Comparison of Patient-Specific Quality Assurance of ۶ MV and ۱۰ MV VMAT plan at Isocenter using Improved Gamma Evaluation Algorithm

Introduction: To compare Patient-Specific Quality Assurance (PSQA) of ۶ MV and ۱۰ MV Volume Modulated Arc Therapy (VMAT) plans performed with Electronic Portal Imaging Device (EPID) kept at Isocenter ۱۰۰ cm (Source to Imager Distance (SID)) using an Improved Gamma Evaluation algorithm. Material and Methods: Previously treated patients with ۶ MV IMRT for Pelvic cancers were planned, on Eclipse TPS, with ۶ MV and ۱۰ MV photon beams using VMAT technology. The PSQA was performed using EPID and investigated the effect on Area Gamma, Maximum Gamma & Average Gamma. Results: The mean Area Gamma passing rate (%GP±Standard Deviation(σ)) for ۶ MV was ۹۷.۰۶±۳.۷۰, ۹۵.۴۲±۵.۳۱, ۹۰.۹۳±۷.۲۹, ۸۶.۵۵±۹.۱۰ and for ۱۰ MV  ۹۷.۱۴±۶.۰۸, ۹۵.۸±۸.۴۷, ۹۴.۶۲±۹.۴۵, ۹۱.۹۷±۱۳.۵۰ using the criteria ۳%/۳ mm, ۳%/۲ mm, ۲%/۳ mm, ۲%/۲ mm respectively.  Similarly, for mean Maximum Gamma value for ۶ MV was ۲.۵۰±۰.۸۹, ۲.۷۲±۰.۹۴, ۳.۳۲±۱.۱۳, ۳.۵۶±۱.۰۲ and for ۱۰ MV ۲.۱۷±۰.۶۲, ۲.۴۲±۰.۷۲, ۲.۸۴±۰.۹۰, ۳.۲۶±۰.۹۴ respectively. For mean average gamma, value for ۶ MV was ۰.۳۶±۰.۰۹, ۰.۴۲±۰.۱۰, ۰.۴۵±۰.۱۲, ۰.۵۳±۰.۱۳ and for ۱۰ MV was ۰.۲۷±۰.۱۶, ۰.۳۲±۰.۱۹, ۰.۳۴±۰.۲۰, ۰.۴۱±۰.۲۴. Conclusion: There is a marked difference between Area Gamma Passing Rate of ۶ MV and ۱۰ MV photon beam. The gamma criteria of ۳%/۲ mm with a ۵% Threshold limit and ۹۵% Area Gamma Passing Rate can be used for PSQA using EPID at Isocenter for ۶ MV and ۱۰ MV photon beam. There is no marked significant difference in values of mean Maximum Gamma and mean Average Gamma for ۶ MV and ۱۰ MV photon beams PSQA

Keywords:

Radiation Dosimetry , Portal imaging , High Energy Radiotherapy , Linear Accelerator