Further experiments to study whether localised fission fragment irradiation of rat lung causes tumours.

Male albino rats inhaled an aerosol of 235UO2 (mass median aerodynamic diameter = 2.8 micrometers and geometric standard deviation = 1.6). Approximately 20 h or 7 d post-inhalation the rats were exposed briefly to 10(12) slow neutrons cm-2 in a nuclear reactor, causing the retained 235UO2 particles of approximate mass 40 or 400 micrograms to emit fission fragments which irradiated the lungs. The mean absorbed doses from the fission fragments were either 80 ot 800 cGy approximately and in addition the lungs were exposed to a background of alpha-rays throughout the rats' life-time from the retained 235UO2 which gave mean doses of about half that from the fission fragments. The animals were kept for their life-time and killed when they became moribund. Malignant tumours were found in the lungs (squamous cell carcinoma and adenocarcinoma) which were probably induced by the alpha-rays rather than the fission fragments. Because of insufficient numbers of animals in the experimental groups, however, some statistical uncertainty exists as to whether the fission fragments were in fact less effective than the alpha-rays per unit absorbed dose in causing malignant tumours of the lung.