Expression of the dose rate effect in clinical curietherapy.

The role of the dose rate on the biological effect can be assessed by a simple formula involving only two parameters related to the repair ability (alpha/beta) and to the repair kinetics (repair time constant t or repair half time Tr). The result of the computation provides a value in which the dose rate is taken into account along with the physical dose. It can be expressed by: the ERD (extrapolated response dose), the equivalent dose at a constant dose rate used as a reference, or by the equivalent dose for a conventionally fractionated dose (5 x 2 Gy/week). The computation permits a description of the treatment by the ERD distribution or the "equivalent dose" distribution which is more significant than the dose distribution. It expresses the relative decline in the effect in the low dose region, and the relative increase in the high dose region which are related to the difference in dose rate: the quantitative comparison of different treatments with identical dose distribution delivered in different times; a rational timing of a partial removal of the sources when a uniform treatment time would result in an overdosage of a part of the treated volume. The computation has been used for an analysis of the conflicting clinical data on the variation of the isoeffect dose as a function of the treatment time. The basic radiobiological mechanisms implied in the method of computation and the values of the parameters which have been used are discussed.