Evaluation of tissue-air ratios and tissue-maximum ratios with the photon energy-fluence absorption dose model.

Values of tissue-air ratio (TAR) and tissue-maximum ratio (TMR) have been calculated with the photon energy-fluence absorption dose model, at depths d greater than dn (depth of dose normalization) in a homogeneous water phantom, for Co-60 gamma-ray, and 4, 6, 10, 14, and 45 MVp x-ray beams of EQS (the side of the equivalent square) up to 20 cm. It has been found that by (i) slightly altering the formula for the depth modification factor so that it depends on the difference (d -- dn) rather than d alone; and (ii) permitting E, the effective energy of the beam, to vary with EQS for some x-ray beams, it is possible for the model to reproduce within 2.5% the measured data at depths d greater than dn, on TAR, TMR and PDD (the central axis percent depth dose), for any source to surface distance.