Cumulative radiation effect. part IV. Normalisation of fractionated and continuous therapy - area and volume correction factors.

In three 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. In this paper, it is intended to further the scope of application of the CRE-system by describing in detail the normalisation between the assessments of fractionated and continuous therapy. To do this, it is necessary to consider area and volume correction factors, whose roles are to modify the values of uncorrected CREs so that a specific biological effect, regardless of the area or volume treated, is described by a unique value of the corrected CRE. The problem arising when the CRE achieved by a treatment regime is not constant over a volume is briefly discussed and a simple hypothesis is proposed which may stimulate interest in this problem. The establishment of a normalisation procedure between fractionated treatment regimes and continuous radiation therapy, which is the primary aim of this paper, serves to integrate the concept of the CRE into a unified system of assessment of biological damage, so that a unique value of the CRE describes a specific level of biological damage regardless of the area or volume treated, or the time course of administration of the radiation employed.