In a preliminary investigation of hot particle carcinogenesis uranium oxide particles were introduced into the lungs of rats either by intubation of a liquid suspension of the particles or by inhalation of an aerosol. Subsequently the animals were briefly exposed to slow neutrons in a nuclear reactor, resulting in localized irradiation of the lung by fission fragments emitted from 235U atoms in the oxide particles. The uranium used in the intubation experiments was either enriched or depleted in 235U. Squamous cell carcinomas developed at the site of deposition of the enriched uranium oxide in many cases but no lung tumours occurred in the rats with the depleted uranium oxide, in which the lung tissue was exposed to very few fission fragments. Only enriched uranium oxide was used in the inhalation experiments. Pulmonary squamous cell carcinomas occurred after the fission fragment irradiation but were fewer than in the intubation experiments. Adenocarcinomas of the lung were seen in rats exposed to uranium oxide without subsequent irradiation by neutrons in the reactor and in rats irradiated with neutrons but not previously exposed to uranium oxide. It is concluded that (i) fission fragments were possibly implicated in the genesis of the squamous cell carcinomas, which only developed in those animals exposed to enriched uranium oxide and neutrons and (ii) the adenocarcinomas in the rats inhaling enriched uranium oxide only were likely to have been caused by protracted irradiation of the lung with αrays emitted from the enriched uranium.
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Acknowledgments We are most grateful to Dr . J . Vennart and Dr . E . V . Hulse for help and encouragement, to Miss M . Cryer who was involved in the preliminary stages of the intubation experiments and to Dr . D . G . Davey and Dr . B . J . Leonard of Imperial Chemical Industries, Pharmaceutical Division, Alderley Park, Cheshire and their staff in the Animal Breeding Unit for providing the rats used in the inhalation experiments . We would also like to thank AWRE Aldermaston for the use of HERALD reactor and assistance of their staff and the Central Electricity Generating Board for partial financial support in this work .