Backscatter towards the monitor ion chamber in high-energy photon and electron beams: charge integration versus Monte Carlo simulation

In some linear accelerators, the charge collected by the monitor ion chambe r is partly caused by backscattered particles from accelerator components d ownstream from the chamber. This influences the output of the accelerator a nd also has to be taken into account when output factors are derived from M onte Carlo simulations. In this work, the contribution of backscattered par ticles to the monitor ion chamber response of a Varian 2100C linac was dete rmined for photon beams (6, 10 MV) and for electron beams (6, 12, 20 MeV). The experimental procedure consisted of charge integration from the target in a photon beam or from the monitor ion chamber in electron beams. The Mon te Carlo code EGS4/BEAM was used to study the contribution of backscattered particles to the dose deposited in the monitor ion chamber. Both measureme nts and simulations showed a linear increase in backscatter fraction with d ecreasing field size for photon and electron beams. For 6 MV and 10 MV phot on beams, a 2-3% increase in backscatter was obtained for a 0.5 x 0.5 cm(2) field compared to a 40 x 40 cm(2) field. The results for the 6 MV beam wer e slightly higher than for the 10 MV beam. For electron beams (6, 12, 20 Me V), an increase of similar magnitude was obtained from measurements and sim ulations for 6 MeV electrons. For higher energy electron beams a smaller in crease in backscatter fraction was found. The problem is of less importance for electron beams since large variations of field size for a single elect ron energy usually do not occur.