Influence of scattered radiation on heterogeneity correction in three-dimensional treatment planning

The first and multiple scattered radiations of 60Co gamma rays and 10 MV X-rays were measured using an ionization chamber, and the values obtained were compared with the calculated values. The experimental values of the angle distributions of the first scattered photons from tissue-equivalent materials in free space agreed well with the theoretical values calculated from the Klein-Nishina probability in the compton scattering process. However, the scattered radiation in phantoms was not represented by only the first scatter components. The results indicate that the weighting factor of all scattered radiations in the calculated three-dimensional dose distribution can be set by the inverse square correction for the distance from the scattering point to the reference point within an accuracy required in clinical dosimetry. The present method provides a simple and practical approach to obtaining the computation speed in three-dimensional treatment planning.