Differences among tissues with respect to iso-effect relations for fractionated irradiation.

An analysis has been made of responses of a variety of normal tissues in animals to fractionated irradiations. It is shown that the influence of fractionation can be described on the basis of a simple formula relating the effectiveness for induction of cellular effects to the dose per fraction Dn:F(Dn) = a1Dn+a2D2n. The ratio a1/a2 is shown to be an essential parameter for the description of fractionation effects. It is concluded that the values of a1/a2 for responses of various tissues range widely from 2 to 10 Gy. On the basis of a review of radiobiological data, a formalism is developed for the analysis and prediction of iso-effect relations for tissue tolerance. An essential characteristic of the formalism ist that three groups of tissue responses are distinguished for which fractionation effects can be described by average values of a1/a2 = 10, 5 and 2.5 Gy, respectively. The first group comprises a.o. skin, intestine, mucosa, the second group a.o. connective tissue, bone marrow, and the third group a.o. lung, vascular system, and kidney. Dose rate effects can be described by a similar formalism. For the calculation of equivalent total doses, which cause early or late normal tissue damage, a concept is introduced, termed Extrapolated Tolerance Dose (ETD) of Extrapolated Response Dose (ERD). This ETD is the tolerance dose for an infinite number of very small fractions. Specific treatments can be expressed as fractions of the corresponding ETD. An important feature of the described formalism is that it is directly based on radiobiological insights and it provides a more logical concept to account for the diversity of tissue responses than the assumption of different exponents of N and T in the NSD formula.