Quantitative assay for the radiosensitivity of malignant melanomas.

There exists a controversy regarding the radiosensitivity of malignant melanomas. Several in vitro and in vivo studies have suggested that the 'radioresistance' of melanomas may be due to a 'large shoulder' on the cell survival curve. As yet, however, there is no consensus relating to the effects of total dose, the fraction size and the time between fractions. In this study, we have used a B16 mouse melanoma model to evoke a response with single doses of irradiation, and have attempted to evaluate the growth kinetics of in vivo irradiated and unirradiated tumour cells implanted in an unirradiated limb. The radiosensitivity of B16 melanoma cells was quantified by comparing the growth of tumour from an inoculum of 10(6) melanoma cells irradiated in vivo with various doses of radiation to the growth of tumour following inoculae of 10(3) to 10(7) cells derived from unirradiated melanoma. Using this bioassay we found that a single dose of 18 Gy leads to close to 99% of the surviving cells becoming nonclonogeneic. It is hoped that this assay will further the development of the most efficacious fractionation scheme in the treatment of malignant melanomas.