The effects of the combination of a perfluorochemical emulsion (Fluosol DA, 20%) and carbogen (95% O2, 5% CO2) on the response of BA1112 rat rhabdomyosarcomas to continuous low-dose-rate irradiation were examined. Tumors were irradiated locally in unrestrained, unanesthetized rats at a dose rate of 0.98 Gy/h, using a specially designed241 Am irradiator system. Cell survival was measured using a colony formation assay. The tumor cell survival curves were fitted to linear relationships of the form ln S = -αD, where α for air-breathing rats was$0.104\pm 0.005\ {\rm Gy}^{-1}$, as compared to$0.137\pm 0.009\ {\rm Gy}^{-1}$ for rats treated with Fluosol plus carbogen. The increase in the slope of the survival curve produced by the treatment with Fluosol and carbogen was highly significant with a P value of 0.0015. The radiosensitization factor for the combination of Fluosol/carbogen plus continuous low-dose-rate irradiation was 1.32 ± 0.11. Slightly less radiosensitization was observed with continuous low-dose-rate irradiation than in previous experiments using acute high-dose-rate irradiation. The diminished sensitization with Fluosol/carbogen during continuous low-dose-rate irradiation probably reflects the intrinsically lower oxygen enhancement ratio (OER) of low-dose/low-dose-rate irradiation, reoxygenation of the tumors during the prolonged treatment times used for continuous low-dose-rate irradiation, and the decrease in the levels of circulating perfluorochemicals during the 30-h irradiations. More importantly, the significant level of radiosensitization observed in the experiments with continuous low-dose-rate irradiation suggests that hypoxic cells persist in BA1112 tumors during continuous low-dose-rate irradiations and that the response of these tumors to continuous low-dose-rate irradiation can be improved by adjunctive treatments which oxygenate these radioresistant hypoxic tumor cells.

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