A simple equivalent tissue-air ratio method for calculating absorbed dose in a heterogeneous medium

Density correction factors for 60Co gamma rays and 8-MV x rays were measured with an ionization chamber in water-equivalent phantoms containing simulated lung or bone inhomogeneities larger or smaller than the beam (beam sizes, 5 x 5 to 30 x 30 cm). Measurements were compared with values calculated from the effective SSD method, the generalized Batho power law method, and a simplified equivalent tissue-air ratio (SETAR) method. The results indicate that the SETAR method gives the best overall agreement with experimental values, while the effective SSD and Batho algorithms are accurate only in certain situations. The SETAR method seems adequate for most clinical applications