The BEAM Monte Carlo code is used to simulate the 60Co beam from an Eldorado 6 radiotherapy unit and to calculate the relative air-kerma output factors as a function of field size. The unit is realistically modeled, including source capsule, housing and collimator assembly. The calculated relative air-kerma output factors at SSD=80.5 cm agree to within 0.1% with measured values. It is shown that the variation of the output factor is almost entirely due to scattered photons from the fixed and adjustable collimators and there is no effect of shadowing primary photons. The influence of the geometry of the collimation system on the photon spectra on-axis is shown to be small but finite. The calculated buildup region of a depth-dose curve in a water phantom irradiated by a narrow and a broad 60Co beam is shown to agree with experimental data at the 2% to 3% level. Unlike previous calculations, the results accurately predict the effects of electron contamination from the surface to dose maximum. The variation of electron contamination with field size is also presented, as are spectra as a function of field size.
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