A Monte Carlo study on neutron and electron contamination of an unflattened 18 MV photon beam

Recent studies on flattening filter (FF) free beams has shown increased dose rate and less out-of- field dose for those beams. On the other hand, changes in contamination electrons and neutron spectra produced through photon (E>10MV) interactions with linac components have not completely studied for FF free beams. The objective of this study was to investigate the effect of removing FF on contamination electron and neutron spectra for an 18 MV photon beam using Monte Carlo (MC) method. The 18 MV photon beam of Elekta SL-25 linac was simulated using MCNPX MC code. The photon, electron and neutron spectra at a distance of 100 cm from target and on the central axis of beam were scored for 10×10 and 30×30 cm2 fields. Our results showed increase in contamination electron fluence (normalized to photon fluence) up to 1.6 times for FF free beam which causes more skin dose for patients. Neuron fluence reduction of 54% was observed for unflattened beams. Our study confirmed the previous measurement results which showed neutron dose reduction for unflattened beams. This property may lead to less neutron dose for patients treated with unflattened high energy photon beams.

A Monte Carlo study on neutron and electron contamination of an unflattened 18 MV photon beam Keywords:

Unflattened photon beam , neutron contamination , electron contamination