Experimental investigation of source-block distance to source-surface distance ratios for cuboid and beam-divergence-shaped shadow blocks for cobalt-60 teletherapy.

The partial transmission zone (PTZ) measured in air and its radiation shadow components are identified quantitatively and qualitatively for both cuboid and beam-divergence-shaped blocks. Source-block distance (SBD) and source-surface distance (SSD) are varied to produce SBD/SSD ratios from 0.71 to 0.86. The variation of the magnitude of the radiation shadow components as a function of SBD/SSD is explained in terms of a radiation shadow model. The clinical significance of PTZ minimization and a comparison between PTZs of 'shaped' and 'nonshaped' blocks is discussed. For small ratios of SBD/SSD the beam-divergence-shaped blocks produce smaller medial and lateral PTZs, whereas for larger ratios in the order of 0.9 the medial PTZ is also small for the cuboid blocks. Both PTZs, in general, decrease with increasing SBD/SSD ratios and for both cuboid and beam-divergence-shaped blocks.