Biological dosimetry was applied to 30 patients with differentiated thyroid carcinoma who were treated with 131I (3.7 GBq) to ablate thyroid remnants after surgery or in case of metastases.
Methods: Chromosomal aberrations were scored in peripheral blood samples obtained before and 4 days after the first administration of 3.7 GBq 131I according to two methods: conventional cytogenetics and chromosome 4 painting. This generated a dosimetric index that reflects the dose to the bone marrow.
Results: Results of both techniques were in close agreement. The mean dosimetric index at Day 4 was 0.54 Gy (95% CI: 0.45-0.62 Gy) by conventional cytogenetics and 0.48 Gy (95% CI: 0.42-0.61 Gy) by chromosome 4 painting. This dose is 2-4 times higher than that derived from the MIRD estimates. Since blood was drawn at Day 4, this technique underestimates the dose by at least a third. The high dose estimate may be related to the hypothyroid status of these patients at the time of 131I administration, a condition which decreases renal clearance of 131I and thus increases whole-body irradiation. This dose estimate was closely related to whole-body retention of 131I at Day 4.
Conclusion: These data should be used to estimate the risk due to 131I exposure.