Sublethal damage repair in squamous cell carcinoma cell lines.

Squamous cell carcinoma (SCC) cell lines recently established from head and neck tumors were studied for their capability of repairing sublethal radiation induced damage (SLDR). Total doses of 1.25, 2.50, 5.00, and 7.50 Gy were used either as a single dose or split in two equal fractions with a 24 h interval. Cell survival was determined using a 96-well plate clonogenic assay based on limiting dilution. Survival data was fitted by linear quadratic model, and the area under the survival curve (AUC) was obtained with numerical integration. The amount of SLDR was expressed as AUC-ratio comparing survivals from split dose vs single dose experiments. SLDR capacity was observed to vary markedly between individual cell lines (AUC-ratios 1.0-1.5). The relatively radiation-sensitive cell lines had a tendency toward higher SLDR proficiency (correlation coefficient 0.85). The differences in repair capacity could not be explained by the differences in doubling times of the cell lines. The inverse relationship between SLDR capacity and inherent radiosensitivity could explain the poor radiocurability of the sensitive donor tumors. Two of the most resistant cell lines were found SLDR deficient. This is a novel finding, since SLDR has been previously reported in all studied cells.