Monte Carlo calculations of scatter to primary ratios for normalisation of primary and scatter dose.

The separation of total absorbed dose into primary and scatter components is a commonly used technique in photon dose calculations. The primary dose component can be characterised by a measured narrow beam attenuation coefficient and a single normalisation value which establishes the relative proportion of the primary to the total dose at some reference depth and field size. The determination of this normalisation value from measured data requires an extrapolation of measured values for finite field sizes to obtain a zero field size value. We have used Monte Carlo simulations to score primary and scatter dose for photon beams of 4, 6, 10, 15 and 24 MV and report values of the scatter to primary ratio at the depth of dose maximum for the circular equivalent of a 10 cm x 10 cm field. These values have an uncertainty of less than 1% and can be used in lieu of extrapolation of measured data to establish the relative magnitude of the primary dose for a wide range of photon beam energies.