Does variation in the in vitro cellular radiosensitivity explain the shallow clinical dose-control curve for malignant melanoma?

In radiotherapy, clinical dose-control curves are generally more shallow than what should be expected from in vitro dose-survival curves for human cells of the same histology. One possible explanation is that a considerable inter-tumor heterogeneity in radiosensitivity flattens out the presumably steep individual dose-control curves. This paper compares dose-control curves for malignant melanomas derived from clinical data with curves derived from in vitro cell-survival experiments. Although inter-tumour variability in the in vitro dose and fractionation sensitivity may explain parts of the discrepancy between the steepness of clinical and in vitro dose-control curves, the present calculation indicates that a considerable additional variability, undetected by current in vitro assays, must be assumed to exist in order to resolve the discrepancy.