Influence of air cavities on central depth dose curves for 33 MV roentgen rays.

The relative central absorbed dose preceding and following air layers and channels in a polytetrafluorethylene (teflon) phantom has been measured with LiF-teflon dosimeters. Focus phantom distance is set to 100 cm and the field sizes range from 3 cm X 3 cm to 6 cm X 6 cm. Absorbed dose decrease and build-up factors in front of and behind the air cavity are evaluated. The build-up factor is strongly dependent on field size. Measurements of absorbed dose in water and polystyrene yield approximately the same results as in teflon if the linear dimensions of the irradiation geometry (including depth in phantom) in water and polystyrene are equal to 1.84 and 1.99 respectively times the corresponding parameter in teflon. The underlying transformation procedure is derived. The absorbed dose correction factors in the region behind the slab are discussed in terms of tissue-air-ratio and effective attenuation formulae.