SU1338154A1 - Method of monitoring beam parameters in proton therapy - Google Patents

Abstract

To understand the accuracy of the radiation beam correspondence to the pathological focus, two identical position-sensitive detectors are installed one near the patient, the other at the exit from the last focusing doublet of magnetic lenses. The beam intensity density is determined, the measured current density densities are quantitatively compared with the allowable values, and when they are exceeded, the beam is turned off.

Description

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The invention relates to the field of melcinia, 1) in particular to medical radiology and, oncologic, and can be used to control the parameters of the beam in proton therapy.

The aim of the invention is to improve the accuracy of the radiation beam correspondence to the pathological focus, which is achieved by determining the density of the beam intensity by means of a sensitive detector with a quantitative comparison of the measured current intensity densities with the allowable values, and if they are exceeded, the beam is turned off.

To achieve the goal, the synchrocyclotron beam parameters are monitored during proton Yr therapy. Two identical position-sensitive detectors (proportional chambers) are used by this, one of the Eichs is installed near the patient, the other is at the output of the last focusing doublet of magnetic lenses.

. Before laying down the patient, a proton beam is switched on and the current from each signal wire of the proportional chambers is measured. The wires are oriented along the horizontal and vertical directions. The currents from all signal wires are measured simultaneously with the registration of all particles of the proton beam macro-bunch. The amplitude spectrum of the currents flowing through the wires gives the desired distribution of the proton intensity density along the horizontal and vertical axis (i.e., one-dimensional projection of the two-chamber intensity density distribution over the beam section ). This distribution is measured using a capacitor on the capacitor and is transmitted in parallel to an additional block for visual control, then by an electronic deviation detection circuit (COO) - to dry out a quantitative control of the dynamic changes in the beam parameters during proton therapy and (if necessary) for shutdown of the beam, and on the computer - to obtain a documented picture of the irradiation.

The visual and quantitative control of the beam parameters with the help of COOs is carried out simultaneously during the whole time of revision. Intensity density distribution on a semi5A2 computer

also during the entire irradiation session,

flo the size of the irradiated target is formed by the proton beam so that it blocks the irradiated target, without affecting the surrounding healthy tissue. The horizontal and vertical coordinates of the maximum of the intensity density distribution and the beam boundary, i.e. the numbers of the corresponding signal wires and channels of the amplitude spectrum. The boundaries of the beam are determined by distribution points at which the intensity density is 10% of the maximum.

At the COO, by adjusting the calibration voltage, two control (permissible) voltage values are set: one corresponds to the value of the current at the maximum of the distribution, and the other is of equal-sum intensity outside the beam boundaries.

Next, turn off the beam. The laser exposes the beam direction along the coordinates of the maxima of the distributions in two proportional chambers.

The patient is placed and the target is placed in the direction of the beam, the center of the target is combined with the maximum of the intensity density distribution and the boundary of the target with the beam boundary.

Include a bunch. At each proton microimpulse, on electronic comparators, the calibrated values of the voltages at the COO are compared with the measured current values of the currents outside the established channels. Finding an amplitude equal to the calibration voltages outside the established channels means that the beam was either displaced from the target, or expanded beyond the target boundary, or the intensity of the beam increased. In any of these cases, a signal is generated to turn off the beam. At the same time, the intensity density distribution in each macrobank is displayed on the oscilloscope screen, which allows the operator to visually monitor the size and position of the beam relative to the target.

PRI m and p r1. Implementation of the method with shutdown of the radiation beam during proton therapy.

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120 Gy with an irradiation floor size of 5 mm and a 50% isodose. It is known that the chiasm is located 5 mm from the center of the pituitary gland. |. The critical dose for chiasm is 10 Gy.

For irradiation under given conditions, a beam of protons with an energy of w 1 GeV was formed on the synchrocyclotron. At the location of the target, the beam diameter, measured at half the height of the intensity density distribution, ensured dose delivery mainly to the pituitary gland. The intensity of the beam was 7 10 s, which corresponded to the intensity density at the center of the beam -2.4 10. The beam was pulsed with a macrobunch duration of 0.4 n and a frequency of trace of 20 Hz.

The irradiation was conducted all the way from one direction in the absence of a finishing collimator directly in front of the patient. The dose rate at the 25th dose center was 5.4 Gy / min. The planned irradiation time is -22.2 min.

In order to reduce the radiation load on tissues lying in the path of the beam, radiation was performed by a convergent beam, and additional measures were taken, such as rotating the irradiation table around the target.

The beam was focused at the irradiation point g with a pair of short-focus quadrupole 20K50 lenses placed at a distance of 2.3 m from the irradiated target. The beam convergence was 0, .4. 40

The main controlled parameter both during the formation of the medical proton beam and in the course of irradiation was the density of the intensity, the horizontal and vertical distribution of which over the beam section was measured using two

position-sensitive detectors (proportional cameras). One chamber was installed on the output 50 lance of the last focusing magnetic element 20K-50 at a distance of 2.3 m from the center of the irradiated target, the second one - in close proximity to the patient at a distance of 0.30 n 55 from the target.

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proportional chambers. (The cords are oriented along the horns nt; r, no1o and vertical directions. Currents from all wires were measured simultaneously under the condition of registration of all particles of the proton bundle macro-bunch. The amplitude spectrum of the currents flowing through the wires gives the desired distributions of the proton intensity density along the horizontal and vertical axis .

The spatial resolution of the propoption of the Jam cameras was 1 mm, which made it possible to measure the intensity density simultaneously at 10–15 points of each distribution and ensured a high accuracy of determining the parameters of the proton beam.

The following parameters of the medical proton beam were determined from the measured four intensity density distributions:

the direction of the proton beam along the line connecting 5 "tsey maxima

spatial density distributions of intensity, measured

two detectors;

full intensity, as the area under the intensity density distribution curve; beam size - by the spatial position of the intensity density distribution at the level of 10% of the maximum;

the position of the beam relative to the irradiated target is specified by combining -. by irradiating the half-width field of the intensity density distribution measured by a detector located near the patient;

beam convergence, characterizing the beam focusing on the mingen, is determined by the formula:

FWHMj - FWHMz

tg "// 2 2L

beam convergence angle;

the full width at half-height of the intensity density distributions measured by two proportional chambers; distance between detectors. per oscillotraf - to visually monitor the beam parameters during the irradiation process; electronic deviation detection circuit (COO) - for quantitative control of the dynamic measurements of the beam parameters during proton therapy and, in the case of leaving the parameters, automatic beam shutdown; on a computer, to obtain a documented picture of the irradiation process. Quantitative control of parameters using COO was carried out simultaneously with visual control, carried out by the operator, during the entire irradiation time. An intensity density distribution on a computer was also obtained during the entire irradiation session. COO was connected to the second proportional chamber, which was installed in front of the patient. The horizontal intensity density distribution had a maximum in the 9th channel, and the beam boundaries in 4 14 channels of the X plane. The maximum of the vertical distribution fell into the 8th channel, and the beam boundaries - in the 3rd and 13th Channels of the U plane. The numbers of these channels were fixed by turning on the toggle switches on the COO scheme. The regulator, the calibration voltage at the COO, set the control (permissible) values of the voltage: V corresponds to the current value at the maximum of the distribution, and Vj is equal to the total intensity outside the beam boundaries. In each macro pulse of the proton, the COO quantitatively compared on electronic comparators the values of V and V and V that were set were compared with the measured current voltages. When finding the amplitude of a larger IL11 equal to V inside the beam boundaries or the amplitude exceeding the Vj outside the beam, the SOO scheme produced a signal to turn off the accelerator, which was turned off before the next macro proton beam arrived. After these preparatory works, the proton beam was turned off. The laser exposed the beam direction (along the coordinates of the distribution maxima in two proportional chambers). The patient was placed on a horizontal table. His bare face was fixed with a plastic 4b mask. The combination of the center of the microscope with the direction of the beam was achieved using an x-ray apparatus-the center of the torus and ensured an accuracy of no less than +0.5 mm. After which the laser was removed, the proton beam was turned on and the irradiation session started. After 10 minutes 40 seconds after the start of irradiation, the beam was turned off. It turned out that in the 4th and 14th channels of the Hin plane of the 3rd and 13th channels of the Y plane, the COO scheme detected amplitudes greater than the calibration voltages Vj. This meant that the beam widened. At the same time, the expansion (approximately 1.5 times) of the horizontal and vertical beam profiles in the second proportional chamber was noted by the operator on the oscilloscope cathode-ray tube. At the same time, the dose to chiasm increased 9 times. After 25 ms after changing the profile, the beam was automatically switched off by the impulse with COO. During the action of the wide beam before shutdown, the dose absorbed by the chiasm did not exceed 2 mGy. The test showed that one of the horizontal focusing lenses located along the beam formation path was turned off. Its mode has been restored. After checking the operating modes of all the magnetic elements of the medical proton channel, the beam was switched on again and the irradiation session was continued. The pause in irradiation was about 3 minutes. 20 min 10 s after the start of the irradiation session, the beam was again. disabled. It turned out that in the 8th channel along the X and in the 9th channel along the Y, values exceeding - were found. V. At the same time, amplitudes larger than V were observed in channels smaller than 4th by X and larger 13th by Y. The corresponding displacements of the horizontal and vertical profiles (approximately 1 channel) were recorded by the operator during visual inspection on an oscilloscope. This led to an increase in the dose per chiasm by a factor of 20 for the time of one Macrobanche 0.4 ms, after which (in the pause between the two macro bunch bunches) the accelerator was turned off by a signal from the COO. The absorbed chiasm dose was mGy. The beam shift from the center of irradiation of the target turned out to be associated with a small change in the current in about