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 value s of the PSF for photon beams ranging from Co-60 to 24 MV are calculated us ing the FGS4 Monte Carlo technique, to avoid measurement difficulties. The calculation shows that the effect of SSD on PSF for high energy photon beam s is not significant for small fields, but can be as high as 1% for large f ields. 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 10x10 cm(2) for Co-60 is 0.6% lower than the present value due to the underestimation of s catters from the source capsule and collimators. For a given field size, PS F is varied by up to 2% when beam quality changes from Co-60 to 24 MV. For normalized PSF, the values of BJR Supplement 25 (which are assumed to be th e same for beams ranging: from Co-60 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 f itting parameters. This equation describes the PSFs within 0.4% (0.15% on a verage). (C) 1999 American Association of Physicists in Medicine.