Impact of electronic portal image quality on Elekta AQUA(®) collimator isocenter

PURPOSE: The collimator radiation isocenter position determined in AQUA(®) v3.0 [Elekta AB, Stockholm, Sweden] software test “MLC Leaf and Jaw Position” was independently validated using an in‐house MATLAB [Natick, MA: The MathWorks Inc.] script. METHODS: The AQUA test determines radiation isocenter using the mean field center of nine 4 cm × 4 cm electronic portal imager (EPID) exposures at equidistant collimator angles. Impact of EPID image quality on AQUA reported isocenter for thirteen Elekta linear accelerators with Agility MLC heads were evaluated. RESULTS: Of the thirteen, three had visually and quantitatively identifiable artifacts. For the ten good EPID's there was a systematic 0.25 mm offset of the MATLAB calculated mean field center relative to AQUA in the X‐axis and Y‐axis. This corresponds to one image pixel and was found to be due to differences in software co‐ordinate convention. After subtracting this offset there was no significant difference in AQUA and MATLAB calculated isocenter. CONCLUSIONS: For the three machines with poor image quality there was a demonstrated variation in AQUA calculated field center and therefore radiation isocenter relative to MATLAB. Restricting the region of interest (ROI) in AQUA software to only the irradiated section of the EPID brought AQUA and MATLAB result for these three machines into agreement.