SU‐E‐T‐442: Comparison of Global versus Local Gamma Criteria for Planar Dose IMRT QA

Abstract

Purpose: To evaluate the effect of using global vs local gamma analysis on planar dose IMRT QA . Methods: Seven Eclipse IMRT plans, with nine fields each, for head and neck and pelvic cancer were evaluated; planar doses from verification plans calculated on a flat phantom were compared with measurements obtained using a 2D detector array (Matrixx). An in‐house developed software was used to evaluate the measured doses using the local gamma scheme where percent dose difference is calculated relative to dose at each point, as well as the global gamma in which percent dose difference is calculated relative to the maximum dose in the measurement plane. Percent of pixels that passed (gamma >1) were compared and points that failed were investigated. Results: With 3%/3mm gamma criteria and 10% threshold dose, overall passing rate with local gamma was 4.9% (+/− 1.6%) smaller on the average than global gamma rates. Maximum difference in the passing rate was as high as 16%, with an average of 10.3% (+/−3.6%) for all fields; the variation was more pronounced for pelvic plans with simultaneous integrated boost which involved multiple planning target volumes with different prescribed doses. Points failing the local gamma criteria were mostly in lower dose (i.e., about 40–60% of maximum planar dose) and low gradient regions. Conclusions: For planar dose IMRT QA, global gamma analysis gives higher passing rates where points in lower dose and low gradient regions pass the global dose criterion with higher actual percent dose differences between measured and planned doses. Higher percent dose deviations in lower dose regions might possibly be due to deficiencies in IMRT commissioning and/or planning which could have been missed using the global gamma analysis. Users need to be aware of the limitations of the techniques used in the dosimetric analysis of IMRT plans.