Angular dependency correction of 2D planar detector I’mRT MatriXX an offline dosimetry system used for IMRT pre-treatment verification

Abstract

I’mRT MatriXX is one of the common offline dosimetry tools used for the pre-treatment verification quality assurance (QA) of intensity modulated radiation therapy (IMRT) treatment plans in radiotherapy. It has 1020 pixel ionisation chambers (PIC) arranged in 32 × 32 grid in a plain. In this study the angular response of the I’mRT MatriXX was measured using treatment planning system (TPS) calculated dose and I’mRT MatriXX measured dose. The angular response correction factor (CF) of the I’mRT MatriXX was found by measuring the ratio of MatriXX measured dose to the treatment planning system (TPS) calculated dose in 5° step size of MatriXX angle from 0° to 355° under the similar set up condition for MatriXX with MULTICube Phantom and TPS plan. This CF can be applied directly in OmniPro software and used if the user have gantry angle sensor. The aim of this work is to apply this CF without gantry angle sensor to each of the 1020 PIC with the help of a simple program on MATLAB and verify with the TPS treatment plan field by field. For the 10 Ca cervix patients which were planned with standard 7 fields IMRT, there is improvement in the pass percentage of gamma index 3% dose difference and 3 mm distance to dose agreement. At 0° IMRT field, there is insignificant improvement but at other fields like 50°, 100°, 150°, 200°, 250° and 300° there are significant improvement in the gamma index pass percentage.