Estimating DNA repair by sequential evaluation of head and neck tumor radiation sensitivity using the comet assay

Background: The alkaline comet assay is a microelectrophoretic technique for detecting single-strand DNA breaks, and may be used as an indirect measure of hypoxia by determining the radiation sensitivity of individual cells. Objective: To assess the ability of the comet assay to estimate the rate of DNA repair after irradiation in patients with head and neck cancer. Methods: The comet assay was used to evaluate DNA damage in fine-needle aspirates of lymph nodes containing metastatic squamous cell carcinoma in patients with head and neck cancer 1, 2, and 3 minutes after treatment with 500 rad (5 Gy) of irradiation. The amount of DNA damage (measured as the "tail moment" of the comet) is proportional to the number of DNA single-strand breaks after irradiation, which in turn depends on the oxygen concentration in each cell. Results: The mean ± SD of the median tail moment of the 1-minute postirradiation comets was 29.4 ± 14.2 (n = 27). After 2 minutes, the mean median tail moment decreased to 25.4 ± 13.6 (n = 25), representing a mean decrease of 11.9% in those patients with both 1- and 2-minute comet assays. Assuming a linear rate of repair, this decrease in DNA damage corresponds to a repair half-life of 4.2 minutes. A 3-minute assay was also performed on samples from a smaller number of patients (n = 9), with a mean value not significantly different from that of the 2-minute assay of the samples from this group. Conclusions: The comet assay is a promising tool for evaluating radiation sensitivity in individual cells. The rate of DNA repair early after irradiation is consistent with data in the literature.