Scatter-air ratios are tabulated quantities which are based on measurement and which are used to calculate the dose to points in the body of a patient exposed to radiation fields of arbitrary shape. For computing purposes it is convenient to represent these quantities by an equation. We have found a nine-parameter empirical function which expresses scatter-air ratio data for 60Co gamma rays and for 4- and 6-MV x rays. To test the equation it was used to calculate tissue-air ratios for a range of rectangular fields. The results are in good agreement with published values.

Cobalt Radioisotopes↗

PubMedDate not supplied

Head-scatter factors and effective x-ray source positions in a 25-MV linear accelerator.

The behavior of the effective source position and the correction factor associated with the collimator opening (head-scatter factor) were investigated for the 6- and 25-MV x-ray beams of a linear accelerator. The primary photon fluence was measured in air for square field sizes from 5 x 5 cm to 40 x 40 cm at distances from the nominal source of 80 to 140 cm, for open and wedged fields (wedge angle 60 deg). An inverse-square analysis shows that, for open fields, the effective source position of the accelerator is about the same (approximately 1 cm downstream) at 6 and 25 MV, for all field sizes. For the wedged fields, the effective source position depends on field size and ranges from about 2 to 4 cm. The head-scatter correction factors for given collimator settings were found to be essentially independent of distance at both energies.

Mathematics↗