The alkaline single-cell gel electrophoresis (SCGE or Comet) assay appears to be 
a promising tool for measuring DNA damage at the individual cell level in both 
in vitro and in vivo studies. To provide further data on the possible 
applicability of this assay in human biomonitoring studies, we have evaluated 
the eventual genetic damage induced by therapeutic exposure to 131I, by 
measuring the Comet length and the amount of DNA damage in peripheral blood 
leukocytes from a group of 28 thyroid cancer patients who received 131I sodium 
iodide via oral administration. Blood samples were taken just before the 
treatment and 1 week after it. From the results obtained after radioiodine 
therapy, a small increase in the Comet length and in the grade of DNA damage is 
observed; however, this increase is not statistically significant because of 
inter-individual variability and the variable responses before and after 131I 
treatment. Considering our previous studies showing significant increases in the 
frequency of cytogenetic damage (when measured as micronuclei) in patients 
treated with relatively low doses of 131I, the results obtained in the present 
work by using the Comet assay could indicate that 1 week after the exposure most 
of the radioiodine-induced DNA lesions, that can be detected with this assay, 
have already been repaired.