Neutron issues in the JANUS mouse program

Over the last 25 years, the JANUS program in the Biological and Medical Research Division at Argonne National Laboratory (ANL) has compiled a database on the response of both sexes of an F1 hybrid mouse, the B6CF1 (C57BL/6 x BALB/c), to external whole- body irradiation by 60Co γ-rays and fission neutrons. Three basic patterns of exposure for both neutrons and γ-rays have been investigated: single exposures, 24 equal once-weekly exposures, and 60 equal once-weekly exposures. All irradiations were terminated at predetermined total doses, with dose calculated in centigrays at the midline of the mouse. Three endpoints will be discussed in this paper: (1) life shortening, (2) a point estimate for cumulative mortality, and (3) the hazard function. Life shortening is used as an analysis endpoint because it summarizes, in a single index, the integrated effect of all injuries accumulated by an organism. Histopathological analyses of the mice used in the ANL studies have indicated that 85% of the deaths were caused by neoplasms. Connective tissue tumors were the dominant tumor in the B6CF1 mouse, with tumors of lymphoreticular origin accounting for approximately 80% of this class. The latter two endpoints will therefore be used to describe the life table experience of mice dying from the lymphoreticular class of tumors. Dose-response models will be applied to the three endpoints in order to describe the response function for neutron exposures, evaluate the effect of dose range and pattern of exposure on the response function for neutrons, and provide a set of neutron relative biological effectiveness (RBE) values of the ANL database. 25 refs