron is input to the accelerating tube and an accelerating electric field is formed in the accelerating tube. Reference numeral 13 denotes a block for feeding back the klystron applied voltage when the calculated energy is different from the normal state.

When the voltage applied to the klystron is increased, the microwave output is increased and the accelerating electric field of the accelerating tube is increased. This increase in the accelerating electric field increases the energy of the electron beam and increases the dose at the center of the beam dose distribution (decreases D5a / D8). By adjusting the voltage applied to the klystron in this way, the energy of the accelerated electron beam can be controlled so that the radiation energy is always constant. The strength of the accelerating electric field is determined by the increase and decrease of the voltage applied to the klystron. Although the accelerating electric field of the accelerating tube is fed back to the klystron applied voltage in order to control the electron beam energy in this example, it may be fed back to the output of the RF oscillator which is the klystron oscillation source.

[0029]

According to the present invention, it is possible to monitor radiation energy during treatment, which was not possible with the conventional apparatus. Therefore, prior to treatment, the radiation energy in the depth direction measured using a measuring device such as a standard dosimeter can be measured. Since it can be confirmed that the same dose distribution as the dose distribution is given to the human body, it is possible to perform more reliable radiotherapy than the conventional device.

[Brief description of drawings]

FIG. 1 is an explanatory diagram of a radiation generator according to a first embodiment of the present invention.

FIG. 2 is an explanatory diagram of the present invention and is an explanatory diagram showing a radiation dose distribution in a depth direction.

FIG. 3 is an explanatory diagram of the present invention, and is an explanatory diagram showing a detection signal in an irradiation field by a dose monitor.

FIG. 4 is an explanatory diagram of the present invention and is an explanatory diagram of a radiation dose distribution in an irradiation field.

FIG. 5 is an explanatory diagram of the present invention and is an explanatory diagram showing a relationship between a ratio of detection signals and energy change.

FIG. 6 is an explanatory diagram of a radiation generator according to a second embodiment of the present invention.

FIG. 7 is an explanatory diagram of a conventional radiation generator.

[Explanation of symbols]

1 high voltage electrode, 2 output dose detection electrode, 3 high voltage electrode, 4 output dose rate detection electrode, 5a, 5b, 5
c, 5d Radiation dose distribution monitoring electrode, 6 Radiation generation unit, 7 Dose monitor, 8 Beam axis electrode (for dose distribution monitoring), 9 Energy calculation unit, 10 Steering magnet (STC) control unit, 11 Klystron (KLY) , 12 Klystron (KLY) power supply, 1
3 Klystron (KLY) applied voltage control section,
14 Steering magnet (STC).

Claims (6)

[Claims] 1. A first radiation detector for detecting a radiation dose in a central portion of the radiation beam, a second radiation detector for detecting a radiation dose in a peripheral portion of the radiation beam, and the first radiation detector. And a beam control means for controlling the energy of the radiation beam based on a comparison between the output from the radiation detector and the output from the second radiation detector. 2. The first radiation detector is arranged on a beam axis of the radiation beam, and the plurality of second radiation detectors are arranged in a peripheral portion of the radiation beam. Item 2. The radiation generator according to item 1. 3. The radiation generator according to claim 2, wherein the plurality of second radiation detectors are arranged at positions symmetrical with respect to the beam axis. 4. The first or second radiation detector,
4. The radiation generator according to claim 1, wherein the radiation generator has a structure of a parallel plate ionization chamber including a high voltage electrode and a detection electrode arranged to face the high voltage electrode. 5. The high-voltage electrode of the first radiation detector and the high-voltage electrode of the second radiation detector are formed by a single metal flat plate, and the detection electrode of the first radiation detector and the first radiation detector are connected to each other. The radiation generating device according to claim 4, wherein the detection electrode of the radiation detector of No. 2 is formed by a separated metal portion formed on one insulating flat plate. 6. A first radiation detector for detecting a radiation dose in a central portion of the radiation beam, a second radiation detector for detecting a radiation dose in a peripheral portion of the radiation beam, and the first radiation detector. A radiation energy detector comprising: a comparison means for comparing the relative magnitudes of the output from the radiation detector and the output from the second radiation detector, and detecting the energy of the radiation beam based on the comparison result.