Studies on the Mechanism of Acquired Radioresistance in Cancer

It is a frequent clinical, and less frequent labora tory, observation that a tumor subjected to a number of sublethal irradiations will gradually acquire radioresistance (5, 11). The usual manifes tation of acquired radioresistance by a tumor is the diminished amount of regression for the same dose, or the increased dose required for an equal amount of regression or for sterilization, as the radiation treatments continue. Three possible mechanisms to explain the phenomenon are commonly pro posed. Firstly, the initial sublethal irradiation kills off all but the few resistant tumor cells in a hetero geneous population. Secondly, the initial irradia tions induce a mutation toward radioresistance in a tumor cell, which is then selected for by subse quent irradiations. Thirdly, the initial irradiations cause changes of one sort or another in the host ("tumor bed changes") that interact to make the