Cumulative Radiation Effect Part III: Continuous radiation therapy — Short-lived sources

Abstract

In two 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 long-lived sources. This scale provides a means of assessing and comparing the biological effects of various treatments. In this paper, the concept of the CRE is extended to include damage to normal connective tissue for continuous radiation from short-lived sources, whose dose rates change considerably during the course of treatment. In particular, the method of assessing the biological damage resulting from radiation by a source whose change in dose rate can be approximated by an exponential decay is described. The application of the CRE to radiation therapy by short-lived sources is described for treatments given as implants or by external applicators and maintained either for a finite time or to infinity. Examples of calculations for treatments with several isotopes are given. The results of these may be considered as predictions to be verified or criticised on the grounds of experience, and offer a possible test of the validity of the analysis leading to the expression for the CRE achieved by short-lived isotopes.