Build-up effects at air cavities measured with thin thermoluminescent dosimeters

The build-up and build-down situation around air cavities of various size in a polystyrene phantom was evaluated by measurements with LiF-Teflon disks with a thickness of about 1.6 X 10(-2) kg/m2. The measurements demonstrate that when irradiating with two opposed beams the surface dose at the cavity (depth 39 mm) is 74% of the maximum dose for 60 Co, 77% for 6 MV and 87% for 42 MV roentgen radiation.