The distribution of inhaled UO2 particles is described in the upper respiratory tract of rats up to 14 d after inhalation. Delayed neutron analysis was used to determine accurately nanogram quantities of UO2 present in excised tissues. The mass of UO2 in the extrapulmonary bronchi and first bifurcation decreased more slowly over 0--8 d after inhalation than the corresponding mass on the trachea. Between 8 and 14 d neither amount altered significantly. Small but significant quantities of UO2 deposited on the trachea during inhalation, remained until at least 14 d after inhalation.
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.
A technique is described by which the distributions of both fissionable and alpha-emitting radionuclides in histological sections of lung are visualised in the nuclear track recording plastic CR-39. Fission fragments and alpha-particles from radionuclides contained within the section register as tracks in CR-39 and an image of the tissue structure is induced, superimposed upon the track distribution, by means of low-energy (less than 1 MeV) alpha-particles from an external 238Pu source. Thus accurate apposition of the track distribution and tissue structure is achieved. The efficiencies of this system for alpha-particle and fission fragment detection are 0.51 +/- 0.09 (SD) and 0.48 +/- 0.08 (SD) respectively.
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