Monte Carlo calculations of scatter dose for small field sizes in a 60Co beam

Photon dose calculations are frequently based on the separation of primary and scatter dose components. For small field sizes it is difficult to make this separation from measured data. Such problems lend themselves to solution by calculational methods, which permit rigorous definition of primary and scatter dose. Using Monte Carlo methods, we verify an equation derived from O'Connor's scaling theorem relating scatter-primary ratios at small depths and field sizes to those at larger depths and fields. This equation may prove to be useful when separating primary and scatter dose components, and consequently in dose calculations involving irregular fields or inhomogeneities.