Peak scatter factors for high energy photon beams.

The peak scatter factor (PSF) for a photon beam is defined as the ratio of the total dose and the primary dose at the depth of dose maximum. The values of the PSF for photon beams ranging from 60Co to 24 MV are calculated using the EGS4 Monte Carlo technique, to avoid measurement difficulties. The calculation shows that the effect of SSD on PSF for high energy photon beams is not significant for small fields, but can be as high as 1% for large fields. For the 60Co beam, the calculation agrees with the data tabulated on BJR Supplement 25 to within 0.8%. The BJR value (1.054) of 10 x 10 cm2 for 60Co is 0.6% lower than the present value due to the underestimation of scatters from the source capsule and collimators. For a given field size, PSF is varied by up to 2% when beam quality changes from 60Co to 24 MV. For normalized PSF, the values of BJR Supplement 25 (which are assumed to be the same for beams ranging from 60Co to 50 MV) agree well with the present calculation for small field sizes, but are higher than our data by up to 2% for large field sizes. The presently calculated PSFs are related to field size(s) by an empirical expression, PSF = 1 + ms/(s + n), where m and n are the fitting parameters. This equation describes the PSFs within 0.4% (0.15% on average).