Collimator scatter in modeling radiation beam profiles.

In computer dose calculations using scatter-air ratio sector summation algorithms, the primary dose from the target to points away from the central axis of a beam is computed using an exponential intensity model of the source and a transmission parameter for the collimator. This model works well inside the beam and near edges but is inaccurate outside the beam at distances of more than 1-2 cm from beam edges. We have modified the standard beam profile model to include a dose contribution representing photon radiation scattered from the collimators. Collimator edges are treated mathematically as line sources and an adjustable parameter is introduced which represents the activity per unit length of the collimator edges. Dose from the collimator edges is assumed to decrease purely geometrically as the inverse of the square of the distance and no modification is made for tissue attenuation. With these assumptions, the total collimator scatter dose to a point is most accurately computed by a line integral over the edges of the beam outline. This modification fits naturally into the standard scatter-air ratio sector summation computer algorithm but adds significantly to dose computation time. Some approximations eliminate the line integration and lead to a collimator scatter term which is proportional to field perimeter and independent of off-axis distance. The modified dose model was tested by comparing measured dose profiles with computed ones using x-ray beams from Philips (6 and 15 MV) and Varian (4 and 6 MV) accelerators. There was significant improvement in fit compared to the standard beam model for points outside the radiation beam.