Analysis of central-axis doses for high-energy x rays

The purpose of this study was to improve on the analytical expressions used to describe central-axis doses for high-energy x-ray beams, in particular, the component due to phantom-scattered photons. The beams were characterized by quantities related to the physical processes, namely, transmission, head-scatter, and phantom-scatter factors, which were described separately with mathematical functions. Transmission in water was measured in a narrow beam and head scatter with a small phantom in air. The phantom-scatter factors, i.e., the ratios between total and primary dose, were deduced from measured central-axis doses per monitor unit. Based on previous work, it was assumed that this scatter factor is proportional to the depth d if the ratio between the depth and the field size s is constant. The proportionality constant was examined as a function of this ratio d/s and the effective linear attenuation coefficient mu. Two quality-dependent parameters were extracted. One expresses the probability of scatter and was numerically close to mu. The other, which has not previously been studied, reflects the directional distribution of the scattered photons and was also found to be a linear function of mu. Thus the scatter factors can be estimated if mu is known. Central-axis doses were described by these formulas with 2.5% maximum error at 6 MV, 0.8% at 25 MV. To achieve this result, only a few measurements were needed for selected d and s, which indicates that the model used for the scatter factor is realistic. When the method was applied to 10-MV and 15-MV x-ray beam data measured by another institution, about +/- 2% accuracy resulted.