Hyperthermic radiosensitization of cells from a human melanoma xenograft

Cells derived directly from a human melanoma xenograft were exposed to radiation and/or hyperthermia under aerobic conditions in vitro. Single cell survival was assayed in soft agar. The activation energies for heat treatment alone were 420 +/- 40 kcal/mole (41.5-42.5 degrees C) and 170 +/- 10 kcal/mole (43.0-45.5 degrees C). Heat doses of 41.5 degrees C (30 min) or 43.5 degrees C (30 min) did not cause a reduction in the shoulder of the X-ray survival curve of the cells, but the D0 value was reduced. The thermal enhancement ratios (the ratio of the D0 values) were in the ranges 1.0-1.3 (41.5 degrees C, 30 min) and 1.1-1.7 (43.5 degrees C, 30 min), depending on the sequence and the time between the treatments. Repair of sublethal radiation damage was not significantly inhibited when treatments with 41.5 degrees C (30 min) or 43.5 degrees C (30 min) preceded irradiation, but was inhibited when 44.5 degrees C (30 min) was given immediately before radiation and when 41.5 degrees C (120 min) was given immediately after radiation. Implications of the results for clinical treatment of malignant melanomas are discussed.