Late somatic effects in mice after fractionated X-ray and neutron exposure

Purpose: To determine the dose rate reduction factor and RBE for X-ray and neutron exposure.

Status: 1987- ongoing

Treatment: Single exposure to 250 kVp X-rays (318 mGy/min, 1.5 mm HVL) and single and fractionated (5 fractions at one day interval) exposure to fission neutrons (0.4 MeV, 4 mGy/min) from the biological facility of the RSV-TAPIRO reactor at Casaccia

Dosimetry: Twin ionization chamber method, ENDIP

Endpoints: Life-span study (spontaneous death) with macroscopic/microscopic pathology

Animal: Male (C57Bl/Cne x C3H/Cne)F1 (BC3F1) mice aged 4-6 weeks

Results: The data were analyzed with the Peto test for myeloid leukemia and for selected solid tumors. Myeloid leukemia was absent in controls and rare in the irradiated groups. Nevertheless, a positive trend with dose for 0-0.17 Gy or more could be established. Epithelial tumors were induced from doses of 0.17 Gy or more. Tumor occurrence was further evaluated as final incidences with age adjustment for the difference in rates of mortality. Survival and incidence for selected classes of tumors after 0.17, 0.36 and 0.71 Gy were compared with those from a previous experiment at corresponding doses given acutely (dose rates between 0.05 and 0.25 Gy/min). The results did not suggest any marked overall influence of the time regimen of neutron irradiation on survival and tumor induction.