Scintillation imaging as a high‐resolution, remote, versatile 2D detection system for MR‐linac quality assurance

Abstract

To demonstrate the potential benefits of remote camera-based scintillation imaging for routine quality assurance (QA) measurements for magnetic resonance guided radiotherapy (MRgRT) linear accelerators. Methods: A wall-mounted CMOS camera with a time-synchronized intensifier was used to image photons produced from a scintillation screen in response to dose deposition from a 6 MV FFF x-ray beam produced by a 0.35 T MR-linac. The oblique angle of the field of view was corrected using a projective transform from a checkerboard calibration target. Output sensitivity and constancy was measured using the scintillator and benchmarked against an A28 ion chamber. Field cross-plane and in-plane profiles were measured for field sizes ranging from 1.68 × 1.66 cm 2 to 20.02 × 19.92 cm 2 with both scintillation imaging and using an IC profiler. Multileaf collimator (MLC) shifts were introduced to test sensitivity of the scintillation imaging system to small spatial deviations. A picket fence test and star-shot were delivered to both the scintillator and EBT3 film to compare accuracy in measuring MLC positions and isocenter size. Results: The scintillation imaging system showed comparable sensitivity and linearity to the ion chamber in response to changes in machine output down to 0.5 MU (R 2 = 0.99). Cross-plane profiles show strong agreement with defined field sizes using full width half maximum (FWHM) measurement of <2 mm for field sizes below 15 cm, but the oblique viewing angle was the limiting factor in accuracy of in-plane profile widths. However, the system provided high-resolution profiles in both directions for constancy measurements. Small shifts in the field position down to 0.5 mm were detectable with <0.1 mm accuracy. Multileaf collimator positions as measured with both scintillation imaging and EBT3 film were measured within AE 1 mm tolerance and both detection systems produced similar isocenter sizes from the star-shot analysis (0.81 and 0.83 mm radii). Conclusions: Remote scintillation imaging of a two-dimensional screen provided a rapid, versatile, MR-compatible solution to many routine quality assurance procedures including output constancy, profile flatness and symmetry constancy, MLC position verification and isocenter size. This method is high-resolution, does not require post-irradiation readout, and provides simple, instantaneous data acquisition. Full automation of the readout and processing could make this a very simple but effective QA tool, and is adaptable to all medical accelerators.

Radioluminescence imaging feasibility for robotic radiosurgery field size quality assurance

“… 23 Daniel A. Alexander et al., demonstrated the potential value of scintillation imaging for MR‐linac QA compared with film and comparable sensitivity and linearity to the ion chamber in response to changes in machine output. 24 RLI has been recently applied also to preclinical small animal radiotherapy for dose distribution verification, showing a good agreement with Monte Carlo dose calculations and GC film measurements for small field of size up to 20× 20 mm 2 . 25 Besides radiation therapy by photons, the good sensitivity and sub‐mm spatial resolution of RLI system, makes it a useful tool for the quality control of scanning proton beams that allows for the detection of deviations of a few percent in relative dose from the expected dose distribution.…”

“…Furthermore, using the compact hardware design and image processing algorithms, a practical, reliable, and comprehensive solution has been provided based on RLI for high dose rate brachytherapy 23 . Daniel A. Alexander et al., demonstrated the potential value of scintillation imaging for MR‐linac QA compared with film and comparable sensitivity and linearity to the ion chamber in response to changes in machine output 24 . RLI has been recently applied also to preclinical small animal radiotherapy for dose distribution verification, showing a good agreement with Monte Carlo dose calculations and GC film measurements for small field of size up to 20× 20 mm 2 25 .…”

Enhanced Cherenkov imaging for real‐time beam visualization by applying a novel carbon quantum dot sheeting in radiotherapy

“…has been adopted in this work to avoid the measurement error as much as possible 29 . Regarding beam profiles, it has been observed that the inline profile exhibits more blurring in this angle 22 . The profile measurements in y‐axis are more subject to skewed perspective angle of the camera, and the oblique viewing angle limits image resolution in the y‐direction.…”

“…The reference points are selected according to the four corners of solid water, as performed in previous studies. 22 Using the light field method of calibrating pixel size, a scalable factor of 0.68 mm/pixel is determined. 23…”

Visual Isocenter Position Enhanced Review (VIPER): a Cherenkov imaging‐based solution for MR‐linac daily QA

“…8 However, Cherenkov emission has been demonstrated as a beneficial tool for QA measurements related to beam geometry, 9 as Cherenkov intensity is monotonically related to dose. Cherenkov and scintillation imaging have been demonstrated as useful methods for various QA procedures in both traditional image-guided radiotherapy and MRgRT, [10][11][12] and this is exploited in this context for highresolution 3D positional measurement of the radiation beam. In this study, we developed a custom Cherenkov imagingbased solution to measuring mechanical-imaging-radiation isocenter coincidence on the MRIdian system, entitled Visual Isocenter Position Enhanced Review (VIPER).…”