The fraction of photons undergoing head scatter in x-ray beams.

The output factor in air for a high-energy x-ray beam varies with the collimator setting. Collimator backscatter and obscuring of the source in the target contribute to this variation, but the main component is photon scatter in structures in the accelerator head. To determine the scatter-to-primary ratio, SPR, between the air kerma from such scattered photons and from those that originate directly from the source, it is essential to know the shape of this function SPR(c) of the square collimator opening c, especially for small c. To determine this, simulated head scatter was generated in lead blocks inside the head. By taking the ratio of the output factors for two such blocks of slightly different thickness, the effects of source obscuring and collimator backscatter were eliminated. Using the measured difference in transmission through the two lead blocks, the limiting value for c-->0 could be determined and hence the scatter-to-primary ratios SPR(c). The results could be fitted well with an error function, which then was applied to data measured for clinical beams of 6 and 25 MV. For c = 20 cm, SPR for one 6 MV beam was determined to be 0.016 without a flattening filter, 0.06 for the open beam, and 0.16 with the built-in wedge. At 25 MV, the SPR was higher, 0.09 and 0.20 for the open and wedged beams, respectively.