Method of characterizing physical properties of attenuating elements in a radiotherapy device

Abstract

Disclosed herein are methods of characterizing a physical property of an attenuating element in a radiotherapy device having a source of radiotherapy radiation and a detector of radiotherapy radiation on respective sides of the attenuating element. The method includes obtaining an average detected radiotherapy radiation intensity at two or more locations around the attenuating element, comparing the detected intensity at one location to the average intensity, and characterizing the respective physical property based on the comparison.

Description

Technical Field

[0001] This disclosure relates to a method for characterizing the physical properties of attenuation elements in a radiotherapy device. Background Technology

[0002] This disclosure relates to machines, devices, or apparatuses for radiotherapy, and methods of implementing such apparatus. Radiotherapy devices are important tools in modern cancer treatment. These devices are adapted to deliver radiation beams to patients to treat tumors. An example of a radiation source used to generate the beam is a linear accelerator (LINAC). Clinical LINAC devices are configured to deliver high-energy radiation to patients.

[0003] To deliver different treatments effectively, radiotherapy machines are becoming increasingly diverse in design. For example, the electron beam generated by a LINAC can be used for treatment itself, or it can be guided to a target to produce X-ray radiation for treatment. Different LINAC designs can be used to produce radiation with different properties.

[0004] Radiation therapy devices are large, complex machines with many moving parts and interoperating mechanisms, requiring precise engineering and rigorous testing. Some components of a radiation therapy machine can interact with other components in complex ways. One such example is the radiation attenuation caused by passing radiation through a cryostat (a component of some machines).

[0005] The Elekta-Unity MR-LINAC (Magnetic Resonance Linear Accelerator) is an example of a system that delivers radiation via a helium-filled cryostat. The LINAC component provides radiotherapy and X-ray imaging capabilities. The MR component provides magnetic resonance imaging capabilities. Typically, radiation from the LINAC is delivered via the cryostat of the MR device. In such systems, defects in the cryostat, such as metal plate thickness tolerances or weld seams, can cause variations in radiation output as the gantry rotates, resulting in undesirable variations in beam attenuation. Typically, these attenuation variations occur in all directions, but are also longer due to the length of the arc (the direction of gantry rotation). These output variations exceed typical specifications for dose output variation (the ratio of the set radiation dose to the radiation dose measured at the isocenter). Typically, the cryostat assembly produces less than 5% output variation, requiring 1% of the typical specification.

[0006] In known solutions, Farmer cavity measurements can be used to obtain the magnitude of variation in order to compensate for the inhomogeneity of cryostats. In this technique, a radiation detector, such as a PTW Farmer ionization cavity available from PTW Freiburg GmbH, is placed at the isocenter. For fixed monitoring unit delivery, the dose is measured every 2 degrees. Farmer cavity measurements are both difficult and time-consuming. Errors can be introduced due to air gaps (e.g., the air gap between the plexiglass cover and the cavity), and expensive facilities are required. Furthermore, when using a Farmer cavity, attenuation is determined relative to a reference angle and on the angular axis only for a single cryostat housing. This approximation ignores any variations caused by beam divergence in multi-housing cryostats, whereby rays can pass through one layer at one angle and then through another layer at a different angle. Summary of the Invention

[0007] The invention is set forth in the independent claims. Optional features are set forth in the dependent claims. Attached Figure Description

[0008] Specific embodiments will now be described by way of example only with reference to the accompanying drawings, in which:

[0009] Figure 1 This is a schematic diagram of an MR-LINAC device;

[0010] Figure 2A and Figure 2B The cross-section of the low-temperature thermostat is depicted;

[0011] Figure 3A and Figure 3B A geometric representation of the relevant features of the cryostat is depicted;

[0012] Figure 4 A comparison of experimental data characterizing the cryostat according to this disclosure with Farmer cavity measurement data is depicted. Detailed Implementation

[0013] Overall, this invention seeks to address the shortcomings encountered in the prior art by providing a method for determining radiation attenuation through a cryostat. This method offers an alternative solution for cryostat characterization that is faster and less error-prone than Farmer cavity measurements.

[0014] In one example, a rack-mounted imaging device (e.g., the rack-mounted imaging device in the Elekta-Unity system) is used to acquire a series of images. This imager captures radiation attenuation through two passes (incident and exit) of a cryostat. A numerical model driven by the fully automated imaging device can be used to derive and resolve perturbations in beam attenuation caused by cryostat inhomogeneities. By acquiring multiple images over a 360-degree rack area, radiation attenuation through the cryostat can be determined. This information can then be supplied to a treatment planning system, where cryostat attenuation can be taken into account during dose calculations. This can be done, for example, in software by performing dose calculations within the treatment planning system that calculate the attenuation of individual photons as they pass through a suitable cryostat model (e.g., a transmission cylinder).

[0015] The proposed method alleviates the time-consuming and laborious task of measuring cryostat non-uniformity using Farmer cavity measurements. This method can be integrated into automated workflows. Although the proposed method measures radiation across both sections of the cryostat, the low average error between the obtained perturbation coefficients indicates that the model well represents Farmer cavity measurements performed at isocenters.

[0016] High-level overview of MR-LINAC

[0017] Figure 1 A radiotherapy device 100 suitable for delivering a radiation beam to a patient during radiotherapy treatment and configured to deliver a radiation beam to a patient is depicted. The device and its constituent parts will be generally described to provide accompanying information useful to the invention. Figure 1 The apparatus depicted herein is based on the present invention and is suitable for use with the disclosed systems and devices. Although Figure 1 The device described is MR-LINAC, but embodiments of this disclosure can be any radiotherapy device, such as a LINAC device.

[0018] Figure 1 The device described herein is an MR-LINAC. This device includes both an MR imaging unit 112 and a radiotherapy (RT) unit, which may include the LINAC unit. In operation, the MR scanner generates MR images of the patient, and the LINAC unit generates and shapes a radiation beam according to the radiotherapy treatment plan, directing it toward a target area within the patient's body. The depicted device does not possess the typical "casing" found in commercial environments (such as hospitals) that would cover the MR imaging unit 112 and the RT unit.

[0019] Figure 1The MR-LINAC device 100 depicted includes a radiation source 106, which may include beam-generating devices such as one or more of the following: a radio frequency source 102, a circulator 118, an electron source 105, a waveguide 104, and a target (not shown). The MR-LINAC device 100 may also include a collimator 108 (e.g., a multi-leaf collimator) configured to collimate and shape the beam, an MR imaging device 112, and a patient support surface 114. The device also includes a housing that, together with the circumferential gantry, defines an aperture. The movable support surface 114 can be used to move a patient or other object into the aperture when an MR scan and / or radiotherapy is about to begin. The MR imaging device 112, the RT device, and the object support surface actuator are communicatively coupled to a controller or processor. The controller is also communicatively coupled to a memory device including computer-executable instructions that can be executed by the controller.

[0020] The RT device includes a radiation source 106 and a radiation detector (not shown). Typically, the radiation detector is positioned radially opposite the radiation source 106. The radiation detector is adapted and configured to generate radiation intensity data. In particular, the radiation detector is positioned and configured to detect the intensity of radiation that has passed through an object. The radiation detector can also be described as a radiation detection device and can form part of an illumination field imaging system.

[0021] Radiation source 106 defines the point at which the treatment beam 110 is introduced into the aperture. Radiation source 106 forms part of a beam generation system that may include an RF energy source 102, an electron gun 105, and a waveguide 104. The beam generation system is attached to a rotatable gantry 116 so as to rotate together with the gantry 116. In this way, radiation source 106 can rotate around the patient, allowing the treatment beam 110 to be applied from different angles around the gantry 116. In a preferred embodiment, the gantry can rotate continuously. In other words, the gantry can rotate 360 ​​degrees around the patient, and in fact, can continue to rotate beyond 360 degrees. The gantry can be annular. In other words, the gantry can be a ring-shaped gantry.

[0022] A radio frequency (RF) source 102 (e.g., a magnetron) is configured to generate RF waves. The RF source 102 is coupled to waveguide 104 via a circulator 118 and configured to pulse the RF wave into waveguide 104. The RF wave can travel from the RF source 102 through an RF input window and into an RF input connection conduit or tube. An electron source 105 (e.g., an electron gun) is also coupled to waveguide 104 and configured to inject electrons into waveguide 104. In the electron source, electrons are thermally emitted from the cathode filament as it is heated. The temperature of the filament controls the number of injected electrons. The number of injected electrons can be additionally controlled by the gate voltage. The injection of electrons into waveguide 104 is synchronized with the pumping of RF waves into waveguide 104. The design and operation of the RF source 102, electron source 105, and waveguide 104 are such that the RF wave accelerates the electrons to very high energies as they propagate through waveguide 104.

[0023] The design of waveguide 104 depends on whether the LINAC uses standing waves or traveling waves to accelerate electrons, although waveguides typically consist of a series of chambers or cavities, each connected by an aperture or "iris" through which the electron beam can pass. These cavities are coupled to generate a suitable electric field pattern that accelerates electrons propagating through waveguide 104.

[0024] To ensure that electron propagation is unimpeded as the electron beam travels toward the target, waveguide 104 is evacuated. Electrons can be accelerated to near the speed of light within the vacuum waveguide 104.

[0025] Radiation source 106 is configured to direct therapeutic radiation beam 110 toward a patient positioned on patient support surface 114. Radiation source 106 may include a heavy metal target toward which high-energy electrons exiting a waveguide are directed. When electrons strike the target, X-rays are generated in various directions. A master collimator may block X-rays traveling in certain directions and allow only forward-traveling X-rays to pass through to generate therapeutic beam 110. X-rays may be filtered and may pass through one or more ion chambers for dose measurement. Before the beam enters the patient's body as part of radiotherapy, it may be shaped in various ways by beam-shaping devices, such as by using a multi-leaf collimator 108.

[0026] In some embodiments, the radiation source 106 is configured to emit an X-ray beam or an electron particle beam. Such an embodiment allows the device to provide electron beam therapy, i.e., an external beam therapy, in which electrons, rather than X-rays, are directed toward a target region. The LINAC's components can be "swapped" between a first mode of emitting X-rays and a second mode of emitting electrons.

[0027] The object or patient support surface 114 is configured to move between a first position substantially outside the aperture and a second position substantially inside the aperture. In the first position, the patient or object can mount the patient support surface. The support surface 114 and the patient can then be moved within the aperture to the second position for imaging the patient by MR imaging device 112 and / or imaging or treating the patient 140 using RT device. The movement of the patient support surface is achieved and controlled by an object support surface actuator, which can be described as an actuation mechanism. The actuation mechanism is configured to move the object support surface in a direction parallel to and defined by the central axis of the aperture. The terms object and patient are used interchangeably herein, such that the object support surface can also be described as a patient support surface. The object support surface may also be referred to as a movable or adjustable recliner or table.

[0028] Figure 1 The radiotherapy device / apparatus depicted also includes an MR imaging device 112. The MR imaging device 112 is configured to acquire images of an object positioned (i.e., located) on an object support surface 114. The MR imaging device 112 may also be referred to as an MR imager. The MR imaging device 112 can be a conventional MR imaging device that operates in a known manner to acquire MR data (e.g., MR images). Those skilled in the art will understand that such an MR imaging device 112 may include a main magnet, one or more gradient coils, one or more receiving coils, and an RF pulse applicator. The operation of the MR imaging device is controlled by a controller.

[0029] A controller is a computer, processor, or other processing device. A controller may be formed from several discrete processors; for example, a controller may include an MR imaging device processor that controls the MR imaging device 112; an RT device processor that controls the operation of the RT device; and an object support surface processor that controls the operation and actuation of the object support surface. The controller is communicatively coupled to memory, i.e., a computer-readable medium.

[0030] As is well known to those skilled in the art, the LINAC device also includes several other components and systems. The entire system is cooled by a water cooling system (not shown in the figures). In particular, the water cooling system can be used to cool the waveguide 104, the target, and the radio frequency source 102. Appropriate shielding is also provided to ensure that the LINAC does not leak radiation. As those skilled in the art will understand, the LINAC device for radiotherapy will have additional equipment such as a gantry for supporting and rotating the LINAC, a patient support surface, and a controller or processor configured to control the LINAC device.

[0031] The LINAC component of the MR-LINAC device can provide radiotherapy and X-ray imaging capabilities. The MR component can provide magnetic resonance imaging capabilities. Figure 2A and Figure 2B An example cross-section of a cryostat in a radiotherapy device is depicted. Typically, radiation from a LINAC is delivered through the field generation unit of an MR device. The radiation beam 110 passes through the cryostat 304, gradient coil 306, and system body coil 308 before being delivered to a patient surrounded by the device. The system body coil 308 may be an orthogonal body coil. As used herein, the general term “cryostat” may be used to refer to the overall field generation unit comprising the cryostat 304, gradient body coil 306, and system body coil 308, any other suitable refrigerant unit, or any other attenuation element. The cryostat may be annular or cylindrical and surrounds the patient during treatment. The cryostat may be fixed in place in a position where a gantry surrounds the cryostat and provides means for the LINAC to rotate 360 ​​degrees to deliver radiation from different angles. This arrangement means that at any given location, radiation 110 will be delivered to the patient by passing through the cryostat.

[0032] The gantry may also feature a radiation detector in the form of an imaging device, which may be located at a point opposite the radiation beam 110 on the gantry diameter and may be used to image the therapeutic radiation generated by the LINAC during radiotherapy treatment. The device may be a digital imaging device (e.g., a CCD camera), other semiconductor-based detectors, and / or a liquid ion chamber. The device may be a megavolt X-ray imager and / or an electronic irradiation field imaging device (EPID). The device may include a flat panel detector, a scintillator, an amorphous silicon (a-Si) based image panel, and / or a scintillator-mirror-camera system. References to EPD-based methods and apparatus in this disclosure should be considered applicable also to megavolt X-ray imaging devices or any other suitable imaging device.

[0033] Imaging device drives numerical model

[0034] The described method allows for the numerical simulation of cryostat perturbations using existing imaging devices outside the cavity, such as megavolt X-ray imaging devices and / or EPID or any other suitable imaging device. Perturbations in beam attenuation caused by cryostat inhomogeneities can be compensated for using a numerical model driven by a fully automated imaging device. The perturbations can be obtained from the images using an appropriate geometric representation of the cryostat. The cavity can be modeled as follows: Figure 3A and Figure 3B As shown in the image.

[0035] Figure 3A A schematic diagram depicts the geometric layout of the device considered in the model. In this example, a device with a radius is used. and thickness The cryostat is modeled using a single outer shell. The radiation source (solid cross) is placed at a distance SI from the isocenter. The source can rotate around the frame at an angle. And at an angle relative to the central axis The delivery beam is represented by the radiating beam (dashed line), which intersects the outer shell (solid black circle) and the EPID (shaded rectangle) at a vertical distance SE (cm) from the radiation source and a horizontal distance E (cm) from the isocenter. The radiating beam can be considered as a diverging beam comprising multiple sub-beams radiating outward from the source, including a central axis sub-beam and additional sub-beams following corresponding paths forming an angular range on either side of the central axis.

[0036] Figure 3B An enlarged schematic diagram of a cryostat is depicted. This is based on an offset angle relative to the central axis. The delivered sub-bundles can use a tilt angle. and shell thickness To approximate the distance the sub-beams travel across the cryostat The tilt angle This represents the angle between the sub-beam path and a line originating from the isocenter of a circle perpendicular to the circumference of the cryostat; it can be expressed by... and sub-bundle cross angle We obtain the result by summing.

[0037] A beam passing through a decaying material (such as a cryostat) will have a decay value. initial strength On MR-LINAC, there is incident intensity. The sub-bundle at the frame angle At the incident ( ) and ejection ( Both are delivered through a cryostat, passing through its two sections. The cryostat can be modeled as... A clustered shell, in which the first The outer shell exhibits a linear attenuation coefficient. and thickness Sub-beam attenuation can be modeled differently depending on whether they are tilted (intersecting the circumference of the cryostat at a non-orthogonal angle) or non-tilted (intersecting the circumference of the cryostat at an orthogonal angle). For non-tilted orthogonal sub-beams, the intensity measured by EPID on the rack... This can be expressed as:

[0038] (1)

[0039] For the angle The tilted sub-bundle, such as Figure 3B As shown, crossing the first Local distance of the outer shell For a thin shell, it is given by the following formula:

[0040] (2)

[0041] Utilizing this, the intensity of the tilted sub-bundle passing through a perfectly uniform cryostat It can be written as:

[0042] (3)

[0043] Equations (1) and (3) can be used to model the intensity of non-tilted or tilted sub-bundles measured by EPID on the rack.

[0044] If it is not possible to sample all rack angles, then sample the two closest available rack angles ( ; Linear interpolation between each of the above exponents.

[0045] Indicates entering and exiting the first Interpolation factors for the incident and exit angles of the outer shell:

[0046] , , , (4)

[0047] Interpolation factors can be obtained by simulating the sub-beam path through a cryostat using ray tracing. Slope cosine factor First, by solving the following simultaneous equations ( Figure 3A Find the angle of the rack Place and No. Sub-bundle intersection of a low-temperature thermostat housing :

[0048] (5)

[0049] (6)

[0050] in, , , and They are the first The source EPID distance, EPID field of view, source isocenter distance, and radius of the cryogenic thermostat housing.

[0051] These two solutions correspond to the sub-beam incident and exit:

[0052] (7)

[0053] (8)

[0054] use Figure 3B The information provided above, along with the two solutions, can be used to calculate the sub-beam crossover angles on the incident and exit sides. and ,as follows:

[0055] (9)

[0056] Then the incident cosines of the inclinations on the incident and exit sides are:

[0057] (10)

[0058] in This refers to the EPD offset angle. It pertains to the rack angle. The calculated cosine of the tilt is valid for all other rack angles. Interpolation factor. It can be based on the angle of the frame The point of intersection of the bundles Confirmed. First, at any rack angle... The beam crossing angle at the point can be determined as:

[0059] (11)

[0060] (12)

[0061] Then the interpolation factor for each rack angle can be... The weights are calculated between the two closest available angles, such that:

[0062] (13)

[0063] (14)

[0064] Using the calculated interpolation factor and the tilted cosine, a sparse linear matrix solver can be used to compute the perturbation of a shell.

[0065] definition Using equation (4), equation (3) can be rewritten as:

[0066] (15) (16)

[0067] Equations (15) and (16) describe the attenuation of the radiation beam by an idealized uniform cryostat. To account for the effects of cryostat defects, the... , Or the sub-bundle intensity of the non-uniform decay in either can be approximated as First-order perturbation in :

[0068] (17)

[0069] Although local non-uniformities will cause disturbances in the decay, the decay of the cryostat as a whole can be approximated as a uniform average of the effects of these disturbances. It can be approximated by the median sub-beam intensity over all rack angles from 0 to 360 degrees (or 2π radians):

[0070] (18)

[0071] This allows for the perturbation beam strength observed at EPID. Calculated as in the given and The intensity detected at that location is similar to that at a given location. The ratio of median sub-beam intensities detected at all rack angles:

[0072] (19)

[0073] Perturbation beam strength Experiments can be conducted using EPID images Obtain, as in the given and The intensity detected at that location is similar to that at a given location. The ratio of median sub-beam intensities detected at all rack angles:

[0074] (20)

[0075] Among them, again, It is the median EPID image value over the entire range of rack angles. In equation (20), it is assumed that... and It is proportional to some constant factor (e.g., detector efficiency), and these factors cancel each other out when the ratio is calculated.

[0076] If the perturbation is small, the Maclaurin expansion can be applied. :

[0077] (twenty one)

[0078] Then, for a shell ( The product is reduced to a factor, and the exponent k can be reduced. The perturbation coefficient can be calculated using the following formula. :

[0079] (twenty two)

[0080] For many enclosures, the perturbation coefficient Similarly, it can be calculated using the following formula:

[0081] (twenty three)

[0082] like Figure 3A The perturbation beam strength calculated from the EPID image is shown in the figure. The attenuation will be caused by the two segments of the cryostat, and the attenuated beam strength at the isocenter is the amount of interest for the treatment plan. The perturbation coefficients can be calculated using equation (22) or (23) with the experimental EPID image values ​​having equation (20). These perturbation coefficients can be used with equation (17), which is modified to consider only one cryostat segment passing through, to calculate the beam attenuation at the isocenter due to the cryostat. For the isocenter sub-beam, the incident light through all the shells is perpendicular, therefore all cosine factors are 1, and the angle... and and interpolation coefficients and It does not depend on the shell radius or the exponent k. Therefore, Equation 17 simplifies to:

[0083] (twenty four)

[0084] This value can then be used to fully characterize the cryostat and to adjust the intensity of the radiation beam 110 accordingly to compensate for the non-uniformity of the cryostat.

[0085] Adjusting the beam intensity to compensate for cryostat inhomogeneities can be done during device calibration or in real-time during treatment. This provides an improvement from typical output variations of less than 5%, allowing for better attainment of treatment plans with the desired specifications of 1%. The reduction in variation can be verified by implementing cryostat inhomogeneity compensation in the treatment planning system, delivering a fixed dose to the isocenter using the system, and confirming that the fixed dose was measured at the isocenter.

[0086] Figure 4 The results of this method are depicted and compared with the more time-consuming Farmer cavity measurements. A very close match was observed between this method and slower conventional measurement techniques.

[0087] A Farmer cavity was used to verify the calculated perturbation. Charge was measured using a non-tilted sub-beam from the Farmer cavity, which was positioned at the isocenter of the system. It can be written as:

[0088] (25)

[0089] Following the same logic as for each EPID measurement, the perturbation coefficient can be expressed as:

[0090] (26)

[0091] The perturbation coefficients calculated based on the EPID-driven numerical model are compared with the fundamental true values ​​in the Farmer cavity measurement. Figure 4 Good consistency was achieved between the values ​​described in the figure. The mean and standard deviation errors of the absolute difference between the EPID and the Farmer cavity perturbation coefficient (at all rack angles) were: wide beam: 0.23 ± 0.16%; thin beam: 0.20 ± 0.18%. On an Intel i7 3540 M processor with 8GB of RAM, the perturbation coefficient was calculated in approximately 25 seconds, providing a surprisingly fast and accurate characterization technique.

[0092] This method provides an alternative solution for cryostat characterization that is faster and less error-prone than Farmer cavity measurements. Negligible differences were observed between this method and the slower Farmer cavity decay measurements. This method provides a way to account for variations (and possible anomalies) in the Y-direction and for different cryostat layers. Because of its ease of use, it allows users to consider the helium level of the cryostat for cryostat characterization.

[0093] This method can also be used to correct changes in beam symmetry when the radiation source rotates around the gantry. This method can be implemented using an imaging device placed inside or outside a cryostat. By appropriately selecting the imaging device, this method can be implemented using kilovolt (kV) and / or megavolt (mV) X-rays.

[0094] This method can be implemented on a device or system configured to perform the steps described above. This method can be executed by a processor configured to execute instructions stored in a computer-readable medium, which cause the processor to perform the disclosed method.

Claims

1. A method for characterizing the physical properties of an attenuation element in a radiotherapy device, the radiotherapy device having a radiotherapy radiation source and a radiotherapy radiation detector on a corresponding side of the attenuation element, wherein, The attenuation element includes a cylindrical component, wherein the cylindrical component comprises one of a cryostat, a gradient coil, or an orthogonal coil, and the method includes: obtaining an average detected radiation intensity at two or more locations around the attenuation element; comparing the detected intensity at one location with the average intensity; and characterizing a corresponding physical property based on the comparison, wherein the characterized physical property is at least one perturbation coefficient given by the following equation: ,in It is derived by comparing the detection intensity at a location with the average intensity; in, The rotation angle of the radiation source for the radiotherapy; This represents the angle between the sub-beam path and the line from the isocenter of the circle perpendicular to the cryostat; Indicates entering and exiting the first Interpolation factors for the incident and exit angles of the outer shell: ， ， ， ； Indicates the angle of incidence on the frame. Indicates the launch frame angle; and , The charge is measured by a non-tilted sub-beam from a Farmer cavity placed at the center of the system.

2. The method according to claim 1, wherein, The radiation source for the radiotherapy includes a linear accelerator.

3. The method according to claim 1 or 2, wherein, The radiotherapy radiation detector is rotatable relative to the attenuation element, and the corresponding physical properties are related to the rotation angle.

4. The method according to claim 1 or 2, wherein, The radiation source for radiotherapy is located radially opposite to the radiation detector for radiotherapy.

5. The method of claim 1 or 2, further comprising using the characterized physical properties to calibrate or control the radiation intensity.

6. The method according to claim 1 or 2, wherein the method is performed as part of a radiotherapy device calibration method.

7. A method for controlling the operation of a radiotherapy device, the radiotherapy device comprising a radiotherapy radiation source and a radiotherapy radiation detector on corresponding sides of an attenuating element, the attenuating element comprising a cylindrical component, wherein the cylindrical component comprises one of a cryostat, a gradient coil, or an orthogonal coil, the method comprising: Radiation is delivered from the radiation source of the radiotherapy to the isocenter; as well as The radiation detector used in the radiotherapy was used to characterize at least one aspect of radiation attenuation. At least one aspect of the radiation attenuation is characterized using the method described in any one of claims 1 to 6.

8. The method of claim 7, further comprising correcting the intensity of radiation delivered to the isocenter based on at least one aspect of radiation attenuation.

9. The method according to claim 8, wherein, The characterization and correction are performed in real time during radiation delivery.

10. The method according to claim 8, wherein, The correction is performed after a calibration process that includes the characterization step.

11. An apparatus comprising a radiotherapy radiation source and a radiotherapy radiation detector on respective sides of an attenuating element, said apparatus being configured to perform the method according to any one of the preceding claims.

12. A computer-readable medium comprising instructions that, when executed by a processor, perform the method as claimed in any one of claims 1 to 10.

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