Loss of repair capacity in density-inhibited cultures of C3H 10T1/2 cells during multifraction irradiation.

Multifraction survival curves for slowly cycling, density-inhibited C3H 10T1/2 cells were shown previously to bend toward lower survival levels with increasing total dose, even for doses per fraction as small as about 2.0 Gy. In an attempt to explain this, we tested the capacity of cells to repair potentially lethal damage (PLD) as fractionation progressed. Plateau-phase cultures were exposed to repeated doses of 4.0 Gy of 137Cs gamma rays delivered at 12-hr intervals. After zero, three, five, and seven fractions, some cultures were put aside, incubated for 12 hr at 37 degrees C, irradiated with a single dose of 9.0 Gy, and subsequently returned to a 37 degrees C incubator. At 0, 2, 4, 6, and 12 hr after the 9.0 Gy dose, cultures were trypsinized and plated for a survival assay. Following three fractions of 4.0 Gy, cells were able to repair PLD as well as those receiving a single dose of 9.0 Gy without prior fractionation. Following five fractions, cells were less able to repair PLD, and after seven fractions, only a very small amount of PLD repair was detectable using this method of measurement.