Poster — Thur Eve — 07: Measurement of Radiation‐Light Congruence Using a Photodiode Array

Many factors affect treatment delivery, including setup errors during the simulation process, dose calculation uncertainty, patient setup errors during treatment, and errors that result from incorrect calibration and geometric setup of the linear accelerator. Among these, the radiation‐light congruence is especially important because the light field is used to simulate the radiation beam. The light field is an integral part of patient setup and jaw calibration, thus radiation‐light congruence is essential for accurate treatment delivery. We have developed a novel device that enables precise and automated measurement of radiation‐light congruence. Using the device, we show that the radiation‐light field congruence for both 6 and 15 MV beams is within standard tolerance limits. However, our results indicate that radiation‐light congruence is dependent on collimator angle and energy. The maximum measured disagreement between radiation field edge and light field edge was 1.290 ± 0.004 mm at collimator angle 270 (X1, 15 MV) and 0.932 ± 0.003 mm at collimator angle 90 (Y2, 15MV). The minimum disagreement was 0.016 ± 0.003 mm for 270 (X2, 6 MV) and 0.102 ± 0.004 mm for 90 (X2, 6MV). This detector and measurement method will give us a better understanding of the radiation‐light congruence dependence on collimator angle and energy. It could also be used to determine location of the x‐ray source within the linear accelerator.