Radioresponsiveness of human melanoma xenografts given fractionated irradiation in vivo--relationship to the initial slope of the cell survival curves in vitro.

The radioresponsiveness of five human melanoma xenograft lines given fractionated irradiation in vivo was studied using specific growth delay and cell survival in vitro as endpoints. Superfractionation (3 fractions of 2.0 Gy with 4-h intervals each day) as well as conventional fractionation (1 fraction of 2.0 Gy each day) were used. The total dose was varied within the ranges 12 to 30 Gy and 10 to 30 Gy, respectively. The rankings of the melanomas in radioresponsiveness were almost identical, irrespective of the endpoint and the fractionation regime considered. The radioresponsiveness was found to be positively correlated to the initial slope of the in vitro cell survival curves, i.e. the alpha and the surviving fraction at 2.0 Gy (conventional dose rate; 3.0 Gy/min) and the D0 (low dose rate; 1.25 cGy/min). There was no relationship between the radioresponsiveness and any known growth or microenvironmental parameter. It is concluded that the differences in radioresponsiveness among the melanomas for the fractionation regimes studied here were governed mainly by the intrinsic repair capacity of the tumour cells and not by microenvironmental factors. The potential of in vitro cell survival curve parameters in predicting the clinical radioresponsiveness of tumours is discussed.