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Although dose is the simplest and most widely used measurement of a radiation field, it does not always lead to an unambiguous estimate of response. This is reflected in the very wide range of relative biologic effectiveness (RBE) values for biological systems. The ambiguity arises from the focus on energy deposition as the source of biological effect, whether in macroscopic or microscopic volumes. The properties of the biological detector play a role equally important to the properties of the radiation field in their interaction. To predict even the most experimentally accessible biological response, cell killing, we must know the probability per unit path length for generating the observed end point. Especially for high LET radiations we need the action cross sections and the particle-energy spectrum. No one parameter reduction of a radiation field can predict biological effect. For cell killing, however, such a prediction can be made from a two-parameter reduction of the interaction between the radiation field and a specific cell line and a specific ambience of the survival curve for the specific radiation field. The determination of these two parameters leads to a suggested new procedure for evaluating the dose equivalent.