Repair of fractionated radiation in plateau phase cultures of human tumor cells and human multicellular tumor spheroids.

We have measured potentially lethal damage repair (PLDR) after fractionated radiation, delivered at 24-h intervals in density-inhibited plateau phase cultures of four human tumor cell lines derived from tumors of differing radiocurability (two melanoma and two breast). The repair of potentially lethal damage conferred significant radioresistance on the human melanoma cells but not on the breast carcinoma cells. We examined the effects of fractionated radiation on human tumor cells adapted to become multicellular tumor spheroids (MTS). MTS derived from a human neuroblastoma were "cured" by a total fractionated radiation dose about 50% of that required for MTS derived from a human melanoma.