An effective correction algorithm for off-axis portal dosimetry errors

Portal dosimetric images acquired for IMRT pretreatment verification show dose errors of up to 15% near the detector edges as compared to dose predictions calculated by a treatment planning system for these off-axis regions. A method is proposed to account for these off-axis effects by precisely correcting the off-axis output factors, which calibrate the imager for absolute dose. Using this method, agreement between the predicted and the measured doses improves by tip to 15% for fields near the detector edges, resulting in passing rate improvements of as much as 60% for gamma evaluation of 3 rum, 3% within the collimator jaws. (C) 2009 American Association of Physicists in Medicine. [DOI: 10.1118/1.3187785]