Estimation of risk based on biological dosimetry for patients treated withradioiodine

A multicentre study was undertaken to assess the cytogenetic damage to peri pheral blood lymphocytes in 31 patients treated with I-131 for thyrotoxicos is using the cytokinesis-blocked micronucleus assay. The results were compa red to those for eight thyroid carcinoma patients using the same method. Fo r each patient, blood samples were taken immediately before and 1 week afte r iodine administration. The first blood sample was divided into three frac tions and each fraction was subsequently irradiated in vitro with 0, 0.5 an d 1 Gy Co-60 gamma rays, respectively. After blood culture for 70 h, cells were harvested, stained with Romanowsky-Giemsa and the micronuclei scored i n 1000 binucleated cells. For both patient groups, a linear-quadratic dose- response curve was fitted through the data set of the first blood sample by a least squares analysis. The mean increase in micronuclei after I-131 the rapy (second blood sample) was fitted to this curve and the mean equivalent total body dose (ETBD) calculated. Surprisingly, in view of the large diff erence in administered activity between thyroid carcinoma patients and thyr otoxicosis patients, the increase in micronuclei after therapy (mean +/- S. D.: 32 +/- 30 and 32 +/- 23, respectively) and the equivalent total body do se (0.34 and 0.32 Gy, respectively) were not significantly different (P > 0 .1). The small number of micronuclei induced by I-131 therapy (32 +/- 29), compared with external beam radiotherapy for Hodgkin's disease (640 +/- 381 ) or cervix carcinoma (298 +/- 76) [1], gave a cancer mortality estimate of less than 1%. This also explains why late detrimental effects in patients after I-131 treatment have not been reported in the literature. ((C) 1999 L ippincott Williams & Wilkins).