Quantitative evaluation of transmission EPID daily imaging on a Halcyon Linac

Abstract

The recently published AAPM TG100 has advocated streamline IMRT/VMAT QA procedure based on Failure Mode and Effects Analysis (FMEA) criteria, i.e., the product of incidence severity, occurrence probability, and detection probability. Transmission EPID has proven to be able to detect dose errors at a detection probability of 60% vs. those based on pre-treatment patient specific QA, at a detection probability of less than 5%. The objective of this study is to retrospectively evaluate the gamma index (3%, 3mm) using composite transmission EPID images for various treatment sites on a Halcyon Linac. Transmission EPID images are recorded during treatment and compared to first day EPID as reference. Over 1000 treatment fractions, the average gamma index and standard deviation of gamma for various treatment sites are 99.04-1.64, 99.95-0.093, 100-0 and 99.7-0.567 and 95.65-3.57 for head and neck, brain, prostate, pelvis and breast respectively. In conclusion: Our data show very high inter-treatment consistency for most treatment sites except breast. For breast cases, using second day treatment images as reference, the gamma index is much higher and standard deviation is lower than using day1 images as reference. Overall a threshold of gamma passing rate of 95% is a reasonable value.