PO-2 IN IRRADIATED VERSUS NONIRRADIATED TUMORS OF MICE BREATHING OXYGEN AT NORMAL AND ELEVATED PRESSURE

Purpose: To determine if prior tumor irradiation influences tumor pO(2) changes in mice breathing oxygen (100%) at normal and elevated pressure. Methods and Materials: Single-point pO(2) measurements were performed in nonirradiated and previously irradiated (72 h) isotransplanted MCaIV tumors in C3H/Sed mice, Continuous recordings were performed at the same tumor locus under air breathing, followed by 100% oxygen and oxygen at three atmospheres pressure. Following decompression and induction of pentobarbital anesthesia, the procedure was repeated at the same locus. Six nonirradiated and five irradiated tumors were evaluated under the three gas breathing conditions +/- anesthesia. Results: The mean, median, and range of pO(2) values did not differ under air-breathing conditions in the nonirradiated vs, previously irradiated tumors, However, prior irradiation substantially enhanced the tumor PO2 increase when the inspired gas phase was switched from air to 100% oxygen at 1 or 3 atmospheres pressure, In four of six nonirradiated tumors, 100% oxygen breathing resulted in a pO(2) increase of < 4 mmHg; in the irradiated tumors, the minimum increase was 16 mmHg, Pentobarbital anesthesia did not significantly influence the results obtained. Conclusion: These data indicate that the efficacy of oxygen breathing increases during tumor treatment, and suggests that oxygen breathing is a simple nontoxic method for reducing or eliminating radiobiologic hypoxia during therapy.