Calibrating TrueBeam jaws by considering collimator walkout to improve the dose uniformity at abutting field junctions

Jaw positions on a linear accelerator are calibrated to have accurate field size values over the range of jaw positions and to have excellent junctions when matching fields. It is sufficient to have field size accuracy on the order of a millimeter for most clinical applications but good junctions require submillimeter precision and accuracy in the jaw positioning. Presented is a method to measure collimator walkout with the MV imager and a mathematical model to determine an optimal origin for calibrating jaws on the TrueBeam accelerator. The calibration procedure uses the jaw position encoders which are sufficiently accurate and precise enough to achieve a homogeneous junction dose for abutting fields.