Rat tracheal epithelial cells exposed directly on planar210 Po sources exhibited exponential cell killing; however, no significant increase in induction of preneoplastic transformation was observed over a range of α-particle fluences ($0.017-0.050\ \mu {\rm m}^{-2}$). In contrast, up to 10-fold increases in frequencies of preneoplastic transformants, above control levels, were observed after exposure of rat tracheal epithelial cells to similar α-particle fluences on${}^{238}{\rm Pu}$ and241 Am sources. Two alternative hypotheses are evaluated as an explanation for this apparent difference in the biological effect of α particles emitted from different sources: (a) possible interactions between effects produced by α particles and by low-energy photons, which occur with${}^{238}{\rm Pu}$ and241 Am but not with210 Po; and (b) the influence of spatial relationships between exposed cells and the surface of the planar source. The data suggest that cell-to-source spatial relationships affect both survival and transformation markedly.

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