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, tissue-air ratio (TAR), etc.) consider only on-axis points, rectangular fields, and inhomogeneous slabs covering the whole irradiating field. A new method is proposed that overcomes the above limitations. The new method uses the principle of the Clarkson method on sector integration 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. The field is divided into angular sectors, and each sector is then treated separately for the presence of inhomogeneities using a conventional correction method. Applying this method, we can predict the correction factors for Co-60 and 6-MV photon beams for irregular fields that include inhomogeneities of lower or 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%. In addition, 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.