Many insults at low doses are defined as nontoxic, even though obvious effects do occur. Among these are changes in colony size when clonogenic survival is assessed. Early attempts to quantify radiation effects on colony size have been reported, but the time-consuming nature of these experiments did not encourage the use of this parameter as an end point. Recently, however, developments in image cytometry technology have provided alternative, less labor-intensive means of measuring colony size. These techniques have been used in our ongoing investigations of radiation effects at low doses. Data accumulated to date show a measurable dose dependence of colony size in clones classed as survivors. This dose dependence is characterized by fluctuations in the 0-1.5-Gy dose range, followed by a gradual decrease in colony size at higher doses. The fluctuations at low doses correspond qualitatively to the concavity, thought to be indicative of inducible repair phenomena, that has been observed in cell survival curves in the same dose range. This concavity was also seen in the current study, but its detection appeared to be dependent on the method used to score survivors.

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