Pretreatment quality assurance (QA) is a major concern in complex radiation therapy treatment plans like intensity modulated radiation therapy (IMRT). Present study considers the variations in gamma index for gantry dependent pretreatment verification and commonly practiced zero gantry angle verifications for ten prostate IMRT plans using two commercial medical linear accelerators (Varian 2300 CD, Varian Clinac iX). Two verification plans (the one with all fields at the actual treatment angles and one with all fields merged to 0 degree gantry angles) for all the patients were generated to obtain dose fluence mapping using amorphous silicon electronic portal imaging device (EPID). The gamma index was found depend on gantry angles but the difference between zero and the nonzero treatment angles is in the confidence level for clinical acceptance. The acceptance criteria of gamma method were always satisfied in both cases for two machines and are stable enough to execute the patient specific pretreatment quality assurance at 0 degree gantry angle for prostate IMRTs, where limited number of gantry angles are used. 1. Introduction Modern cancer treatments using radiation therapy is mostly employed with multileaf optimized plans as in intensity modulated radiation therapy. Higher monitor units and continuous motion of multi-leafs during the beam on time need to be strictly monitored for beneficial outcome from IMRT treatments. Complex IMRT plans are widely used in routine clinical practice which requires pretreatment patient specific quality assurance tests [1, 2]. Pretreatment quality assurance in complex treatment techniques like intensity modulated radiation therapy is carried out using electronic portal imaging devices attached to the medical linacs. This ensures the accuracy of treatment plan generated using commercial planning systems for complex IMRT treatments. Pretreatment quality verification is advisable in all IMRT treatment plans to compare the beam fluence maps delivered using continuous motion of multileaf collimators (MLC). Most centers execute the patient specific verification plans generated prior to the first treatment fraction, where all the fields with various gantry angles are merged to zero degree gantry angle and dose fluence generated from TPS is cross-checked using EPID outputs [3, 4]. The linacs attached with EPID make the pretreatment QA easier and faster than other commercial tools like 2D detector arrays, which is time consuming for machinery settings. EPIDs provide high resolution two-dimensional data. It requires virtually no
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