Cumulative radiation effect part V: Time gaps in treatment regimes

Abstract

In four previous papers, a scale of accumulated sub-tolerance radiation damage, the Cumulative Radiation Effect (CRE), was described for fractionated treatment regimes and continuous radiation therapy from both long-lived and short-lived sources. This scale provides a means of assessing and comparing the biological effects of various treatments. The concept of the CRE has been integrated into a unified system of assessment of biological damage so that a unique value of the CRE describes a specific level of radiation effect. In this paper, the range of application of the CRE-system is further extended by the derivation of an expression to allow for gaps occurring in the course of radiation therapy. The original empirical equations for the assessment of CRE as described in previous papers do not explicitly involve a term for the decay of CRE. From a consideration of the fundamental premises and biological reasoning underlying the development of the CRE-system, a more general equation for the CRE has been derived, from which it has been found possible to obtain an expression which describes the decay of CRE occurring during an interval in treatment. The application of this expression to clinical situations is demonstrated by several examples for simple regimes, Sambrook-type techniques and more complex treatments where both continuous and fractionated regimes are given. These examples have been chosen, where possible, for commonly occurring situations in practical radiotherapy so that the results found by the CRE-system can be compared with those expected on the grounds of clinical judgement and experience. The effect of the position of a gap in a treatment regime on the additional dose required to allow for that gap has been discussed and predictions made which can be tested on the basis of clinical experience or in a qualitative way be experiments on animals. To enable a comparison to be made between the method of allowing for gaps in treatment on the CRE-system and others previously proposed, a brief review and analysis of the various methods are made and their applicability to clinical situations is discussed.