Cubic electronic detector array for radiotherapy verification

Abstract

A volumetric radiation dose detector provides radiation sensors distributed along plates of a laminated stack including materials with different Compton scattering to provide the stack with a water equivalence. The plate configuration allows ready adjustment of separation between the photodetectors to provide varied resolutions across the area of the detector and with depth. Printed circuit techniques may be used to produce the plates holding the photodetectors for simplified signal management.

Description

CUBIC ELECTRONIC DETECTOR ARRAY FOR RADIOTHERAPY VERIFICATIONSTATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

[0001] CROSS REFERENCE TO RELATED APPLICATION

[0002] The present application claims the benefit of us provisional application 63 / 484,582 filed February 13, 2023, and hereby incorporated by reference.BACKGROUND OF THE INVENTION

[0003] The present invention relates generally to radiation therapy equipment and in particular to a dosimeter for quality assessment of radiotherapy equipment and patient treatment plan.

[0004] Prior to clinical service, a new radiotherapy linear accelerator (linac) must undergo extensive data acquisition measurements to establish that linac’s dosimetric performance that will set the baseline for subsequent quality assurance (QA) checks throughout its service lifetime. Also, the acquired data serves as input to treatment planning software (TPS) that provides an accurate simulation and dose calculation of a patient’s radiotherapy treatment to be delivered with that linac. This extensive data acquisition process is known as commissioning.

[0005] The commissioning data consists of percent depth doses (PDDs), tissue maximum ratios (TMRs), beam profiles, output factors, off-axis factors, wedge factors, and cone factors acquired for different field sizes, photon energies, and electron energies.

[0006] Currently, the commissioning data is measured in a tank of water using an ion chamber (IC) or diode detector mounted on a carriage system to move along the cross-plane, inplane, and vertical axes of an approximately 50 x 50 x 50 cm3volume. Beam profiles are acquired with cross-Zin-plane beam scans, while PDDs / TMRs are acquired with vertical scans. The carriage is computer-controlled, and its software can provide the acquired data in real-time. Also, the software can be programmed to execute multiple sequential scans according to predetermined parameters.

[0007] A linac that generates 6 MV, 10 MV, 15 MV, 6FFF MV, and 10FFF MV photon beams and 6 MeV, 9 MeV, 12 MeV, 15 MeV, 18 MeV, and 22 MeV electron beams may have a total of 2,255 commissioning measurements. Despite the automation in the water scanningsystem for commissioning data acquisition, it is estimated that two to three weeks of around-the- clock work could be needed to acquire this data, barring no equipment malfunction. On average, data collection time could be close to a month, working 10-12 hour days, six or seven days a week.SUMMARY OF THE INVENTION

[0008] The present invention provides a solid-state dose verification instrument that can replace water tank measurements to provide practically instantaneous PDD, profiles, and output factor data acquisitions throughout a volume of interest. Solid-state detectors are embedded in a water equivalent material at fixed depth locations with varied spacing to implement a trade-off between detector complexity, resolution, and areas of particular interest. The deviation from water equivalents of the detectors, the detector supporting structure, and the large amount of wiring is minimized through the use of integrated circuit techniques decreasing component and conductor sizes and compensated for by a calibration process that models the structure of the various materials of the instrument.

[0009] More specifically, in one embodiment, the invention provides a radiotherapy dose verification instrument constructed of a radiolucent stack of planar plates assembled together along an axis to define a measurement volume defined by an area of the plates and a cumulative thickness of the plates in the stack. A plurality of solid-state electronic radiation detectors are distributed through the measurement volume supported on a set of the plates, and the plates are selected to provide a radiolucent stack with water-equivalent absorption at mega voltage x-rays.

[0010] It is thus a feature of at least one embodiment of the invention to provide an instrument that permits rapid radiation therapy verification over a volume using a dense array of solid-state detectors.

[0011] The plates may include a first set of plates holding the solid-state electronic radiation detectors, the plates of the first set separated by a second set of plates of a different material having lower Compton scattering than a material of the first set of plates at megavoltage radiation levels.

[0012] It is thus a feature of at least one embodiment of the invention to permit tailoring some of the plates for the support of circuitry for the solid-state detectors such as may require a higher Compton scattering material while maintaining a water equivalent by proper selection of the remaining plates.

[0013] In one example, the first set of plates are printed circuit boards supporting photodetectors as electrically joined with applied copper conductors having a thickness of less than 0.5 mm measured along the axis.

[0014] It is thus a feature of at least one embodiment of the invention to allow tractable interconnection of the electronic photo detecting elements using axially thin high-density copper possible through integrated circuit techniques avoiding the manufacturing problems associated with comparably thin discrete wiring.

[0015] The spacing of the plurality of solid-state electronic radiation detectors along the axis increases from an upper side of the volume to a lower side of the volume along the axis.

[0016] It is thus a feature of at least one embodiment of the invention to provide a dose verification instrument well adapted to both photon and electron beams having relatively shallow penetration as well as x-ray radiation operating at greater depths.

[0017] In some embodiments, a spacing of the plurality of solid-state electronic radiation detectors on each plate may be closer near a center of an area of the plate than at edges of each plate.

[0018] It is thus a feature of at least one embodiment of the invention to boost resolution in an area likely to be aligned with a treatment region of interest and reference field size for beam dose calibration.

[0019] The solid-state electronic radiation detectors maybe staggered with respect to paths along the axis passing through the stack.

[0020] It is thus a feature of at least one embodiment of the invention to minimize deviations from a water equivalent caused by the relatively high density photo detecting elements by reducing the chance that multiple photo detecting elements will be in alignment.

[0021] The second plates are an elastomeric material conformally covering the first plates.

[0022] It is thus a feature of at least one embodiment of the invention to provide an instrument having inherent shock resistance and minimizing air voids.

[0023] The instrument may further include an electronic circuit receiving signals from the plurality of solid-state electronic radiation detectors and applying a calibration factor to the signals being a function of radiation energy and radiation angle.

[0024] It is thus a feature of at least one embodiment of the invention to provide an accuracy comparable to an ionization detector in a homogenous water bath by preparing calibrationweights offsetting deviations caused by the inhomogeneity of the stack.

[0025] These particular objects and advantages may apply to only some embodiments falling within the claims and thus do not define the scope of the invention.BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Fig. 1 is a simplified perspective diagram of a radiotherapy machine as may be positioned above a cubic detector of the present invention supported on a patient table and in communication with a computer for providing quality assurance reports;

[0027] Fig. 2 is an exploded perspective view of the cubic detector of Fig. 1 showing support of detector arrays on planar integrated circuit substrates sandwiched between water equivalent material, the detector arrays having multiple regions of different detector spacings;

[0028] Fig. 3 is an elevational, fragmentary cross-sectional view through the assembled cubic detector of Fig. 2 showing a varied vertical spacing of the integrated circuit substrates to provide varying resolution of the detection at different detector depths;

[0029] Fig. 4 is fragmentary cross-section of the detector of Fig. 3 showing close conformity of the water equivalent material to the detector elements and a staggering of the detector elements;

[0030] Fig. 5 is a flowchart of a calibration process to reduce the effect of non-water equivalent material in the cubic detector; and

[0031] Fig. 6 is a schematic diagram showing a multiplexing system for the detector elements to minimize wire interconnections.DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0032] Referring now to Fig. 1, a radiation detector system 10 of the present invention may be used with radiotherapy equipment 12 of a type providing a radiation beam 14 that may be directed toward a patient table 16 at a variety of angles. These angles may be distributed in both azimuth and altitude about a treatment region occupied by the radiation detector system 10, and the radiation may be photon radiation or electron radiation. In this regard, the invention contemplates use with radiotherapy linacs as well as intensity-modulated radiotherapy (IMRT) and volumetric-modulated art therapy (VMAT) treatment modalities. .

[0033] In one embodiment, the detector system 10 may be placed for support on the patienttable as stabilized by its own weight and, for example, elastomeric feet (not shown) and positioned with respect to a known fiducial of the radiotherapy equipment 12. As so positioned, the radiation detector system 10 may communicate via electrical cable 26 or wirelessly to a computer 28 that may receive electrical signals from the detector system 10 indicating radiation dose at a variety of locations within a measurement volume of the detector system 10. Using that data, the computer 28 may provide for the display of dose information in a variety of forms as will be discussed below.

[0034] Referring now also to Fig. 2, the detector system 10 may be constructed of alternate layers of a water equivalent material 30, for example, a silicone rubber, acrylic, or other well- known water equivalent, and detector arrays 32, for example, being thin planar substrates supporting electronic components and conductive traces assembling the components together. For example, the detector arrays 32 may employ substrates using bakelite, which is a plastic made from synthetic materials and density comparable to water, for example, having a thickness of 1.57 mm or less or a range between 0.78 mm and 2.36 mm. These substrates may support thin copper traces 34, for example, less than 0.150 mm thick, interconnecting electronic components of detecting pixel elements 36 providing point measurements of radiation dose. Furthermore, a pixel element (diode, capacitor, MOSFET [transistor]) could be made of graphene which has a comparable biological composition as the human body, and further reduced calibration factor corrections.

[0035] Generally the water equivalents of a material may be assessed by measurements comparing the material to water with respect to measurements by an ionization chamber at an equivalent physical depth, percentage depth dose (PDD), or the like with water equivalents being materials that have less than 5% and typically less than 2% deviation from water with respect to these measurements.

[0036] In one embodiment, the detector system 10 may have outer dimensions of 40 cm width x 40 cm length x 35 cm height with twenty, 30 x 30 cm detector arrays 32 each providing 1200 spaced pixel elements 36 along their surfaces.

[0037] A central area 38 of the detector arrays 32 may have a closer spacing of the detector pixel elements 36 than an outer peripheral region 40 of the detector arrays 32. Thus, for example, the pixel elements 36 in the 10 x 10 cm2central area 38 may have a 0.5 cm separation in width and length dimensions, while elsewhere the pixel element 36 separation is 1.0 cm. Theentire detector system 10 may have a total of 24,000 pixel elements 36.

[0038] The layers of water equivalent material 30 each may be, for example, approximately 40 by 40 centimeters on a side and 1 cm thick.

[0039] Referring now to Fig. 3, in one embodiment, the spacing between the detector arrays 32 may vary with height being positioned at depths of 1.0 cm, 1.5 cm, 2.0 cm, 2.5 cm, 3.0 cm, 3.5 cm, 4.0 cm, 6.0 cm, 8.0 cm, 9.0 cm, 9.5 cm, 10.0cm, 10.5 cm, 11.0 cm, 12.0 cm, 14.0 cm, 18.0 cm, 22.0 cm, 26.0 cm, and 30.0 cm. Thus, in a first range 31 between 1.0 cm and 4.0 cm depths, the detector arrays 32 have a 0.5 cm separation which covers photon and electron beams’ dose maximum depths. Also, detector arrays 32 have a 0.5 cm separation in a second range 33 at depths between 9.0 cm and 11.0 cm. This is correlated to photon beam quality at a reference depth of 10 cm.

[0040] The inventors contemplate that in one embodiment individual or groups of the detector arrays 32 supported by one or more layers of water equivalent material 30 may be detached physically to be used in isolation. For example, the bottommost detector array 32 may be used individually for daily linac quality assurance. The upper detector layers 32 and water equivalent material 30, within a 10 cm depth, may be used individually for quality assurance of a clinical plan. All of the detector arrays 32 depicted in Fig. 3 and water equivalent material 30 assembled together may be used for monthly quality assurance. These individually detachable elements may be retained, for example, by the weight of the elements in the stack as aligned with radiolucent alignment pins or the like or may be assembled with a radiolucent band or other known attachment mechanism.

[0041] Referring to Fig. 4, pixel elements 36 on each detector array 32 may be staggered vertically so as to reduce cumulative scattering or absorption in the received x-ray beam 14 along a single trajectory that would differ from the water equivalent material 30. Generally, the water equivalent material 30 may be constituted to flow around the electrical components of the pixel elements 36 during manufacture providing a conformal coating to eliminate air pockets or voids that could also create discontinuities in the water equivalency of the detector system 10. Subsequent to this flowing, the material may harden for convenient transportation of the device 10 and resistance against shock particularly when the water equivalent material 30 is an elastomer.

[0042] Referring now to Fig. 5, deviations from a homogenous water equivalency of thedetector system 10 caused by the materials of the detector arrays 32 may be accommodated through a calibration process in which the physical properties of the detector system 10 arc modeled, for example, to include the dimensions and sizes of each of the constituent components of the water equivalent material 30, the substrates of the detector arrays 32, the circuitry of the pixel elements 36, and the traces 34. For various angles and energies of photon radiation and energy radiation, a Monte Carlo simulation in water equivalent and detector array material per process block 42 may be conducted to compute expected relative dose values at each pixel element 36 since each pixel may have manufacturing variations and respond differently to a specific beam quality. At process block 44, another Monte Carlo simulation in water, of the locations of the pixel elements 36 may then be taken for each of those photon and electron energies and angles, and a weighting factor calculated for each of the pixel elements 36 and photon and electron energy and angle, and stored as a calibration factor 46 associated with each detector system 10.

[0043] In one embodiment, Monte Carlo simulations that incorporate each detector array imbedded in acrylic blocks and silicon rubber will be used to get a characterization function for each pixel in an array. Thereafter, using MC data in water, correction factors for that medium are determined for each pixel. These data can be transformed to absolute dose using dose measurement data collected with a calibrated Farmer chamber.

[0044] During use of the detector system 10, as indicated by process block 48, the readings from the pixel elements 36 taken from a radiotherapy machine 12 may be adjusted per process block 49 by the weighting or calibration factors stored at process block 46 to provide a final output 50 corrected for the minor deviations caused by the non-water equivalency of the materials of the traces 34, the substrate of the detector arrays 32, and the electronic components of the pixel elements 36. A particular weighting factor may be identified by input by the user indicating the radiation intensity and angle of the radiotherapy machine 12 being tested so that the correct weighting factors may be identified from the table.

[0045] Referring now again to Fig. 1, electrical signals from the traces 34 may be collected and digitized by an interface circuit 64 to send along the electrical cable 26 to the computer 28 (shown in Fig. 1) where the process steps 49 and 50 may be implemented using stored values of the weighting factors of process block 46.

[0046] The data collected by the present invention may be used to provide a variety of outputdisplays on the computer 28 directly including, for example, percent depth doses (PDDs), tissue maximum ratios (TMRs), beam profiles, output factors, off-axis factors, wedge factors, and cone factors acquired for different field sizes, photon energies, and electron energies.

[0047] Referring now to Fig. 5, the pixel (consisting of a diode, capacitor, MOSFET [transistor]) elements 36 may be arranged to be multiplexed in an active array to reduce the number of traces necessary to connect each of these detectors to the computer 28. In each pixel element 36, strobe lines 74 may connect to the gate of a thin film transistor 78. The transistor 78 may connect an output line 76 for diode 36 to the anode of a capacitor 80 that stores the radiation sensitivity in the form of charge. Each capacitor is discharge by opening the transistor gate. In one embodiment the diodes 80 may be PIN photodiodes. The cathode of the diode 80 may be connected to a ground wire (not shown) that passes in a raster fashion through each pixel element 36 of the rows 70 and columns 72. A MOSFET transistor 82 shunts the pixel capacitor 80 sending its charge to a data acquisition unit (DAQ), resetting it to zero charge, and getting it ready for the next data acquisition 36. The diodes 80 may each have an area of approximately 0.5 x 0.5 cm2in a plane of the pixel element 36 and a thickness of approximately 0.3 mm along the surface normal.

[0048] For further information, each pixel may consist of a diode, capacitor, and MOSFET. When a diode is irradiated the MOSFET is in an open state to allow the charge to be stored in the capacitor. Then, the MOSFET closes this gate and opens the readout gate that discharges the capacitor resetting to zero charge and get it ready for the next acquisition.

[0049] Since each detector array is an active matrix, all detectors in a row are read sequentially, raster fashion, and a number of readout channels equal to the number of rows.

[0050] To interrogate the dose received by the diode 80 at a given pixel element 36 (time integrated in the capacitor 82), the corresponding strobe line 74 is raised by the output of the first multiplexer 55 of the interface circuit 54 (multiplexed among the strobe lines 74). An output line 76 from the pixel element 36 of the pixel element 68 is read by a second multiplexer 57 and provided to analog-to-digital converter 52 and received by electrical cable 26 to be sent to the computer 28. This process may be repeated for other pixel elements 36, for example, by parallel similar circuits.

[0051] Data is received at the computer 28 through interface circuits 64 and processed by one or more processors 90 executing a stored program 92 in a computer memory 94 to provideoutput data on an associated terminal screen 100, for example, selected by the user through a keyboard 102.

[0052] The term "cubic" as used herein is intended to describe a volumetric parallelepiped shape not necessarily limited to an equal sided cube.

[0053] Certain terminology is used herein for purposes of reference only, and thus is not intended to be limiting. For example, terms such as "upper", "lower", "above", and "below" refer to directions in the drawings to which reference is made. Terms such as "front", "back", "rear", "bottom" and "side", describe the orientation of portions of the component within a consistent but arbitrary frame of reference which is made clear by reference to the text and the associated drawings describing the component under discussion. Such terminology may include the words specifically mentioned above, derivatives thereof, and words of similar import. Similarly, the terms "first", "second" and other such numerical terms referring to structures do not imply a sequence or order unless clearly indicated by the context.

[0054] When introducing elements or features of the present disclosure and the exemplary embodiments, the articles "a", "an", "the" and "said" are intended to mean that there are one or more of such elements or features. The terms "comprising", "including" and "having" are intended to be inclusive and mean that there may be additional elements or features other than those specifically noted. It is further to be understood that the method steps, processes, and operations described herein are not to be construed as necessarily requiring their performance in the particular order discussed or illustrated, unless specifically identified as an order of performance. It is also to be understood that additional or alternative steps may be employed.

[0055] References to "a microprocessor" and "a processor" or "the microprocessor" and "the processor," can be understood to include one or more microprocessors that can communicate in a stand-alone and / or a distributed environment(s), and can thus be configured to communicate via wired or wireless communications with other processors, where such one or more processor can be configured to operate on one or more processor-controlled devices that can be similar or different devices. Furthermore, references to memory, unless otherwise specified, can include one or more processor-readable and accessible memory elements and / or components that can be internal to the processor-controlled device, external to the processor-controlled device, and can be accessed via a wired or wireless network.

[0056] It is specifically intended that the present invention not be limited to the embodimentsand illustrations contained herein and the claims should be understood to include modified forms of those embodiments including portions of the embodiments and combinations of elements of different embodiments as come within the scope of the following claims. All of the publications described herein, including patents and non-patent publications, are hereby incorporated herein by reference in their entireties.

[0057] To aid the Patent Office and any readers of any patent issued on this application in interpreting the claims appended hereto, applicants wish to note that they do not intend any of the appended claims or claim elements to invoke 35 U.S.C. 112(f) unless the words “means for” or “step for” are explicitly used in the particular claim.

Claims

CLAIMSWhat wc claim is:

1. A radiotherapy dose verification instrument comprising: a radiolucent stack of planar plates assembled together along an axis to define a measurement volume defined by an area of the plates and a cumulative thickness of the plates in the stack; and a plurality of solid-state electronic radiation detectors distributed through the measurement volume supported on a set of the plates; wherein the radiolucent stack provides water-equivalent absorption at mega voltage x- rays.

2. The radiotherapy dose verification instrument of claim 1 wherein the plates include a first set of plates holding the solid-state electronic radiation detectors, the plates of the first set separated by a second set of plates of a different material having lower Compton scattering than a material of the first set of plates at megavoltage radiation levels.

3. The radiotherapy dose verification instrument of claim 2 wherein the second set of plates is selected from materials in the group consisting of silicone rubber and acrylic.

4. The radiotherapy dose verification instrument of claim 2 wherein the first set of plates are printed circuit boards supporting applied copper conductors having a thickness of less than .5 mm measured along the axis.

5. The radiotherapy dose verification instrument of claim 4 wherein the first set of plates are selected from materials consisting of glass-reinforced epoxy and glass -reinforced polytetrafluoroethylene.

6. The radiotherapy dose verification instrument of claim 1 wherein the spacing of plurality of solid-state electronic radiation detectors along the axis increases from an upper side of the volume to a lower side of the volume along the axis.

7. The radiotherapy dose verification instrument of claim 1 wherein a spacing of the plurality of solid-state electronic radiation detectors on each plate is closer near a center of an area of the plate than at the edges of each plate.

8. The radiotherapy dose verification instrument of claim 1 wherein the number of solid-state electronic radiation elements is in excess of 20,000.

9. The radiotherapy dose verification instrument of claim 1 wherein the solid-state electronic radiation detectors are staggered with respect to paths along the axis passing through the stack.

10. The radiotherapy dose verification instrument of claim 1 wherein the second plates are an elastomeric material conformally covering the first plates.

11. The radiotherapy dose verification instrument of claim 1 wherein the stack is free from air voids between the plates.

12. The radiotherapy dose verification instrument of claim 1 further including an electronic circuit receiving signals from the plurality of solid-state electronic radiation detectors and applying a calibration factor to the signals being a function of radiation energy and radiation angle.

13. The radiotherapy dose verification instrument of claim 1 wherein the radiolucent stack of planar plates are releasably assembled along the axis to permit a subset of the solid-state electronic radiation detectors supported on a subset of the plates to be independently positioned for use in a radiation beam.

14. The radiotherapy dose verification instrument of claim 13 wherein a single lowermost planar plate may be releasably detached from the stack for individual use.

15. The radiotherapy dose verification instrument of claim 13 wherein an upper set of planar plates comprising multiple planar plates may be rclcasably detached from the stack for individual use.

16. The radiotherapy dose verification instrument of claim 1 wherein the plurality of solid-state electronic radiation detectors are composed of graphene

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