Dose-rate effects in normal and malignant cells of human origin.

Human cell lines derived from normal tissue and malignant tumours were irradiated in plateau phase under acute (81 Gy/h) or protracted (0.1-0.7 Gy/h) exposure to determine initial survival curve slopes, assess sublethal damage repair (SLDR) capability, and establish differences in survival due to SLDR over a range of dose rates. No correlation was found between clinical resistance of the tumor types and initial slope or survival at 2 Gy. Sublethal damage repair was assessed by comparing survival curves for acute and continuous irradiation. All cell lines showed significant SLDR, with the more clinically resistant tumour types demonstrating the best recovery rates. The survival data also demonstrated little difference in cell killing over the 0.1-0.7 Gy/h range. Using a modified linear-quadratic model with provisions for SLDR, it was found that for these cell lines the repair half-time was less than 1 h, with no significant change in the amount of recovery from sublethal damage at dose rates below 1 Gy/h.