The sensitivity to X-ray dose fractionation was assessed for hepatocytes forming colonies in fat pads in mice. When the cells were assayed 1 day after the last irradiation the α/β ratio was 1.0-1.6 depending on the method of analysis. The α/β ratio describing the shape of the single-dose survival curve was much higher, and it did not predict the response to fractionation. When the assay was delayed for 10 months after the fractionated irradiation, the α/β ratio was 1.9-2.1, and the β component showed the greatest change with time. It is concluded that hepatocytes respond to dose fractionation in a manner expected of a late-responding tissue, even when the cells are assayed as early as 1 day after the last dose.

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