An inhomogeneity correction algorithm for irregular fields of high energy photon beams based on Clarkson integration and the 3D Beam Subtraction Method

Abstract

A number of treatment planning systems still use conventional correction methods for body inhomogeneities. Most of these methods (Power law method, Ratio of TARs, etc) consider only on-axis points, rectangular fields and inhomogeneous slabs covering the whole irradiating field.
A new method that overcomes the previous mentioned limitations of the conventional methods is proposed. This method uses the principle of Clarkson integration combined with the 3D Beam Subtraction Method. The method is able to take into account the position, and lateral extent of the inhomogeneity with respect to the point of calculation as well as the shape of the irradiating field by dividing the irradiating field into angular sectors and treating each one separately for the presence of inhomogeneities using a conventional correction method. Applying this method, the correction factors predicted for Co-60 and 6MV photon beams using various field shapes and inhomogeneity materials of lower and higher densities relative to water. Validation of the predicted corrections factors was made against Monte Carlo calculations for the same geometries. The agreement between the predicted correction factors and the Monte Carlo calculations was within 1.5%. Also, the new method was able to predict the behavior of the correction factor when the point of calculation was approaching or moving away from the interface between two materials.