Reoxygenation in a rat rhabdomyosarcoma tumor following X-irradiation

The paired survival curve technique was used to characterize the rate at which the fraction of hypoxic cells in rat rhabdomyosarcoma R-1 tumors returns to the preirradiation value of 37% following a single dose of 225-kVp X rays. Tumors were administered a conditioning x-ray dose of 15-Gy, followed at 0, 3, 6, 12, 24, or 48 hr by a 5-Gy, 10-Gy, or 15-Gy dose of X rays under air-breathing conditions or under hypoxic conditions produced by nitrogen-gas asphyxiation 5 min prior to irradiation. Cellular surviving fractions were determined by the tumor excision assay following in vivo irradiation. From the ratio of the survival fractions measured for tumor cells from air-breathing and hypoxic animals, the fraction of hypoxic cells was determined as a function of time postirradiation. These results indicated that immediately following a 15-Gy dose of X rays, essentially 100% of the viable cells remaining were hypoxic. The tumors reoxygenated rapidly, returning to the preirradiation level of 37% during the first 6 hr postirradiation.