4D scintillation dosimetry for the MRI-linac: proof of concept

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

PURPOSE

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 cm² to 20.02 × 19.92 cm² 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 (R²  = 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 ± 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.