Purpose: The aim of this study was to test under which conditions non-repaired DNA double-strand breaks (dsb) could be used as an indicator of cellular radiosensitivity of normal human fibroblasts.
Materials and methods: The experiments were performed with a primary normal skin fibroblast line (NFHH) derived from a healthy donor. Cells were X-irradiated either in exponential or confluent state with high (4 Gy/min) or low dose rate (0.04 Gy/min) and either plated immediately or delayed after irradiation. The fraction of clonogenic cells was determined after doses up to 12 Gy using colony forming assay and the number of non-repaired dsb were measured 24 h after X-irradiation with doses up to 180 Gy using constant-field gel electrophoresis.
Results: Cellular radiosensitivity of NFHH cells was found to depend on all three conditions tested. In contrast, the number of non-repaired dsb was found to depend on dose rate and growth state only. There were, however, no differences for the plating conditions tested. This result was attributed to the almost complete inhibition of cell-cycle progression when cells were plated immediately after irradiation. For the two dose rates and growth conditions, differences in non-repaired dsb were found to correspond with the respective differences measured for the cellular radiosensitivity, and these data agreed fairly well with the correlation previously found for 11 fibroblast lines varying in dsb repair capacity.
Conclusions: For irradiation followed by delayed plating only, non-repaired dsb can be used to predict the cellular radiosensitivity.