Contamination of high-energy photon beams by scattered photons

Absorbed dose and energy distribution of scattered photons that contaminate high-energy photon beams have been calculated for 6-MV and 21-MV bremsstrahlung sources using the Monte Carlo method. The percentage of the absorbed dose at the central axis is less than 3%. The results show that the main sources of scattered photons are the primary collimator for 6-MV and the flattening filter for 21-MV bremsstrahlung sources respectively. The absorbed dose from the phantom scattered photons has been calculated by a semi-empirical method. These calculations show that the dominating contribution to the absorbed dose outside the primary beam is due to these phantom scattered photons. In the surface region of the phantom, however, electrons from the air and collimators may give rise to a high absorbed dose.