Evaluation of gamma index analysis reliability of IMRT and VMAT QA using the incorrect dose method

Abstract Gamma index analysis is the most commonly used method for dosimetric control. However, if the dose given in the measurements is incorrect, it is not clear how the gamma analysis will give results. The purpose of this work is to determine the consistency of gamma index analysis under incorrect dose conditions and compare the different equipment results. Fifty intensity-modulated radiation therapy (IMRT) (150 fields) and 50 volumetric modulated arc therapy (VMAT) (150 fields) plans were randomly selected. Quality assurance was performed for all plans with both electronic portal imaging device (EPID) and MapCHECK 2 (MC2). Plans were irradiated with two different beam output (100 cGy = 100 MU and 104 cGy = 100 MU). Gamma passing rate (GPR) > 95%, 3%/3 mm were used for the evaluation. All our fields were passed at 100 cGy = 100 MU for the two equipment. In the case of 4% error (104 cGy = 100 MU), according to the measuring euipment; 210 fields were acceptable for EPID and 80 fields were acceptable for MapCHECK 2. According to the plan type; 178 IMRT fields and 131 VMAT fields passed gamma index analysis. In our study, although the dose was incorrect, GPR of some plans was found to be feasible. Additionally, GPR may vary depending on the equipment used. We think that the presence of different results for each variable reduces the reliability of the results. For this reason innovative approaches should be investigated to improve gamma index analysis.