Influence of surface shape on tissue-air ratio for cobalt-50.

The shape of the surface of incidence could affect the scatter and change the effective tissue-air ratio (TAR) value. The curvature of the body is considerable within the large beams used for total body irradiation. The effect of the surface curvature on the effective TAR value was studied using a computational approach that sums up the differential scatter air ratios from elemental beams. It was observed that the difference between the effective TAR and the standard flat surface TAR increased with depth to reach a maximum value of about 2% around 8--11 cm depth. It decreased for larger depths, and the effect of surface shape was not perceivable beyond 18 cm depth.