Beam profile generator for asymmetric fields

A boundary factor technique for predicting beam profiles has been described by Chui and Mohan [Med. Phys. 13, 409 (1986)]. Boundary factors are calculated as the ratio of the intensity measured in the central plane of a collimated field to the intensity measured in the same location in a 40 X 40 cm field. However, significant discrepancies arise if these factors are applied to the 40 X 40 cm intensity function at off-axis points to predict the beam profile for independently collimated fields. These discrepancies are primarily due to the assumption that the 40 X 40 cm profile approximates an unperturbed intensity function that would exist in the absence of the collimators or scattering media. Two techniques to reduce the residual perturbation are presented. The resulting refined boundary factors allow further factoring so as to permit the calculation of beam profiles for any field size and offset from three discrete functions.