A measuring device and a measuring method for a photon beam reference dose magnetic field correction factor in a magnetic field

Abstract

The application discloses a kind of magnetic field in photon beam reference dose magnetic field correction factor measuring device and measuring method. Wherein, the measuring device includes first electromagnet;Second electromagnet, the central axis of first electromagnet and second electromagnet coincides and exists gap between the two;Water tank, water tank is placed in gap;Ionization chamber, ionization chamber is inserted into water tank, and the central axis of ionization chamber and the central axis of first electromagnet form plane A;Photon source, photon source is located outside plane A, and photon beam formed by photon source is perpendicular to plane A.The measuring device provided by the application can realize the accurate measurement of the magnetic field correction factor of photon beam reference dose in strong external magnetic field.The application also provides a kind of measuring method of magnetic field in photon beam reference dose magnetic field correction factor.

Description

Technical Field

[0001] This invention relates to the technical field of radiotherapy equipment, and in particular to a measuring device and method for measuring the magnetic field correction factor of a photon beam reference dose in a magnetic field. Background Technology

[0002] Magnetic resonance-guided radiotherapy (MRIgRT) is a novel hybrid radiotherapy system that integrates a magnetic resonance scanner and a traditional linear accelerator. It has advantages such as high soft tissue resolution, no radiation, and real-time imaging. As a new type of precision treatment, it has been widely used in clinical practice.

[0003] Currently, the main magnetic field strength of clinical MRI gRT equipment includes 0.35T, 0.6T, 1.0T and 1.5T. The influence of strong external magnetic fields on the trajectory of secondary charged particles increases the uncertainty of accelerator output dose and dose distribution deviation, which can easily cause radiotherapy accidents.

[0004] The radiotherapy reference dose is a crucial physical quantity for the accurate characterization of accelerator beam output dose and the effective implementation of treatment planning. Unlike conventional radiotherapy accelerators, the strong magnetic field of clinically deployed MRI gRT equipment cannot be easily shut off, making it impossible to measure the beam reference dose under magnetic field-free conditions. The assessment of the MRI gRT reference dose needs to consider the changes in the ionization chamber response caused by the magnetic field during measurement (expressed as kJ / kb). B -1 (This indicates that) in order to quantify the influence of the magnetic field on the ionization chamber itself, it is necessary to test the k in the magnetic field environment. B -1 The study investigated the variation of the reference dose in MRI gRT under different magnetic field intensities, laying the foundation for accurate measurement of the reference dose in MRI gRT. Summary of the Invention

[0005] This invention aims to at least solve one of the technical problems existing in the prior art. To this end, this invention proposes a measuring device for the magnetic field correction factor of the photon beam reference dose in a magnetic field. The measuring device provided by this invention can accurately measure the magnetic field correction factor of the photon beam reference dose in a strong external magnetic field.

[0006] The present invention also provides a method for measuring the magnetic field correction factor of the reference dose of a photon beam in a magnetic field using the above-described measuring device.

[0007] According to an embodiment of the first aspect of the present invention, a measuring device for a photon beam reference dose magnetic field correction factor in a magnetic field is provided, the measuring device comprising:

[0008] First electromagnet;

[0009] The second electromagnet has the central axes of the first electromagnet and the second electromagnet coinciding and there is a gap between them;

[0010] A water tank, which is placed within the gap;

[0011] An ionization chamber is inserted into the water tank, and the central axis of the ionization chamber and the central axis of the first electromagnet (110) form a plane A.

[0012] A photon source is located outside the plane A, and the photon beam formed by the photon source is perpendicular to the plane A.

[0013] The measuring device according to embodiments of the present invention has at least the following beneficial effects:

[0014] (1) In the measuring device provided by the present invention, the central axes of the first electromagnet and the second electromagnet coincide, and the water tank is placed in the gap formed by the two electromagnets. Thus, in the gap, the magnetic field lines are parallel to the central axis of the first electromagnet, and the magnetic field strength at the water tank placement position is relatively uniform, providing a stable magnetic field environment for testing the photon beam reference dose magnetic field correction factor in the magnetic field.

[0015] (2) In the measuring device provided by the present invention, the positions of the photon source, ionization chamber and magnetic field are similar to those in clinical settings (the difference is that the ionization chamber is equivalent to the position of radiotherapy), so the magnetic field correction factor of the photon beam reference dose in the magnetic field measured by this invention has more practical guiding significance.

[0016] According to some embodiments of the present invention, the first electromagnet includes a first magnetic pole head and a first electromagnetic coil wound around the first magnetic pole head.

[0017] According to some embodiments of the present invention, the first magnet pole head is cylindrical.

[0018] According to some embodiments of the present invention, the diameter of the first magnet tip is 200 mm.

[0019] According to some embodiments of the present invention, the first magnet tip is made of iron; preferably, the iron is of type DT4. This satisfies the lateral scattering condition.

[0020] According to some embodiments of the present invention, the second electromagnet includes a second magnet pole head and a second electromagnetic coil wound around the second magnet pole head.

[0021] According to some embodiments of the present invention, the second electromagnet is identical to the first electromagnet. This results in a more uniform magnetic field formed in the gap, and the magnetic field directions at different locations within the gap are more consistent.

[0022] According to some embodiments of the present invention, the current of the first electromagnet and the second electromagnet is ≤120A.

[0023] According to some embodiments of the present invention, the power of the first electromagnet and the second electromagnet is ≤10.8kW.

[0024] In other words, the rated maximum current of the first electromagnet and the second electromagnet is 120A, and the rated maximum power is 10.8kW.

[0025] According to some embodiments of the present invention, the distance of the gap along the central axis of the second electromagnet is ≤170mm. For example, it can be 50mm, 90mm or 100mm.

[0026] According to some embodiments of the present invention, the magnetic field strength formed by the second electromagnet and the first electromagnet is ≤2.3T. For example, it can be 1.5T, 1.0T, 0.8T, 0.6T, 0.35T, etc. The magnetic field strength can be adjusted arbitrarily within the above range.

[0027] The magnetic field strength refers to the magnetic field strength within the gap range.

[0028] According to some embodiments of the present invention, the magnetic field strength formed by the second electromagnet and the first electromagnet is ≤1.52T.

[0029] According to some embodiments of the present invention, the magnetic field strength formed by the second electromagnet and the first electromagnet is ≤0.8T. Since excessively high magnetic field strength may affect the vacuum level of the photon source (within the accelerator cavity), it is necessary to improve the magnetic shielding conditions. To save costs and considering the commonly used clinical dose range, the magnetic field strength range is ≤0.8T.

[0030] The magnetic field strength is related to the size of the gap to a certain extent, for example:

[0031] Along the central axis of the second electromagnet, when the gap is 5mm, the maximum magnetic field strength is 2.3T; when the gap is 50mm, the maximum magnetic field strength is 1.525T.

[0032] Furthermore, when the distance of the gap is fixed, the magnetic field strength can be adjusted by adjusting the current of the electromagnet.

[0033] According to some embodiments of the present invention, within the gap region, the magnetic field uniformity inside the 50mm diameter sphere is ≤0.25%. This uniformity means that within the corresponding region, the proportion of the magnetic field strength deviating from the average value is within 0.25%. This indicates a high degree of magnetic field uniformity within the gap region.

[0034] The uniformity of the magnetic field is positively correlated with the size of the gap. For example:

[0035] Along the direction of the central axis of the second electromagnet, when the distance of the gap is 50mm, 4cm 2 The magnetic field uniformity within the spherical region is approximately 0.07%.

[0036] According to some embodiments of the present invention, the measuring device further includes a gaussmeter for real-time monitoring of the magnetic field strength within the gap range.

[0037] According to some embodiments of the present invention, the gaussmeter is located near the ionization chamber and is traversed by the central axis of the electromagnet. This allows for a more accurate reflection of the magnetic field strength at the location of the ionization chamber.

[0038] According to some embodiments of the present invention, the measuring device further includes a magnetic field control system. Because the magnetic field strength is high, the current in the first and second electromagnets is also high. To avoid the dangers of manual operation, the magnetic field strength and direction can be adjusted using the magnetic field control system.

[0039] According to some embodiments of the present invention, the walls of the water tank are made of plexiglass.

[0040] According to some embodiments of the present invention, the water tank is filled with water. Preferably, the water is at least one of deionized water or ultrapure water.

[0041] Because the human body has a high water content, this invention uses plexiglass and water to prepare a water tank, which can effectively simulate the interaction between the photon beam and the human body in a specific environment. The magnetic field correction factor of the photon beam reference dose obtained by measuring the magnetic field is more clinically significant.

[0042] According to some embodiments of the present invention, the external dimensions of the water tank are (50-90) mm × 200 mm × 200 mm. In the measuring device provided by the present invention, the space for placing the water tank is very limited, and a magnetic field exists within the measuring device, making it unsafe for many ferromagnetic materials to exist there. Therefore, conventional water tanks are not suitable. The present invention uses a water tank of a suitable size and made of specific materials, thereby enabling better measurement of the photon beam reference dose magnetic field correction factor in a magnetic field.

[0043] According to some embodiments of the present invention, the width of the water tank along the central axis of the electromagnet is the distance of the gap. Thus, the sidewall of the water tank is in close contact with the first and second magnet poles.

[0044] According to some embodiments of the present invention, the photon beam travels d cm in the water tank before acting on the ionization chamber, where d ≈ 10. That is, when the dimensions of the water tank are (50–90) mm × 200 mm × 200 mm, the ionization chamber is located at the middle depth of the water tank along the direction of the photon beam. This establishes sufficient forward and backscattering conditions.

[0045] According to some embodiments of the present invention, the ionization chamber is a waterproof ionization chamber. Therefore, it can operate normally within the water tank.

[0046] According to some embodiments of the present invention, the ionization chamber includes at least one of the PTW TW30013 Farmer waterproof ionization chamber and the TW31010 Semiflex waterproof ionization chamber. These two detectors are commonly used in clinical practice, and therefore the results obtained from these two ionization chambers can provide good clinical guidance.

[0047] According to some embodiments of the present invention, the ionization chamber includes a sensitive part and a supporting part. The sensitive part is used to test the intensity of the photon beam, and the supporting part is used to support the sensitive part, as well as auxiliary components such as circuitry for transmitting information from the sensitive part.

[0048] According to some embodiments of the present invention, the volume of the sensitive part of the ionization chamber is 0.125 cm³. 3 ~0.6cm 3 The volume is related to the model of the ionization chamber; essentially, selecting the model of the ionization chamber determines the volume of the sensing element.

[0049] According to some embodiments of the present invention, the sensitive part of the ionization chamber is simultaneously traversed by the central axis of the electromagnet and the photon beam, thereby making the measurement of the photon beam reference dose magnetic field correction factor in the magnetic field more accurate.

[0050] According to some embodiments of the present invention, the angle formed by the central axis of the ionization chamber and the central axis of the first electromagnet is θ, and 0°≤θ≤270°. Therefore, the measuring device can be used to test the photon beam reference dose magnetic field correction factor in a magnetic field under different θ angles. For example, test conditions are provided for vertical angles (90° or 270°) and parallel angles (0° or 180°).

[0051] According to some embodiments of the present invention, the measuring device further includes a data acquisition and processing system for acquiring and processing readings from the ionization chamber.

[0052] According to some embodiments of the present invention, the data acquisition and processing system includes a Keithley 6517B high-resistivity electrometer.

[0053] According to some embodiments of the present invention, the distance between the photon source and the plane A is 105 to 145 cm. For example, it can be approximately 110 cm or 143.5 cm.

[0054] According to some embodiments of the present invention, the photon source includes a medical electron linear accelerator.

[0055] According to some embodiments of the present invention, one of the manufacturers of the medical linear accelerator includes Elekta AG of Sweden, and one of the models includes the Syngery medical linear accelerator.

[0056] According to some embodiments of the present invention, the dose rate of the photon source is 0 to 800 MU / min, specifically about 300 MU / min.

[0057] According to some embodiments of the present invention, the energy of the photon beam is 4 to 25 MV.

[0058] According to some embodiments of the present invention, the energy of the photon beam is 4 to 10 MV.

[0059] According to some embodiments of the present invention, the energy of the photon beam is 6 to 7 MV.

[0060] According to some embodiments of the present invention, the field size of the photon beam is (3-10)cm × 10cm.

[0061] According to some embodiments of the present invention, the field size of the photon beam is one of 5cm×10cm, 3cm×10cm, 4cm×10cm and 10cm×10cm.

[0062] According to some embodiments of the present invention, the measuring device further includes a positioning bracket for supporting the water tank, ionization chamber, and other components, ensuring they are positioned appropriately.

[0063] According to some embodiments of the present invention, the positioning bracket is made of a non-ferromagnetic material. This ensures the safety of experiments under a magnetic field.

[0064] According to some embodiments of the present invention, the non-ferromagnetic material includes at least one of aluminum alloy and brass.

[0065] The measuring device provided by this invention is used for measuring the reference dose magnetic field correction factor in MRI gRT.

[0066] According to an embodiment of a second aspect of the present invention, a method for measuring the magnetic field correction factor of a photon beam reference dose in a magnetic field is provided. The measurement method employs the aforementioned measuring device and includes the following steps:

[0067] The photon source is turned on, and the reading M of the ionization chamber is collected.B＝0 ;

[0068] Turn on the first electromagnet, the second electromagnet, and the photon source, and collect the reading M of the ionization chamber. B＝k ;

[0069] The reference dose magnetic field correction factor k of the photon beam in the magnetic field is obtained. B =M B＝0 / M B＝k .

[0070] The measurement method according to embodiments of the present invention has at least the following beneficial effects:

[0071] Since the measurement method adopts all the technical solutions of the measurement device in the above embodiments, it has at least all the beneficial effects brought about by the technical solutions in the above embodiments.

[0072] Furthermore, due to the structural design of the measuring device, the measurement method can obtain the correspondence between the photon beam reference dose magnetic field correction factor and the magnetic field strength and angle θ in the magnetic field through simple magnetic field adjustment and angle adjustment.

[0073] According to some embodiments of the present invention, the magnetic field strength formed by the first electromagnet and the second electromagnet is between -2.3T and 2.3T.

[0074] According to some embodiments of the present invention, the magnetic field strength formed by the first electromagnet and the second electromagnet is between -1.52T and 1.52T.

[0075] According to some embodiments of the present invention, the magnetic field strength formed by the first electromagnet and the second electromagnet is between -1.5T and 1.5T.

[0076] According to some embodiments of the present invention, the magnetic field strength formed by the first electromagnet and the second electromagnet is between -1.0T and 1.0T.

[0077] According to some embodiments of the present invention, the magnetic field strength formed by the first electromagnet and the second electromagnet is between -0.8T and 0.8T.

[0078] According to some embodiments of the present invention, the magnetic field strength formed by the first electromagnet and the second electromagnet is between -0.6T and 0.6T.

[0079] According to some embodiments of the present invention, the magnetic field strength formed by the first electromagnet and the second electromagnet is between -0.35T and 0.35T.

[0080] A positive value of the magnetic field strength indicates that the value of θ is between 0° and 180°, while a negative value indicates that the value of θ is between 180° and 270°.

[0081] According to some embodiments of the present invention, the angle between the central axis of the ionization chamber (from the support to the sensitive part) and the central axis of the first electromagnet (from the N pole to the S pole) in the measurement method is in the range of 0°≤θ≤270°.

[0082] According to some embodiments of the present invention, the measurement method yields k B The difference between the actual value and the simulated value is ≤1.05%.

[0083] According to some embodiments of the present invention, by maintaining θ = 90° or 270° and adjusting the magnetic field strength, the relationship between the photon beam reference dose magnetic field correction factor and the magnetic field strength in an orthogonal magnetic field can be obtained.

[0084] The photon source (linear accelerator) is selected with an energy of 6MV, a field size of 5cm×10cm, an output beam of 100MU, and an output mode of Interlock7 (to avoid the influence of the edge magnetic field).

[0085] The ionization chamber is placed in a water tank and positioned so that the center of the sensitive volume of the ionization chamber, the center of the magnetic field, and the center of the radiation source coincide.

[0086] The closest distance between the surface of the water tank and the photon source is 100cm. The sensitive part of the ionization chamber is located at the equivalent center of the water tank, where the water is equivalent to a depth of 10cm.

[0087] The side wall of the water tank is in close contact with the first and second magnet poles of the electromagnet.

[0088] At this point, the magnetic field strength is adjusted, and the reading of the ionization chamber is obtained. The relationship between the photon beam reference dose magnetic field correction factor and the magnetic field strength in the orthogonal magnetic field is calculated.

[0089] According to some embodiments of the present invention, by maintaining θ = 0° or 180° and adjusting the magnetic field strength, the relationship between the photon beam reference dose magnetic field correction factor and the magnetic field strength in a parallel magnetic field can be obtained. The parameter settings are similar to those used in the measurement of the photon beam reference dose magnetic field correction factor in an orthogonal magnetic field, except for the following parameters:

[0090] Adjust the size of the gap to ensure that the water tank is positioned when θ = 0° or 180°, and that the side wall of the water tank is in close contact with the first and second magnet poles.

[0091] Unless otherwise specified, the parameters of the data collection and processing system of the ionization chamber in this invention are as follows:

[0092] The micro-current signal of the ionization chamber was collected using a 6517B high-resistance electrometer from Keithley. A high voltage of -400V was applied, the measurement range was selected as 20nC, the acquisition time was 80s, the discharge time was 5s, and the delay was 3s. The linkage control, signal acquisition, and preprocessing of multiple instruments were implemented on the LabVIEW platform.

[0093] Unless otherwise specified, the "about" in this invention actually means that the allowable error is within the range of ±2%. For example, about 100 actually means 100 ± 2% × 100.

[0094] Other features and advantages of this invention will be described in the subsequent specification. And, in part, they will become obvious from the specification or be understood by implementing this invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0095] The above and / or additional aspects and advantages of this invention will become obvious and easy to understand from the description of the embodiments in conjunction with the following drawings, where:

[0096] Figure 1 is a partial schematic view of the measuring device in Embodiment 1 of this invention;

[0097] Figure 2 is a top view of the measuring device in Embodiment 1 of this invention;

[0098] Figure 3 is Figure 1 a left view of the structure shown.

[0099] Figure 4 is a comparison chart of the measured results and simulated results of the correction factor obtained in Embodiment 3 of this invention;

[0100] Figure 5 is a comparison chart of the measured results and simulated results of the correction factor obtained in Embodiment 4 of this invention;

[0101] REFERENCE SIGNS:

[0102] The first electromagnet 110, the first magnet pole head 111, the first electromagnetic coil 112;

[0103] The second electromagnet 120, the second magnet pole head 121, the second electromagnetic coil 122;

[0104] The water tank 200,

[0105] The ionization chamber 300;

[0106] The photon source 400, the photon beam 410. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0107] Embodiments of the present invention are described in detail below. Examples of these embodiments are shown in the accompanying drawings, wherein the same or similar reference numerals denote the same or similar elements or elements having the same or similar functions throughout. The embodiments described below with reference to the accompanying drawings are exemplary and are only used to explain the present invention, and should not be construed as limiting the present invention.

[0108] In the description of this invention, the use of terms such as "first," "second," etc., is for the purpose of distinguishing technical features only and should not be construed as indicating or implying relative importance, or implicitly indicating the number of technical features indicated, or implicitly indicating the order of the technical features indicated.

[0109] In the description of this invention, it should be understood that the orientation descriptions, such as up, down, etc., are based on the orientation or positional relationship shown in the drawings and are only for the convenience of describing this invention and simplifying the description, and are not intended to indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation, and therefore should not be construed as a limitation of this invention.

[0110] In the description of this invention, it should be noted that, unless otherwise explicitly defined, terms such as "setting," "installation," and "connection" should be interpreted broadly, and those skilled in the art can reasonably determine the specific meaning of the above terms in this invention in conjunction with the specific content of the technical solution.

[0111] Example 1

[0112] This embodiment provides a device for measuring the reference dose magnetic field correction factor of a photon beam in a magnetic field. Figures 1-3 As shown, the above-mentioned measuring device includes the following components:

[0113] First electromagnet 110;

[0114] The central axes of the second electromagnet 120, the first electromagnet 110, and the second electromagnet 120 coincide and there is a gap between them.

[0115] Water tank 200, water tank 200 is placed in the gap;

[0116] Ionization chamber 300 is inserted into water tank 200, and the central axis of ionization chamber 300 and the central axis of first electromagnet 110 form plane A;

[0117] Photon source 400 is located outside plane A, and the photon beam 410 formed by photon source 400 is perpendicular to plane A.

[0118] It is understandable that the parameters of the first electromagnet 110 and the second electromagnet 120 are the same, specifically:

[0119] The first electromagnet 110 includes a first magnetic pole head 111 and a first electromagnetic coil 112 wound around the first magnetic pole head 111. The first magnetic pole head 111 is a cylinder with a diameter of 200 mm and is made of DT4. Near the gap side, a part of the first magnetic pole head 111 is not wound by the first electromagnetic coil 112; the length of this part is about 10 cm.

[0120] Overall, the first electromagnet 110 has a maximum current of 120A and a maximum power of 10.8kW;

[0121] The second electromagnet 120 also includes a second magnet pole head 121 and a second electromagnetic coil 122 wound around the second magnet pole head 121.

[0122] It is understandable that the first electromagnet 110 and the second electromagnet 120 are connected to a magnetic field control system to control the strength of the magnetic field and the positions of the first electromagnet 110 and the second electromagnet 120.

[0123] It is understandable that the gap between the first electromagnet 110 and the second electromagnet 120 is 50mm along the central axis of the electromagnet.

[0124] It is understandable that the magnetic field uniformity within the 2cm diameter spherical region of the gap region formed by the first electromagnet 110 and the second electromagnet 120 is approximately 0.07%.

[0125] It is understood that the water tank 200 includes a tank wall made of plexiglass with a thickness of 5mm, and is filled with deionized water; the dimensions of the water tank 200 are 50mm×200mm×200mm, so the axis of the ionization chamber 300 is parallel to the 200mm×200mm wall, and the two 200mm×200mm wall surfaces in opposite directions are in close contact with the first magnet pole head 111 and the second magnet pole head 121 respectively.

[0126] It is understood that the ionization chamber 300 is a PTW TW30013 Farmer type waterproof ionization chamber. The sensitive part of the ionization chamber 300 is traversed by the central axis of the first electromagnet 110 and the central line of the photon beam 410. Along the direction of the photon beam 410, the sensitive part is located at a water depth of approximately 10 cm, that is, at the middle depth of the water tank. The parameter information of the ionization chamber 300 used in this embodiment is shown in Table 1.

[0127] It is understandable that the angle between the central axis of the ionization chamber 300 (from the support to the sensitive part) and the central axis of the first electromagnet 110 (from the N pole to the S pole, determined by the direction of the current in the coil) is 90° or 270° (perpendicular).

[0128] It is understandable that the vertical distance between the photon source 400 and plane A is 110cm.

[0129] Understandably, the photon source 400 uses the Syngery medical linear accelerator from Elekta, Sweden, and the field size of the photon beam 410 is 5cm × 10cm.

[0130] It is understood that the measuring device provided in this embodiment also includes a gaussmeter located near the sensitive part, a positioning bracket (made of brass or aluminum) supporting each component, and a data acquisition and processing system for processing the signal of the ionization chamber 300; wherein the data acquisition and processing system is a Keithley 6517B high-resistance electrometer.

[0131] Example 2

[0132] This embodiment provides a device for measuring the magnetic field correction factor of the reference dose of a photon beam in a magnetic field. The specific difference between this embodiment and Embodiment 1 is as follows:

[0133] The water tank 200 has dimensions of 90mm×200mm×200mm, so the axis of the ionization chamber 300 is parallel to the 200mm×200mm wall, and the two 200mm×200mm wall surfaces in opposite directions are in close contact with the first magnet pole head 111 and the second magnet pole head 121, respectively.

[0134] The ionization chamber 300 is a PTW TW31010 Semiflex waterproof ionization chamber; the parameter information of the ionization chamber 300 used in this embodiment is shown in Table 1.

[0135] The angle between the central axis of the ionization chamber 300 and the central axis of the first electromagnet 110 is 0°, 180° (horizontal direction), or 90° and 270° (vertical direction);

[0136] Correspondingly, the gap between the first electromagnet 110 and the second electromagnet 120 was adjusted to accommodate the water tank 200 placed horizontally in the ionization chamber 300.

[0137] Table 1. Parameter information of ionization chamber 300 in Examples 1-2

[0138]

[0139] Example 3

[0140] This embodiment provides a method for measuring the reference dose magnetic field correction factor of a photon beam in a magnetic field. The method uses the measuring device provided in Embodiment 1 and includes the following steps:

[0141] The measuring device is positioned according to the setup given in Example 1.

[0142] Photon source 400 is turned on, and reading M is collected from ionization chamber 300. B＝0Collect M B＝0 Beforehand, the ionization chamber needs to be pre-irradiated with 300-1000 MU;

[0143] The first electromagnet 110, the second electromagnet 120, and the photon source 400 are activated to collect the reading M from the ionization chamber 300. B＝k ;

[0144] The reference dose magnetic field correction factor k of the photon beam in the magnetic field is obtained. B =M B＝0 / M B＝k .

[0145] The parameters of the photon source 400 are as follows: energy is selected as 6MV, field size is 5cm×10cm, output beam is 100MU, output mode is selected as Interlock7 mode, and dose rate is 300MU / min.

[0146] The operating parameters of the 6517B high-resistance electrostatic meter are: high voltage -400V, range selection 20nC, acquisition time 80s, discharge 5s, and delay 3s.

[0147] The magnetic field strengths formed by the first electromagnet 110 and the second electromagnet 120 are 0T, ±0.2T, ±0.35T, ±0.4T, ±0.6T, ±0.8T, ±1.0T, ±1.1T, ±1.2T, ±1.4T, and ±1.5T. Each magnetic field strength is repeated three times, and the average value is calculated. This embodiment is repeated three times a day for three days to verify repeatability.

[0148] The measurement results of this embodiment are shown in Table 2 and... Figure 4 As shown. Among them Figure 2 A negative value of the magnetic field strength indicates an angle θ of 270°, while a positive value corresponds to θ = 90°.

[0149] The measurement results of this embodiment show that when the magnetic field strength is in the range of -1.0T to 1.0T, the response reading of the ionization chamber 300 is positively correlated with the absolute value of the magnetic field strength. When the absolute value of the magnetic field strength is greater than 1.0T, the reading of the ionization chamber 300 decreases as the magnetic field strength increases.

[0150] The results also showed that the measurement results using different serial numbers of TW30013 were in good agreement, with a difference of only 0.54% at a magnetic field strength of 0.8T. At other magnetic field strengths, the results obtained using different serial numbers of TW30013 ionization chambers were basically the same.

[0151] In this embodiment, the result for a magnetic field direction of 90° is slightly higher than the measurement value for 270°. Taking SN009165 (a serial number of TW30013 ionization chamber) as an example, at magnetic field strengths of 0.2T, 0.35T, 0.4T, 0.6T, and 0.8T, k is higher when θ is 90° and 270°. B The differences were 1.43%, 0.83%, 0.98%, 0.87%, and 0.15%, respectively.

[0152] To verify the k obtained in this embodiment B To ensure the accuracy of the values, this invention models the geometry of the TW30013 ionization chamber 300 using the Monte Carlo method and performs particle transport simulations according to the experimental conditions set in the embodiment. The reference dose within the sensitive volume of the ionization chamber is statistically analyzed under different magnetic field intensities in the vertical direction and normalized to the results under no magnetic field conditions. Simulation results show that the measurement results of this invention agree well with the simulation results.

[0153] Example 4

[0154] This embodiment provides a method for measuring the magnetic field reference dose correction factor of a photon beam in a magnetic field, which differs from Embodiment 3 in that:

[0155] The measuring device used in this embodiment is from Embodiment 2; the k-values ​​in the vertical and parallel directions were tested within the magnetic field strength range of -0.8T to 0.8T. B .

[0156] The test results of this embodiment are shown in Table 2 and... Figure 5 As shown. Among them Figure 5 The ordinate is k B The result relative to the magnetic field strength (i.e., k) B The reciprocal of k B -1 ).

[0157] The results show that, using an ionization chamber (model TW31010) to measure the magnetic field correction factor for the photon beam reference dose in a magnetic field, under parallel conditions, the ionization chamber response reading increases with increasing magnetic field strength, but the change is less than 0.01. For the vertical direction, when the angle between the magnetic field and the ionization chamber axis is 90°, the ionization chamber response reading still shows a positive correlation with the magnetic field strength, and is higher than the reading without a magnetic field. However, when the angle is 270°, the ionization chamber response reading first decreases and then slowly increases with increasing magnetic field strength, but remains lower than the reading without a magnetic field.

[0158] The trend of the curves presented by these measurements is strongly correlated with the type of sensitive part in the ionization chamber 300. For the vertical direction, the resulting curves are typically asymmetrical, which is related to the deflection direction of secondary electrons under the influence of a strong external magnetic field. The secondary electrons may deflect either to the core of the ionization chamber or to the tip of the sensitive part.

[0159] To verify the k obtained in this embodiment B The accuracy of the values ​​is the same as in Example 3. This example also models the geometry of the TW301010 ionization chamber using the Monte Carlo method, performs particle transport simulations according to experimental conditions, statistically analyzes the reference dose within the sensitive volume of the ionization chamber under different magnetic field intensities in the parallel direction, and normalizes the results to the no-magnetic-field condition. Simulation results show that the measurement results of this invention agree well with the simulation results.

[0160] Table 2 shows the portion of k obtained from Examples 3-4. B and related statistical results

[0161]

[0162] In Table 2, Δk B This indicates the difference between the measured value and the simulated value (calculated as the ratio of the difference between the measured value and the simulated value to the simulated value). The parentheses at the end of the measured value and the simulated value indicate the uncertainty of the value. For example, 0.9668(3) actually means 0.9668±0.0003.

[0163] Table 2 shows the results of the measuring device provided by the present invention and the k measured by it. B The results show good agreement with the simulated values, with a difference rate of ≤1.04%, of which most differences are ≤1%.

[0164] Table 2 also shows that there are certain differences in the test results obtained from different models of ionization chambers, and the influence of the magnetic field on the ionization chamber response reading reaches its peak when the absolute value of the magnetic field strength is close to 1T. Furthermore, when the angle θ is different, k... B The values ​​may also vary slightly. The above phenomena can be explained by the secondary electron trajectory theory. Under the influence of a magnetic field, the secondary electrons generated when the photon beam passes through the sensitive volume of the ionization chamber will have their trajectories deflected by the Lorentz force, potentially forming trajectories parallel to the axis of the cylindrical sensitive part. This increases the probability that the secondary electrons will remain within the sensitive part rather than penetrate it. Therefore, under a 1T magnetic field, the dose deposited within the sensitive part of the ionization chamber is the highest. When the magnetic field strength is higher, the deflection angle of the secondary electron trajectory is larger, which may cause them to leave the ionization chamber again, resulting in less dose deposition. Simultaneously, it can be observed that when the magnetic field is parallel to the axis of the ionization chamber, the ionization chamber response is less affected by the magnetic field. This is because the direction of electron deflection caused by the magnetic field is not parallel to the axis of the ionization chamber, preventing more electrons from depositing energy within the sensitive volume of the ionization chamber. Ionization chambers with smaller sensitive parts are less affected by the magnetic field, which can also be verified by the aforementioned secondary electron trajectory deflection theory.

[0165] Therefore, during the quality control of MRI gRT equipment, it is recommended to keep the ionization chamber axis parallel to the magnetic field as much as possible to reduce the influence of the magnetic field on the ionization chamber response. At the same time, a magnetic field factor should be considered to correct the ionization chamber reading. If conditions permit, using an ionization chamber with a smaller sensitive part can also be considered for measurement.

[0166] In summary, this invention establishes strong magnetic field radiation conditions for magnetic resonance-guided radiotherapy (MRIgRT) using electromagnets on a conventional medical linear accelerator, and provides a measuring device for measuring the reference dose magnetic field correction factor in MRIgRT. Using this measuring device, the influence of parameters such as magnetic field direction (angle with the ionization chamber) and magnetic field strength on the ionization chamber response was studied. The influence of the strong magnetic field was quantified using the reference dose magnetic field correction factor, and the uncertainty of the measurement results was systematically evaluated. A magnetic field correction measurement method for the photon beam reference dose under a strong magnetic field was established.

[0167] The verification results of the embodiments show that, compared with clinical MRI gRT equipment, the measurement device provided by the present invention offers flexible adjustment of magnetic field strength and direction, covering the magnetic field strength of all MRI gRT equipment on the market, and possesses radiation experimental conditions without a magnetic field. It can be used to study reference doses under magnetic field-free conditions, and subsequently evaluate the magnetic field correction factor for the photon beam reference dose caused by a strong magnetic field. The magnetic field correction factor measured using the measurement method of the present invention shows a high degree of agreement with the simulated magnetic field correction factor. This indicates that the measurement method provided by the present invention has high clinical reference value.

[0168] The embodiments of the present invention have been described in detail above with reference to the accompanying drawings. However, the present invention is not limited to the above embodiments. Within the scope of knowledge possessed by those skilled in the art, various changes can be made without departing from the spirit of the present invention.

Claims

1. A measuring device for the magnetic field correction factor of a photon beam reference dose in a magnetic field, characterized in that, The measuring device includes: First electromagnet (110); The second electromagnet (120) has the central axes of the first electromagnet (110) and the second electromagnet (120) coinciding and there is a gap between them; A water tank (200) is placed in the gap; An ionization chamber (300) is inserted into the water tank (200), and the central axis of the ionization chamber (300) and the central axis of the first electromagnet (110) form a plane A; A photon source (400) is located outside the plane A, and the photon beam (410) formed by the photon source (400) is perpendicular to the plane A.

2. The measuring device of claim 1, wherein, The angle between the central axis of the ionization chamber (300) and the central axis of the first electromagnet (110) is θ, and 0°≤θ≤270°.

3. The measuring device of claim 1, wherein, Within the specified gap region, the magnetic field uniformity within the 50mm diameter sphere is ≤0.25%.

4. The measuring device according to claim 1, characterized in that, The walls of the water tank (200) are made of plexiglass.

5. The measuring device of claim 1, wherein, The photon beam (410) travels d cm in the water tank (200) and then acts on the ionization chamber (300), where d≈10.

6. The measuring device according to any one of claims 1 to 5, characterized in that The energy of the photon beam (410) is 4–25 MV.

7. A method of measuring a magnetic field reference dose magnetic field correction factor for a photon beam in a magnetic field, characterized by, The measurement method employs the measuring device as described in any one of claims 1 to 6, and the measurement method includes the following steps: Turning on the photon source (400), collecting the reading M of the ionization chamber (300) B＝0 ; Turning on the first electromagnet (110), second electromagnet (120) and photon source (400), collecting the reading M of the ionization chamber (300) B＝k ; Photon beam reference dose magnetic field correction factor k in magnetic field B = M B＝0 / M B＝k .

8. The measurement method according to claim 7, characterized in that, In the measurement method, the magnetic field strength formed by the first electromagnet (110) and the second electromagnet (120) is between -2.3T and 2.3T.

9. The measurement method according to claim 7, characterized by, In the measurement method, the angle between the central axis of the ionization chamber (300) and the central axis of the first electromagnet (110) is in the range of 0°≤θ≤270°.

10. The measuring method according to any one of claims 7 to 9, characterized in that, The k B value obtained by the measurement method differs from the simulated value by ≤ 1.05 %.

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