The pertinence of proportional counter microdosimetry to radiobiology rests on the idea of the site model; in view of the current emphasis on formalisms based on the distance model, it would appear that the role of experimental microdosimetry as a predictive tool is compromised. In this paper we challenge this opinion. It is shown that, to the extent that the site model is not only limited to convex sites of simple geometry, (a) the site model and the distance model become only complementary aspects (i.e., two possible interpretations) of the same formalism, and current biophysical theory is not equipped to discriminate, based on experimental evidence, between the two; (b) proportional counter microdosimetry retains its validity; and (c) for any cellular system [characterized by a function γ(x)] the ratio α/β between the linear and quadratic components of the dose-response function can be calculated as a weighted sum of dose-averaged specific energies measured in a series of spherical sites (of different dimensions). An algorithm is provided for calculating the weighting factors.

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