Several techniques are now available for the growth of mammalian cells in culture as three-dimensional "spheroids." Use of these spheroids as in vitro models of solid tumors is increasing, yet little characterization of most systems, other than histological appearance, has been forthcoming. Data are presented here which indicate that spheroids produced with different sublines of V79 Chinese hamster cells can exhibit vastly different radiobiological properties, despite having a similar histological appearance. Quantitative characterization of the radiation response of these spheroids indicates that the responses are highly dependent upon cell type, whether spheroids are irradiated in stirred or unstirred cultures, and the techniques used to recover cells from spheroids. Further, the extent and/or degree of hypoxia within spheroids does not always correlate with spheroid radiosensitivity or development of central necrosis. These results clearly indicate the necessity of adequately characterizing such model systems, and thus suggest, that results obtained with one spheroid system cannot be used as a baseline for another.

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