SU-E-T-379: Evaluation of An EPID-Based System for Daily Dosimetry Check by Comparison with a Widely-Used Ionization Chamber-Based Device

Purpose: To examine the feasibility of using Varian's EPID-based Machine Performance Check (MPC) system to track daily machine output through comparison with Sun Nuclear's DailyQA3 (DQA) device. Methods: Daily machine outputs for two photon energies (6 and 16MV) and five electron energies (6, 9, 12, 16, 20MeV) were measured for one month using both MPC and DQA. Baselines measurements for MPC were taken at the start of the measurement series, while DQA baselines were set at an earlier date. In order to make absolute comparisons with MPC, all DQA readings were referenced to the average of the first three DQA readings in that series, minimizing systematic differences between the measurement techniques due to baseline differences. In addition to daily output measurements, weekly averages were also calculated and compared. Finally, the electron energy dependence of each measurement technique was examined by comparing energy-specific measurements to the average electron output of all energies each day. Results: For 6 and 16MV photons, the largest absolute percent differences between MPC and DQA were 0.60% and 0.73%, respectively. Weekly averages were within 0.17% and 0.23%, respectively. For all five electron energies, the greatest absolute percent differences between MPC and DQA for each energy ranged from 0.49%–0.83%. Weekly averages ranged from 0.07%–0.28%. DQA energy-specific electron readings matched the average electron output within 0.29% for all days and all energies. MPC energy-specific readings matched the average within 0.21% for 9–20MeV. However, 6MeV showed a larger distribution about the average with four days showing a difference greater than 0.30% and a maximum difference of 0.51%. Conclusion: MPC output measurements correlated well with the widely-used DQA3 for most beam energies, making it a reliable back up technique for daily output monitoring. However, MPC may display an energy dependence for lower electrons energies, requiring additional investigation