A general formula for computation of tissue-air ratios for radionuclides commonly used in brachytherapy.

Tissue-air ratio values for different radionuclides have been computed from first principles. The computed data have been used to derive a general formula applicable to most commonly used radionuclides in brachytherapy. There is good agreement between the values obtained from the present formulation and other reported values. For depths beyond 10 cm, no reported values are available for comparison, but on the basis of the self-consistency of the formalism it is reasonable to expect the accuracy of the computed values to be within +/- 2% at any depth up to 30 cm.