IMPLEMENTATION AND GAMMA PASS LIMIT OF EPIQA PORTAL DOSIMETRY SOFTWARE FOR VOLUMETRIC MODULATED ARC THERAPY PRE-TREATMENT VERIFICATION

Abstract

EPIQA is a program converting a dosimetric image acquired by an EPID into a dose map using the GLAaS conversion algorithm. Due to the implementation of the new QA device, the new gamma pass limit which is employed for the evaluation should be determined. The purpose of this study is to determine the gamma pass limit of EPIQA software for volumetric modulated arc therapy pre-treatment verification. The calibration and configuration of EPIQA (EpiDos s.r.o, Bratislava, Slovakia) were performed and evaluated for the accuracy of simple field techniques before the clinical implementation. The VMAT patient plans were recalculated for homogeneous medium in Eclipse treatment planning and delivered to EPID aS1000 equipped with Varian Clinac iX linear accelerator. The raw PV images in air for each patient were converted to dose in homogeneous phantom in EPIQA and compared with Eclipse treatment plans using gamma pass criteria of 3% dose difference and 3 mm distance to agreement. This new tool was compared with ArcCHECK (Sun Nuclear Corporation, Melbourne, FL) which is used routinely in clinic for 43 VMAT plans. The 70 VMAT plans in various organs using EPIQA verification were evaluated to determine the gamma limit using confidence limit of (100 mean)+1.96σ. For simple techniques in open, wedge fields and pyramid IMRT, EPIQA showed good agreement of dose distribution with TPS. The percent gamma pass ranged from 95.25 to 99.79 and 99.64 to 100 for 6 and 10 MV. For 43 clinical VMAT plans, the average gamma pass were 99.29±0.63% and 98.69±1.14% for EPIQA and ArcCHECK, respectively. The result demonstrated slightly better gamma pass of EPIQA than ArcCHECK. For the study of 70 VMAT plans the average gamma pass was 99.29±0.60%, our results agreed with the two previous works which illustrated the average gamma pass of 97.10±2.4% and 99.10±0.6%. The calculated gamma limits in each region were 98.84%, 97.66%, 98.09%, 98.96% and 97.28% for head, H&N, chest, pelvis and the cases using SRS/SBRT technique, respectively. This study demonstrates the gamma limits of 98% for head, chest, pelvis regions and 97% for head and neck regions and SRS/SBRT techniques. EPIQA portal dosimetry is a simple and reliable quality assurance tool for VMAT dose verification which provides the efficient results and suitable for a fast growing number of patients.