Mouse C3H 10T1/2 cells were exposed to single or fractionated doses of charged particles of defined linear energy transfer (LET) from 25 to 200 keV/μm. Dose fractionation with prolonged time intervals enhanced the yield of transformed foci compared with a single acute dose for a range of LET values between 40 and 120 keV/μm. Radiations of lower or higher LET did not show the enhancement that is commonly referred to as the inverse dose-rate effect. The fractionation scheme that was used consisted of three dose fractions; the maximum enhancement of transformation occurred with an interval of 150 min between dose fractions. This inverse dose-rate effect, demonstrated for cycling cells in log phase, was not seen for cells in plateau phase.

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