Cumulative radiation effect part II: Continuous radiation therapy—long-lived sources

Abstract

In a previous paper, a scale of accumulated sub-tolerance radiation damage, the Cumulative Radiation Effect (CRE), was described for fractionated regimes. This scale provides a means of assessing and comparing the biological effects of varied treatment regimes. In this paper, the concept of the CRE is extended to include an equivalent scale of damage to normal connective tissue for continuous radiation of constant dose rate (a particular case of a more general application of the CRE to time-varying continuous radiation to be introduced in a following paper). The properties of the CRE function for continuous radiation are discussed in some detail, and its application to various clinical situations described and illustrated by examples. The CRE approach is made applicable to a variety of sources by considering the relative biological effectiveness of the radiation. The generality of the definition of the CRE for all forms of radiation treatment strategy has been assured by normalising the CRE for continuous radiation to that for fractionated treatment regimes achieving the same biological effect. The equivalence conditions existing at the junctions between schedules are considered. A knowledge of these conditions is essential since the effect of additional radiation damage cannot be evaluated without taking into consideration the CRE already achieved. A derivation is given of the approximate correction factor to be applied to implants and insertions to adjust the treatment time for dose rates different from those intended; this value of the correction differs to some extent from that currently used. In a previous paper, a scale of accumulated sub-tolerance radiation damage, the Cumulative Radiation Effect (CRE), was described for fractionated regimes. This scale provides a means of assessing and comparing the biological effects of varied treatment regimes. In this paper, the concept of the CRE is extended to include an equivalent scale of damage to normal connective tissue for continuous radiation of constant dose rate (a particular case of a more general application of the CRE to time-varying continuous radiation to be introduced in a following paper). The properties of the CRE function for continuous radiation are discussed in some detail, and its application to various clinical situations described and illustrated by examples. The CRE approach is made applicable to a variety of sources by considering the relative biological effectiveness of the radiation. The generality of the definition of the CRE for all forms of radiation treatment strategy has been assured by normalising the CRE for continuous radiation to that for fractionated treatment regimes achieving the same biological effect. The equivalence conditions existing at the junctions between schedules are considered. A knowledge of these conditions is essential since the effect of additional radiation damage cannot be evaluated without taking into consideration the CRE already achieved. A derivation is given of the approximate correction factor to be applied to implants and insertions to adjust the treatment time for dose rates different from those intended; this value of the correction differs to some extent from that currently used.