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 alpha-rays emitted from the enriched
uranium.