The universal use of dose as a normalizing parameter in radiobiology is based entirely on the availability of measuring instruments. It is a poor basis for predicting or understanding the relationship between an irradiation and the resulting end point. Energy deposited is not the cause of an interaction. It is a secondary effect. The interaction is best described by fluence and cross section. Energy deposited depends principally upon inelastic collision cross sections for the interaction of electrons with molecules. Especially for heavy-ion bombardments, for high-LET radiations, inelastic electron collision cross sections relate only remotely to the observed end points of interest. When dose is used to describe effects observed with radiations of different "quality," response predictions can be very wide of the mark. One way to describe such a relationship is through the relative biological effectiveness (RBE). If we consider the RBE to be a correction factor to be applied to a prediction of response based on dose, we find that its values range from 0.01 to infinity. It is apparent that in general dose is a useless predictor of response, except in narrowly defined circumstances.

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