Monte Carlo simulation of a typical Co-60 therapy source

The BEAM Monte Carlo code is used to simulate the Co-60 beam from an Eldora do 6 radiotherapy unit and to calculate the relative air-kerma output facto rs as a function of field size. The unit is realistically modeled, includin g source capsule, housing and collimator assembly. The calculated relative air-kerma output factors at SSD = 80.5 cm agree to within 0.1% with measure d values. It is shown that the variation of the output factor is almost ent irely due to scattered photons from the fixed and adjustable collimators an d there is no effect of shadowing primary photons. The influence of the geo metry of the collimation system on the photon spectra on-axis is shown to b e small but finite. The calculated buildup region of a depth-dose curve in a water phantom irradiated by a narrow and a broad Co-60 beam is shown to a gree with experimental data at the 2% to 3% level. Unlike previous calculat ions, the results accurately predict the effects of electron contamination from the surface to dose maximum. The variation of electron contamination w ith field size is also presented, as are spectra as a function of field siz e. (C) 1999 American Association of Physicists in Medicine. [S0094-2405(99) 01111-6].