The definition of damage interaction in International Commission of Radiation Units and Measurement Report 30

There is some clarification required in the present definition of synergism and antagonism as published in report No 30 by the International Commission on Radiation Units and Measurements for quantitative concepts in radiobiology. In the report, damage interaction appears to be defined relative to additive action rather than to independent action. While the concept of independent action is completely independent of the action mechanism of radiations, it is shown that the concept of additive action requires that isoeffective doses of two different radiations be 'mechanistically equivalent' so that one can 'switch' from one response curve to another at the corresponding effect level. This makes the meaningful use of additive action strongly dependent on a supporting model. At present, most of the current models of radiation action cannot accommodate additive action. The report also states that, due to population selection, cell killing may be greater than that due to independent action if the survivor of one radiation is sensitive to the other radiation. An in-depth study indicates that the analysis is more complicated, depending on whether or not the population is known to be mixed. Lastly, an amended definition of damage interaction for the report is proposed.