Second scatter contribution to dose in a cobalt-60 beam

Semi-analytic and direct numerical integration approaches have been used to study the contribution of doubly scattered photons to dose at a point P in a homogeneous water medium irradiated by a cobalt-60 point source. The semi-analytic analysis was used for an infinite field radius to calculate the variation of second scatter dose with depth, the angular dependence of the second scatter dose, and the spectra. Direct numerical integration was used to calculate the second scatter dose at a depth of 10 cm for various finite field radii ranging from 0 to 25 cm. Maps were made of the locations of scattering sites that were important to the second scatter dose. These calculations show that the second scatter contribution is generally less than the first scatter dose with the exception of a few sites remote from the point of interest. In many ways, the pattern of dose deposition by second scatter is similar to first scatter with less pronounced features. In other respects, the second scatter appears more isotropic. The implications of these calculations for approximate radiotherapy dose calculations are discussed.