Comments on ''V79 survival following simultaneous or sequential irradiation by 15-MeV neutrons and 60Co photons'' by Higgins et al. [Radiat. Res. 95, 45-56(1983)]

In a recent paper Higgins et al. reported survival data for V79 Chinese hamster cells exposed simultaneously or sequentially to 15-MeV neutrons and 60Co photons. In each case the results showed the combined modality to be more effective at cell killing than would be expected from the simple multiplication of the effects due to each radiation alone. Thus the effects of the two radiations are synergistic. In addition, simultaneous exposures yielded lower surviving fractions than sequential ones. Both these results are easy to understand, at least qualitatively, if the concepts of sublethal damage and sublethal damage repair are used. The proposition that simultaneous exposure is more effective than sequential ones thus proposition that simultaneous exposure is more effective than sequential ones thus becomes evident. Theoretical expressions have been derived also to quantify these phenomena. Higgins et al. rederive several of the equations previously obtained, however, the interpretation that they give to these equations is not appropriate. It is the purpose of this note to comment on some of these problems and offer a simple analysis and interpretation of the results based on sublethal damage repair