An investigation of a model of percentage depth dose for irregularly shaped fields

A significant component of the total dose delivered to tumor and surrounding tissue during a radiation treatment arises from the scattering of the primary beam. Accounting for this component accurately and efficiently is a necessity. In this study we investigate a method for calculating the phantom-scatter contributions to the total dose by simple summation of scatter dose from a set of individual triangles that span an irregular field. The calculation of phantom scatter is based on a two-parameter model, which is applicable to regions where electron equilibrium is established. The two physical parameters are the dose-averaged linear attenuation coefficient and the beam-hardening coefficient. The advantage of this model is that it is a natural method when an irregular field is shaped by a multi-leaf collimator (MLC). Accuracy is not compromised by the triangulation since the irregular field is defined by the straight edges of the MLC leaves. The model predicts the percent depth dose with acceptable accuracy for any arbitrary shape of fields. We report on results for 6- and 18-MV photon beams and for a number of irregularly shaped fields.