SEQUENTIAL BIOLOGICAL DOSIMETRY AFTER A SINGLE TREATMENT WITH I-131 FOR DIFFERENTIATED THYROID-CARCINOMA

To determine the cytogenetic and genotoxic risk associated with therap eutic exposure to I-131 (3.7 GBq) in 50 patients with differentiated t hyroid carcinoma, we estimated the dosimetric index that reflects,the dose to the circulating lymphocytes on Day 4 and al several time inter vals after exposure over a period of 2 yr, Methods: Chromosomal aberra tions were scored in peripheral lymphocytes obtained before and then 4 days, 3 mo, 6 mo, 1 yr and 2 yr after the first administration of 3.7 GBq I-131 according to two methods: conventional cytogenetics and chr omosome 4 painting. Results: The dosimetric index was 0.52 Gy on Day 4 , 0.49 Gy at 3 mo, 0.45 Gy at 6 mo, 0.44 Gy at 1 yr and 0.42 Gy at 2 y r by conventional cytogenetics and 0.47 Gy on Day 4, 0.45 Gy at 3 mo, 0.44 Gy at 6 mo, 0.43 Gy at 1 yr and 0.42 Gy at 2 yr by chromosome 4 p ainting, We found a decrease in the frequency of chromosomal aberratio ns between Day 4 and 3 mo after exposure. This may be due to the decre ase of lymphocyte counts shortly after I-131 administration, which wil l recover later on, In contrast, the number of anomalies remained cons tant starting 3 mo after I-131 administration. Conclusion: These techn iques permit retrospective biological dosimetry for up to 2 yr after t herapeutic exposure to I-131.