The differential scatter-air ratio and differential backscatter factor method combined with the density scaling theorem.

Using the O'Connor density scaling theorem, two basic equations have been derived to express scatter dose calculations in both homogeneous and heterogeneous phantoms: (i) the differential scatter-air ratio for calculating the frontscatter, and (ii) the differential backscatter factor for calculating the backscatter. In the derivation of both equations the relative electron density along the line between the scattering element and the calculation point has been averaged to account for scatter attenuation. Each equation expresses the amount of front or backscatter at the point of calculation per unit primary dose, per unit relative electron density, per unit volume at the scattering element. Primary dose calculations in both a homogeneous and a heterogeneous phantom have been carried out using a simple exponential attenuation law. Except in the area of or near interfaces and the area around field borders where there is electron disequilibrium, satisfactory dose calculations have been obtained using the primary and scatter dose calculation procedure for experiments done with Co-60 gamma rays in homogeneous soft tissue phantoms and in heterogeneous cork and aluminum phantoms.