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 press ure. Methods and Materials: Single-point pO(2) measurements were perfo rmed 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 ind uction of pentobarbital anesthesia, the procedure was repeated at the same locus. Six nonirradiated and five irradiated tumors were evaluate d under the three gas breathing conditions +/- anesthesia. Results: Th e mean, median, and range of pO(2) values did not differ under air-bre athing conditions in the nonirradiated vs, previously irradiated tumor s, However, prior irradiation substantially enhanced the tumor PO2 inc rease 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 an esthesia did not significantly influence the results obtained. Conclus ion: These data indicate that the efficacy of oxygen breathing increas es during tumor treatment, and suggests that oxygen breathing is a sim ple nontoxic method for reducing or eliminating radiobiologic hypoxia during therapy.