Clinical Stress Test of a Portal Imager Dosimetry
Method for IMRT Verification
Weimin Chen, Ph.D., Gang Yu, M.S.
Weimin
Medical Physics Group,
Algorithms using EPID as a dosimeter for IMRT plan verification have been studied by medical physicists. Commercial products (PDIP) have been developed based on theses research results. However, the up-to-date publications mostly focused on the validation of the calculation algorithm and the benchmark tests such as reproducibility, dose rate dependence, etc. Reports evaluating the robustness of the algorithm and its ability to detect errors have been scarce.
We have commissioned a Varian R-arm AS500 EPID device according to the Varian protocol. Using Varian¡¯s criteria of 3% dose tolerance and 3mm DtoA, our commissioning test scores were 0.9822 for 6MV and 0.9855 for 18MV photon beams. We then conducted experiments to assess the clinical effectiveness of the PDIP. Specifically, three types of stress tests were generated: (1) dose deviation v.s. test score, in which the MUs generated by the treatment planning system were deliberately altered, the resulting dose map was then compared with the measured dose map delivered with the original MUs; (2) position error v.s. test score, in which measured dose map was
shifted in location, then compared with the original planned dose map; (3) degree of modulation v.s. test score, in which the total level of intensities was varied in a step-wise fashion, the resulting test scores were collected and studied. Finally, several real clinical cases were studied and presented.
This work has not been presented at any other conferences.