Iso-effect tables for tolerance of irradiated normal human tissues

Available literature on radiation injury to human tissues (lung, brain, kidney and intestine) was surveyed. A parameter search program (RAD3) was used to derive best-fitting cell kinetic parameters, on the assumption that radiation injury arises from depletion of parenchymal cells in the irradiated organs. From these parameters iso-effect tables were constructed for a wide range of treatment schedules, including daily treatment as well as fractionation at longer intervals, for each tissue. The tables provide a set of limiting doses, above which the risk of radiation injury becomes substantial. Tolerance limits and dose-time-factors were substantially different in the four tissues. It is concluded that computed iso-effect tables provide a more reliable guide to treatment than conventional time-dose equations.