Biologic dosimetry in thyroid cancer patients after repeated treatments with iodine-131.

Unlabelled: To estimate a cumulative dosimetric index that reflects the dose to the circulating lymphocytes after repeated treatments with 131I, biologic dosimetry was applied to 18 patients with differentiated thyroid carcinoma and neck relapse or lung metastases.

Methods: Chromosomal aberrations were scored in peripheral blood samples that were obtained before and 4 days after each administration of 3.7 GBq 131I according to two methods, conventional cytogenetics and chromosome 4 painting.

Results: The mean dosimetric index was equal to 0.5 Gy by both methods after the administration of 3.7 GBq 131I. Repeated administrations of 131I delivered the same dose each time, resulting in a cumulative dose from 1-3.5 Gy in the patients who had two to seven treatments. However, the estimated dose, based on the number of chromosomal aberrations on Day 4 and, above all, from the third treatment on, was considerably lower than the real dose absorbed by the lymphocytes. This may be linked to the phenomenon of apoptosis, which results in a loss of information during the course of repeated irradiation.

Conclusion: Both chromosomal painting and conventional cytogenetics underestimate the cumulative dose after repeated 131I treatments. A complementary test measuring apoptosis may improve the dose estimates.