RU2747365C1 - Method for controlling beam parameters during proton therapy and device for its implementation - Google Patents

Abstract

FIELD: medicine.

SUBSTANCE: invention is used to control the beam parameters during proton therapy. The essence of the invention lies in the fact that the measurement of the permissible values of the density of the beam intensity before irradiation of the patient is carried out using coordinate-sensitive detectors installed in front of the patient and behind the last scanning magnet, the quantitative comparison of the measured parameters of the intensity density with the permissible values and the shutdown of the beam when the parameters deviate from the permissible values, while measuring the permissible values of the density of the beam intensity additionally includes measuring the depth of the Bragg peak by means of a device for measuring energy release from ionizing radiation, installed at the patient's site, with the possibility of its subsequent removal from the beam axis when the patient is irradiated, and pad detectors are used as coordinate-sensitive detectors cameras on "warm liquid".

EFFECT: invention is aimed at increasing the conformity of proton therapy due to constant monitoring of the dose field boundary in the target.

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Description

Technology area

The proposed invention relates to the field of radiation therapy using for the treatment of protons and heavy ions and can be applied for practical dosimetry in determining the absorbed dose from a radiotherapeutic proton beam in a tissue-equivalent phantom for medical purposes, in particular, to proton therapy for irradiation with a scanning "pencil beam" small pathological foci with a high conformity of the high dose to the area of the pathological focus along the coordinate and dose.

Prior art

From patent RU2654838, publ. 05/22/2018, there is a known method for measuring the energy release from ionizing radiation in a tissue equivalent of a phantom, which consists in the fact that the energy release from ionizing radiation is measured by direct measurement of the particle path length by the Bragg peak to determine the absorbed dose from the calculated path-energy ratio when visualizing the glow of the energy release region by the matrix a CCD television photodetector with coupling optics, using the modes of combining pixels and adding television frames.

From patent RU179244, publ. 05/07/2018, a television detector of ionizing radiation is known, containing: a tissue-equivalent phantom for receiving a beam of ionizing radiation; a television camera on a CCD matrix, equipped with an optical lens system and configured to combine signals from a group of pixels of the CCD matrix and add frames to the output stage of the CCD matrix; a light-shielding casing with an entrance window made of a material impervious to light, but transmitting a beam of ionizing radiation; In this case, the tissue-equivalent phantom is installed inside the light-shielding casing and faces with its longitudinal axis towards the entrance window, and the lens of the optical system, which, together with the television camera, is placed inside the light-shielding casing, faces the tissue-equivalent phantom perpendicular to its longitudinal axis.

The device for measuring the energy release from ionizing radiation, disclosed in the above patents, namely, the digital image acquisition detector (DCPI), with the accuracy regulated by the IAEA, restores the depth of the Bragg peak for the forthcoming control of the dose field in depth and width before irradiating a patient for an accelerator pulse [Series of technical reports No. 398. Determination of Absorbed Dose in External Radiation Therapy: International Practice Guidelines for Dosimetry Based on Absorbed Dose Unit Standards in Water, IAEA, Vienna, 2004].

The closest is the method for controlling the beam parameters during proton therapy, known from the patent SU1338154, publ. 05/23/1988, in which on two identical gas proportional chambers (position-sensitive detectors) control the intensity of target irradiation. One camera is installed after the last quadrupole lens, and the second one in front of the patient.

The disadvantages of this method include low accuracy in determining the flux of protons passing through them, and the inability to determine the total dose of particles passing through them per accelerator pulse, and in addition, it is not possible to measure the Bragg peak depth along the z coordinate per pulse.

Disclosure of the essence of the invention

The objective of the present invention is to increase the conformity of proton therapy, namely the accuracy of monitoring the correspondence of the radiation beam to the pathological focus, when determining the profile of the absorbed dose by the depth and width of the Bragg peak before proton therapy sessions and monitoring the patient's irradiation during the session.

The technical result of the proposed invention is to increase the accuracy in determining the dose released in the target for each accelerator pulse. In addition, increasing the reliability, reducing the cost of the device (detector), simplifying the design by increasing the sensitivity of the chambers, since less expensive charge-sensitive amplifiers are used, and the chambers based on "warm liquid" are small in comparison with conventional air chambers.

The claimed technical result is achieved in a method for controlling the beam parameters during proton therapy, including measuring the permissible values of the density of the beam intensity by measuring the width of the Bragg peak using pad cameras on the "warm liquid" installed in front of the patient and behind the last scanning magnet and the depth of the Bragg peak by means of the device measuring the energy release from ionizing radiation, installed at the patient's site, with the possibility of its subsequent removal from the beam axis when the patient is irradiated, quantitative comparison of the measured intensity density parameters with the permissible values, and switching off the beam when the parameters deviate from the permissible values. In this case, a digital image acquisition detector is used as a device for measuring the energy release from ionizing radiation. The claimed technical result is ensured precisely due to the joint operation of a digital image acquisition detector (CDPI) and pad cameras on a "warm liquid" to measure the width and depth of the Bragg peak and adjust the profile of the Bragg peak in width due to information received from the pad cameras on the "warm liquid" ...

The claimed technical result is achieved by using a device for monitoring the parameters of the beam in the process of proton therapy, which includes coordinate-sensitive pad cameras on a "warm liquid" installed in front of the patient and behind the last scanning magnet, with the ability to determine the width of the Bragg peak, and a device for measuring energy release from ionizing radiation, installed at the patient's site, with the possibility of its subsequent removal from the beam axis when the patient is irradiated. In this case, as a device for measuring the energy release from ionizing radiation, a digital image acquisition detector - CDPI is used, with the help of which the depth of the Bragg peak is determined.

A distinctive feature of the claimed invention is the use of two plane-parallel coordinate-sensitive pad chambers in the method for controlling the beam parameters during proton therapy, the active substance of which is "warm liquid". The advantages of chambers based on "warm liquids" (i.e.) in comparison with ionization chambers in air and gas mixtures are well known.

Cameras based on "warm liquids", in addition to a two-dimensional distribution along the coordinate, give an instantaneous dose distribution over the width and height of the beam directed to the target per pulse.

The characteristics of the "warm liquids" with which pad chambers can be filled are shown in Table 1.

Table 1. Electronic properties of "warm liquids".

Liquid Boiling point (Т), ° С Relative dielectric constant (ε) Mobility of free charge carriers (μ), m ² / V / s Free electron pair output (G _if ) Density of liquid (d), g / cm ³ Isooctane 99.24 1.936 5.3 · 10 ^-4 0.33 0.69 Tetramethylpentane 140.3 2.05 29 · 10 ^-4 0.42 0.72 Tetramethylsilane 26.65 1.84 105 · 10 ^-4 0.74 0.65 Tetramethylgermanium 44.0 2.01 90 · 10 ^-4 0.63 1.006

The yield of electrons for every 100 eV of energy lost to ionization is calculated by the formula (1):

(1)

(one)

where D is the dose absorbed in the working volume of the chamber (mR / imp);

d is the density of the "warm liquid" (g / cm ³ );

v is the working volume of the chamber (cm ³ ).

From formula (1) it can be seen that at the same dose released in the water phantom, in the same volume of the chamber, the amount of the collected charge Q from the chamber on the "warm liquid" will be 100 times greater, and, consequently, the dose determination accuracy will be much higher.

The proposed invention uses two coordinate-sensitive cameras PK1 and PK2, operating on a "warm liquid", sensitive in width (x coordinate) and height (y coordinate). The anode of such a chamber is divided into separate pads, the charge from which is output to its charge-sensitive amplifier. Due to the use of "warm liquid" as an active substance, the accuracy of determining the density of the beam (dose field) increases by 100 times, and as a result, the accuracy of determining the released dose in the target increases in comparison with the analogue.

To increase the conformity of proton therapy, namely the accuracy of the correspondence of the beam to the area of the pathological focus in the patient, two coordinate-sensitive pad chambers are used on the "warm liquid"; one PC1 is installed at the outlet of the last scanning magnet, and the second PC2 is in front of the patient. The density of the beam intensity (dose profile) at the entrance to the target is determined, the measured current intensity densities (dose profile) are quantitatively compared with the permissible values, and when they are exceeded, the beam is turned off.

The device operates not only in the Bragg peak registration mode, but also in the “all the way” mode, i.e. without stopping the proton beam in the target - when the beam scans the target.

Description of the invention

At the location of the patient before the proton therapy session, a device is installed, consisting of jointly working cameras PC1 and PC2 and a digital detector for obtaining images of the CDPI.

The general diagram of the inventive device is shown in Fig. 1. The device includes a synchrotron-type accelerator (1) with a proton beam (2) for irradiating a target (3), for example, a tumor. To bind the position of the target (3) to the beam axis, a fixed coordinate system x, y, z (4) is introduced. The proton beam ejected from the accelerator (1) is focused by two quadrupole lenses (5) and passes through the scanning magnets (6). Two coordinate-sensitive pad chambers on the "warm liquid" PK1 (7) and PK2 (8), are set with their central pads along the z-axis (9), which is called depth, the x-axis (10), which is called the width, and the y-axis ( 11) - height. The planes of the cameras PK1 and PK2 are perpendicular to the z axis (9). The PK1 camera is located immediately behind the second scanning magnet, and the PK2 camera is located in front of the CPPI detector (when measuring the permissible values of the beam intensity density) or in front of the patient. In Fig. 1, the elements included in the CDPI schematically fall into the area indicated by the dashed line (12) and consist of: a light-shielding sealed housing (13), inside which there is a water phantom (14) facing its surface towards the lens (15), and adaptive electronic path of the CDPI with a charge-sensitive matrix (16). The center of the water phantom (14) passes through the z axis, and the input plane of the water phantom, into which the beam enters, is parallel to the x, y plane. Pad cameras PK1 and PK2 during the passage of the beam determine the trajectory of the beam entry into the water phantom of the CPPI (patient's target). And the CDPI, based on the measured coordinate energy release of the beam in the water phantom, determines the region of the dose field and the point where the beam stops (17) - the Bragg peak or the region of the beam boundary “all the way”.

“Throughout” is an irradiation mode without the formation of a Bragg peak (for example, at a proton energy of 1 GeV), when the beam leaves the phantom (or patient) without stopping and the CPDI will register only the region of the beam trajectory and the boundaries of its passage through the target. In the case of operation in the Bragg peak mode (17), this is the point where the beam stops in a given voxel of the water phantom, where the Bragg peak energy is released and (18) is the boundary of the target area, which is scanned by the scanning magnets according to a given program (6). The pads of cameras PK1 and PK2, which recorded the passage of the beam per pulse, and which determine the direction of the beam, are designated as (19).

The operation diagram of the device revealing the method for controlling the beam parameters and stabilizing the dose is illustrated in Fig. 2. The accelerator (20) outputs a proton beam (21), which is focused by quadrupole lenses (22) and (23), and then, using scanning magnets (24) and (25), directs the beam to the target (26) bounded by the boundary (27). The beam (21) deflected by the last scanning magnet (25) stops and releases energy in the voxel region (28), which is recorded by the CPDI detector (29), the information from which is transmitted to the readout electronics of the SE (30). To control the position of the patient during the session (the position of the CDPI before the session), a coordinate system is introduced: x, y, z (31), which coincides with the position of the cameras PK1 (32) and PK2 (33). The CDPI detector (29) determines the distribution of the energy release (dose profile) of the Bragg peak along the depth during the accelerator pulse and transmits the current measured value to the control unit (34) for the correspondence of the irradiation to the set values along the depth. Each chamber PK1 (32) and PK2 (33) during an accelerator pulse measures the direction of the beam to the target and measures two-dimensional profiles (x, y) of intensity density (dose profiles). The readout electronics of the PK1 (35) and PK2 (36) cameras make it possible to process the signals coming from the cameras and obtain dose profiles and beam intensity densities. The signal from the sensor of the proton beam intensity meter (37) is fed to the SKOO comparison electronic circuit (38). Information from blocks (34), (35) and (36) also enters the electronic circuit for comparison and control of the detection of deviations of the RMSD (38). In the SKOO scheme (38), a comparison and analysis takes place according to several measured and specified parameters. Parameter algorithms for comparison are specified using the so-called FPGA programmable matrix (39). The programmable matrix controls the autoregulator of the proton beam intensity (40), which is connected to the acceleration unit (41) of the accelerator. The SCOO scheme operates and is controlled according to a program specified by a programmable FPGA matrix (39), where changes in beam characteristics during a proton therapy session are set and controlled by several parameters, and, if these parameters deviate from the specified ones, the beam is turned off. The entire process of the state of these parameters is recorded in a PC (42) during the entire session of radiation therapy.

The process of controlling the intensity of the beam during the preparation and conduct of the proton therapy session using the device is carried out as follows. Before laying the patient down, the proton beam is switched on and the exact correspondence between the position of the target and the coordinate profile of the energy release required for its irradiation is determined with the help of the CPPI detector. Those. whether the area of energy release in the water phantom of the CPPI corresponds to the area of the pathological focus, while taking into account the data obtained from the cameras PK1 and PK2. The PK1 and PK2 cameras control the width of the dose field (coordinates x and y) irradiating the target during the accelerator pulse.

The accuracy of determining the depth of the dose field in a water phantom that simulates a patient, before his irradiation, due to the CPDI is achieved by 0.5 mm. Then, using the readout electronics of the SE (35) and (36), the charge from each pad of the PK1 and PK2 chambers is measured. The pads of the PK1 and PK2 cameras make it possible to measure the beam profile along the x and y axes perpendicular to the z axis of the beam. The pads of the cameras determine the horizontal and vertical profile of the beam and the absolute dose of the beam that has passed during a given accelerator pulse through PK1 and PK2 (with an accuracy of 100 times higher than other detectors), and through the CPPI detector located at the patient's site, and confirm that this is the same the dose was released in the water phantom simulating the patient during the same accelerator pulse. The entire density of the beam intensity per pulse, which passed through the pads of both chambers and dose profiles along both coordinates, is also recorded for each pulse and displayed on a computer. The measured profiles along the x and y axes and the calculated direction of the beam to the target are entered into the SKOO scheme (38), which quantitatively controls the dynamic changes in the beam parameters during the proton therapy session. If the beam leaves the area of the pathological focus, the SKOO system will turn off the beam, which can also be observed on the PC monitor to obtain a documentary record of the irradiation process.

Visual and quantitative control of the beam parameters using the SKOO scheme is carried out simultaneously during the entire irradiation time. The intensity density distribution on PC1 and PC2 is also obtained during the entire irradiation session. The target is irradiated with a scanning "pencil beam", which, according to a predetermined program built into a programmable FPGA, is irradiated so that the beam does not go beyond the boundaries of the irradiated target and does not affect healthy tissues.

Example 1. Implementation of a method for monitoring deviation detection.

The operator sets the controlled parameters in the programmable matrix program (39).

The dose profile obtained by the PK1 and PK2 cameras is a two-dimensional dose distribution from the x and y coordinates, we denote them as:

d (PK1) _{x, y} = f (x, y) - dose profiles measured by the PK1 camera for coordinates xy; (one)

d (PK2) _{x, y} = f (x, y) - dose profiles measured by the PK2 camera for coordinates xy; (2)

Z _edited (TSDPI) = f (z) - depth of the Bragg peak, measured per pulse detector TSDPI accelerator; (3) is the first monitored parameter.

Further, the algorithm of the SCOO operation is as follows. The operator turns on the beam and instead of the patient installs the CPPI detector, setting its central axis along the z axis, while the central pads of the PK1 and PK2 cameras also coincide with the z axis. Dose profiles and parameters indicated in equations (1) - (3) are measured in one or more pulses.

Control # 1 is as follows. From the dose profiles measured by the chambers, doses are calculated at the level of 10% of the maximum distribution, the so-called medical parameter d10.

d (PK1) _{x, y} 10 _L = f (x, y) is the dose on the left wing of the dose profile at the level of 10% of the maximum measured by the PK1 camera for coordinates xy;

d (PK1) _{x, y} 10 _R = f (x, y) is the dose on the right wing of the dose profile at a level of 10% of the maximum measured by the PK1 camera for coordinates xy;

d (PK2) _{x, y} 10 _L = f (x, y) is the dose on the left wing of the dose profile at the level of 10% of the maximum measured by the PK2 camera for coordinates xy;

d (PK2) _{x, y} 10 _R = f (x, y) is the dose on the right wing of the dose profile at a level of 10% of the maximum measured by the PK2 camera for coordinates xy;

We calculate the following relations,

K (PC1) _{x, y} = d (PC1) _{x, y} 10 _L / d (PC1) _{x, y} 10 _R ; (four)

K (PC2) _{x, y} = d (PC2) _{x, y} 10 _L / d (PC2) _{x, y} 10 _R ; (five)

(4) and (5) are the second and third monitored parameters. Parameters K (PC1)_{x, y}and K (PC2)_{x, y}controlled by the SKOO circuit and recorded in the computer for each accelerator pulse. It is the ratio of these parameters that is most sensitive for any deflection of the beam when the target is irradiated according to a given program. In case of deviation of parameters (3), (4) and (5) from the specified ones, the accelerator will shutdown.

Control No. 2. According to the measured parameters for two chambers, the following parameters of the medical proton beam are determined:

- direction of the proton beam along the line connecting the maxima of the dose distributions of the measured PK1 and PK2;

- the total intensity as the area under the dose curves;

- the size of the beam, determined by the spatial position of the boundaries of the distribution of the dose field at the level d10;

The position of the beam relative to the irradiated target is set by combining the irradiation field with the half-width of the dose field distribution measured by the PK2 camera. The convergence of the beam is determined by the formula:

tg (β / 2) = (FWHM ₁ -FWHM ₂ ) / 2L (6)

where β is the angle of convergence of the beam; FWHM _1,2 - full widths at half-height of the dose field distributions measured by cameras PK1 and PK2; L is the distance between PC1 and PC2.

Parameter (6) is determined on the RMS diagram from the measured profiles on cameras PK1 and PK2 and is compared with a given parameter. If parameter (6) leaves, the accelerator is also turned off.

Then the beam is turned off. Laser beam direction expose the coordinates of the maximum of two chambers PC1 and PC2 and monitor the patient's z-axis position of the parameter (3) Z _edited (TSDPI).

Then the CDPI detector is moved away from the z axis, while the position of the PK1 and PK2 cameras relative to the x, y, z axes remains unchanged. The patient is placed in the place of the CPDI detector, so that the center of his target coincides with the center of the target in the CPDI. The center of the patient's target is set to the center of the beam scanned area, the left border of the beam scanned area is set from the laser, which coincides with the left side of the patient's target. The same is carried out for the right border of the target irradiation zone in the patient. Since, despite the fact that the beam is "pencil", it has its own dimensions and when the patient is installed instead of a phantom, the beam at the d10 level should not go out to the left and right beyond the target area (ie, to healthy tissues).

Include a bunch. For each accelerator pulse, the SKOO circuit displays and writes to the PC all the main control parameters, namely parameters (3) - (6).

If these parameters go beyond the limits set by the operator in the programmable FPGA matrix, the SKOO circuit gives a signal to turn off the accelerator, in which case the beam went beyond the irradiated target. In this case, the distribution of the dose field in the target plane (x, y, and z) and parameters (3) - (6) are simultaneously displayed on the PC screen, which allows the operator to observe the dimensions of the beam on the target for each accelerator pulse and control the delivered dose to the target for every impulse.

The method makes it possible to achieve complete correspondence between the planned parameters of the medical proton beam and those actually used in the patient's irradiation session. This is achieved through the use of controlled parameters of the medical proton beam, which is achieved when two chambers PK1 and PK2 work together with an increased dose determination accuracy and a CPPI detector that simulates a patient's target before irradiation.

Claims (4)

1. A method for controlling beam parameters during proton therapy, including measuring the permissible values of the beam intensity density before irradiation of the patient using coordinate-sensitive detectors installed in front of the patient and behind the last scanning magnet, quantitative comparison of the measured intensity density parameters with the permissible values and switching off the beam when deviation of the parameters from the permissible values, characterized in that the measurement of the permissible values of the density of the beam intensity additionally includes the measurement of the depth of the Bragg peak by means of a device for measuring the energy release from ionizing radiation, installed at the patient's site, with the possibility of its subsequent removal from the beam axis when the patient is irradiated, and as Coordinate-sensitive detectors use "warm liquid" pad cameras. 2. The method according to claim 1, characterized in that a digital image acquisition detector is used as a device for measuring the energy release from ionizing radiation. 3. A device for monitoring beam parameters during proton therapy, including coordinate-sensitive cameras installed in front of the patient and behind the last scanning magnet, characterized in that it additionally includes a device for measuring energy release from ionizing radiation, installed at the patient's site, with the possibility of its subsequent removal from beam axes during irradiation of the patient, and pad chambers on "warm liquid" are used as chambers. 4. The device according to claim 3, characterized in that a digital image acquisition detector is used as a device for measuring the energy release from ionizing radiation, with the ability to work in the "all the way" mode.

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