The equivalent tissue-air ratio method for making absorbed dose calculations in a heterogeneous medium.

The CT scanner makes three-dimensional anatomical information available for treatment planning, the calculational algorithm being the limiting factor in dose calculations. A method has been developed, called the "effective tissue-air ratio method," that uses all the information but reduces it to manageable proportions. The accuracy is satisfactory for a wide range of photon beam energies and clinical applications. It has been implemented in such a way that the CT scan can be used directly without the intermediate step of manually obtaining structures from either a viewing screen or a hard copy of the CT image.