Modifiers of radiation response in tumor therapy: strategies and expectations

The administration of two (or more) cytotoxic agents to widen the differential between the responses of tumor and normal tissues depends upon the biological properties of the agents in the cells and tissue, their interactive potential, and the strategy employed in their administration. Assuming that one agent is ionizing radiation and considering response modification in broad terms, the qualitative features of various strategies are developed for physical as well as chemical modifiers. The heterogeneity of human tumor cells and the compensatory mechanisms of normal tissues following injury are identified as topical areas requiring sustained research effort. Finally, estimates are developed for the degree of improvement required from a response modifier to effect significant improvements in tumor cure rates.