Sensitivity of gamma analysis method for the evaluation of pre-treatment quality assurance of IMRT plans using portal dosimetry

To evaluate the sensitivity of gamma analysis method for the evaluation of pre treatment quality assurance of IMRT plans using portal dosimetry. We had generated two sets of portal dosimetry QA plans of six IMRT plans with 33 fields for varian clinic iX (6 MV photons) equipped with millennium MLC 120 and portal dosimetry, in which one set of plans calculated without changing dose and the other set of plans calculated with intentionally introduced 3% dose difference. All these plans were delivered and acquired portal image. Area gamma index were analysed with and without applied lateral shifts by 1 mm increment to portal doors with 3 mm 3% criteria as well as 2 mm 3% and 1 mm 3% criteria. Their results are tabulated and compared with generated and analysed dynalog files. Average area gamma passing rate with criteria 3 % and 3 mm, 3 % and 2 mm and 3% and 1 mm was 99.25±0.75, 96.7±0.84 and 89.8±3.1 respectively. Average area gamma passing rate of laterally shifted portal dose by 1 mm, 2 mm, 3 mm was 97.68±0.8318, 93.98±1.65 and 87.9±1.627. The compared central axis point dose of 3% error introduced predicted dose and measured dose was -1.64±1.7%. From the dynalog files the average % of counts of less than 1 mm error of mlc position at the time of beam delivery was 95.628±4.1% and more than 2 mm leaf position error was not recorded. We conclude that the Varian portal dosimetry (aS-1000) tool is very sensible tool with lesser than 3% 3 mm error. It's a very effective tool to evaluate IMRT patient specific QA with 3 mm 3% criteria. (author)