In 28 blood samples from 21 patients undergoing 131I treatment after surgical thyroidectomy for cancer, the micronucleus (MN) frequency observed in peripheral blood lymphocytes (MN-test on binucleated cells) had a weighted mean of 0.044 ± 0.006 (SEM), which was significantly different (p < 0.001) from that observed in 93 healthy individuals (0.025 ± 0.001). The MN frequency (F((MN))) of the patients correlated fairly well (R = 0.736) with the modified activity (A(mod)) calculated by the following equation: where A(i) is the 131I activity on a determined day, e the logarithm base, d(i) the number of days that have passed between the determined day and the day when the blood was drawn, and k is a day coefficient, defined in this context as the 'daily attenuation factor'. The use of the value of 0.0003 for k allowed the following equation to be obtained: F((MN)) = 7.9 · 10 -5 (± 1.4.10-5)·A(mod) + 0.014 (R = 0.736) The MN frequency was used to estimate, by our DOSIME program, the dose (Gy) received at the individual level in the 131I treatment. In these subjects the calculated dose was well correlated with A(mod) by the relationship: D(Bio) = 0.0009 (± 0.0002)·A(mod) + 0.0675 (R = 0.755) 3-aminobenzamide (3AB), an inkibitor of the poly(ADP-ribose)polymerase enzyme involved in DNA repair, induced and increase in X-ray cytogenetic damage (MN yields), evaluated at the individual level using the 3AB index (I-3AB). The index was obtained from the MN-yield count after x-irradiation with (MN(+3AB)) and without (MN(-3AB)) 3AB, using the following formula: I = 1-(MN(-3AB)/MN(+3AB)). The weighted average of I-3AB obtained for 15 patients was greater than that found for 38 healthy subjects, being 0.320 ± 0.046 and 0.217 ± 0.032, respectively. We discuss the possibility of using the MN-yield to evaluate on an individual level the dose absorbed, and attempt to determine the significance of the 3AB-index as an adaptative response to 131I therapeutic irradiation.

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