Investigating the potential of conical scintillation detectors for patient-specific verification of intensity-modulated radiotherapy plans

Background and purpose

Conical scintillation detectors are frequently used to measure geometric characteristics of radiotherapy modalities. However, their application to verify intensity-modulated radiotherapy plan delivery has not been investigated and requires a more detailed understanding of device response. This work evaluated the novel application of a conical scintillation detector to plan-specific quality assurance (QA) for intensity-modulated photon plans by evaluating device dependence on beam delivery and device acquisition parameters.

Materials and methods

Measurements were made with a conical scintillation detector using beam delivery parameters of five photon beams (6–15 MV, including flattening filter free), three field sizes (1 × 1–5 × 5 cm²), and several dose rates (100–2000 MU/min) combined with device acquisition parameters of two frame rates (10 and 20 fps) and three gains (18–22 dB). A standardization equation to correct for gain and frame rate was investigated, and the remaining dose rate dependence was characterized. Device precision was evaluated using replicate measurements, and spatial uniformity was determined by irradiating different parts of the device.

Results

For each parameter combination, measurement reproducibility was 1.3%, and spatial uniformity was 1–2%. Scintillation intensity varied with gain, frame rate, and dose rate. Standardizing measurements for gain and frame rate was effective, but a dependence on dose rate caused errors at non-reference conditions (root mean squared error, RMSE: 0–152%). An additional dose rate correction specific to each combination of gain and frame rate improved accuracy (RMSE 0–17%).

Conclusions

To consider the detector for plan-specific QA of intensity-modulated radiotherapy plans, correction factors are imperative to mitigate effects of delivery and acquisition parameters.