The Effect of Intercellular Contact on the Radiation Sensitivity of KHT Sarcoma Cells

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Abstract
The recent finding that some cells grown in vitro as spheroids demonstrate an intercellular contact effect which increases their capacity to accumulate and repair sublethal radiation damage has important implications for the interpretation of results obtained from in vivo irradiations. The intercellular contact effect can be observed in an animal tumor system in vivo. The studies were performed with mouse KHT sarcoma cells, irradiated in contact as small lung colonies in situ or as individual cells in vitro. The cells for the in vitro irradiations were obtained from subcutaneous tumors which were excised from their host mice, immediately made into cell suspensions and then irradiated as a function of time after excision of the tumors from their host animals. Survival curves obtained at 2 or 24 h after tumor excision have identical slopes (D0 = 129 .+-. 7 and 121 .+-. 12 rad, respectively), but extrapolation numbers which differ by a factor of 7 (17 and 2.5, respectively). The cells irradiated in contact as small lung colonies have a survival curve with a shoulder at least as large as that seen at 2 h after tumor excision. Split-dose experiments performed for the various conditions discussed indicate that the size of the shoulder of the survival curve reflects the ability of the cells to repair sublethal radiation damage.