Issues about neutron effects: the JANUS program

The impact of the biological end point selected as a synoptic measurement of mortality for the detection of protraction effects and the estimation of RBE values was investigated. Life shortening was chosen as an end point because it summarizes, in a single index, the cumulative effect of all injuries experienced by an organism. Cumulative mortality at a single time point and the hazard function (age-specific failure rate) were chosen to incorporate progressively more information, respectively, about the distribution of mortality through time. Data for both sexes of the B6CF1 mouse exposed to fission neutrons from the JANUS biomedical research reactor and 60Co gamma rays were analyzed. Three basic patterns of exposure were compared: single exposures, 24 equal once-weekly exposures, and 60 equal once-weekly exposures. The interpretation of results was influenced by the biological end point used as the synoptic measurement. An augmented response with protraction of the neutron exposure depended on the accumulated dose using the life-shortening response while this effect was nonexistent when cumulative mortality or the hazard function was used. A reduced effectiveness of protracted gamma-ray exposures was apparent for all end points. The hazard function analyses suggest that the time domain must be considered in the detection of protraction effects for either radiation quality and the subsequent estimation of RBE values.