Radiosensitivity of Four Human Tumor Xenografts. Influence of Hypoxia and Cell-Cell Contact

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Abstract
Contact effect (CE) and hypoxia were studied in human tumor cell lines transplanted in athymic nude mice. Four cell lines, 1 melanoma (Bell) and 3 colorectal adenocarcinomas (HT29, HRT18 and HCT8), were studied. Cell survival was determined with an in vivo to in vitro colony-forming assay. Survival curves were obtained under 3 different conditions: tumor cells irradiated in air-breathing mice, tumor cells irradiated in animals asphyxiated for 10 min; and tumor cells plated and irradiated either immediately or 5 h later. For all cell lines, radiosensitivity appeared to be lower when cells were irradiated in vivo than when they were irradiated in vitro. Only with the HCT8 tumor did the relative in vivo radioresistance seem to be linked to hypoxia; in the other cell lines, hypoxia alone could not account for the lower in vivo radiosensitivity. A CE apparently plays an important role in the response of human tumor xenografts to irradiation.