NEUTRON-INDUCED TUMORS IN BC3F(1) MICE - EFFECTS OF DOSE FRACTIONATION

An experimental study of the biological effectiveness of multifraction ated low doses of high-LET radiation was carried out using BC3F(1) mal e mice. They were treated with whole-body irradiation with five equal daily fractions of fission neutrons to yield cumulative doses of 0.025 , 0.05, 0.10, 0.17, 0.25, 0.36, 0.535 and 0.71 Gy at the RSV-TAPIRO re actor (mean neutron energy 0.4 MeV, in terms of kerma, ($) over bar y( D) = 51.5 keV/pm, dose rate 0.004 Gy/min) and were followed for their entire life span. The statistical method described by Peto et al. (IAR C Monograph, Suppl. 2, 1980) to establish the existence of a carcinoge nic effect in long-term animal experiments was applied to the data set s. This analysis was done for myeloid leukemia and for the presence of selected solid tumors. Myeloid leukemia was absent in the control gro up and was rarely found in irradiated animals. However, a positive sig nificant trend was found in the desk ranges 0-0.17 Gy and higher. Epit helial tumors were induced at doses from 0.17 Gy on. Tumor occurrence was evaluated further as final incidences with age adjustment for the differences in mortality rates. Survival and incidence data for select ed classes of tumors after 0.17, 0.36 and 0.71 Gy were compared with t hose from a previous experiment at corresponding doses given acutely ( dose rate between 0.05 and 0.25 Gy/min). This indicated no marked over all influence of the time regimen of neutron irradiation on survival a nd tumor induction.