Relationship between cellular radiosensitivity and non-repaired double-strand breaks studied for different growth states, dose rates and plating conditions in a normal human fibroblast line

Purpose: The aim of this study was to test under which conditions non-repai red DNA double-strand breaks (dsb) could be used as an indicator of cellula r 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-irr adiated either in exponential or confluent state with high (4 Gy/min) or lo w dose rate (0.04 Gy/min) and either plated immediately or delayed after ir radiation. The fraction of clonogenic cells was determined after doses up t o 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-fi eld 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 f ound to depend on dose rate and growth state only. There were, however, no differences for the plating conditions tested. This result was attributed t o the almost complete inhibition of cell-cycle progression when cells were plated immediately after irradiation. For the two dose rates and growth con ditions, differences in non-repaired dsb were found to correspond with the respective differences measured for the cellular radiosensitivity, and thes e data agreed fairly well with the correlation previously found for 11 fibr oblast 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.