Radiation-induced micronuclei in human fibroblasts in relation to clonogenic radiosensitivity

As part of our programme for developing predictive tests for normal tissue response to radiotherapy, we have investigated the efficacy of the cytokine sis-block micronucleus (MN) assay as a means of detecting interindividual d ifferences in cellular radiosensitivity. A study was made of nine fibroblas t strains established from vaginal biopsies of pretreatment cervical cancer patients and an ataxia telangiectasia (A-T) cell strain. Cells were irradi ated in plateau phase, replated and treated with cytochalasin B 24 h later. MN formation was examined 72 h after irradiation as the number of MN in 10 0 binucleate cells. The method yielded low spontaneous MN yields (<7 per 10 0 cells), and mean induced MN frequencies after 3.5 Gy varied between cell strains from 18 to 144 per 100 cells. However, in repeat experiments, consi derable intrastrain variability was observed (CV = 32%), with up to twofold differences in MN yields, although this was less than interstrain variabil ity (CV = 62%). An analysis was made of the relationship between MN results and previously obtained clonogenic survival data. There was a significant correlation between MN yields and clonogenic survival. However, when the A- T strain was excluded from the analysis, the correlation lost significance, mainly because of one slow-growing strain which was the most sensitive to cell killing but had almost the lowest MN frequency. With current methodolo gy, the MN assay on human fibroblasts does not appear to have a role in pre dictive testing of normal tissue radiosensitivity.