Organ Dose Estimates for Hyperthyroid Patients Treated with ¹³¹I: An Update of the Thyrotoxicosis Follow-Up Study

The Thyrotoxicosis Therapy Follow-up Study (TTFUS) is comprised of 35,593 hyperthyroid patients treated from the mid-1940s through the mid-1960s. One objective of the TTFUS was to evaluate the long-term effects of high-dose iodine-131 (¹³¹I) treatment (1–4). In the TTFUS cohort, 23,020 patients were treated with ¹³¹I, including 21,536 patients with Graves disease (GD), 1,203 patients with toxic nodular goiter (TNG) and 281 patients with unknown disease. The study population constituted the largest group of hyperthyroid patients ever examined in a single health risk study. The average number (±1 standard deviation) of ¹³¹I treatments per patient was 1.7 ± 1.4 for the GD patients and 2.1 ± 2.1 for the TNG patients. The average total ¹³¹I administered activity was 380 ± 360 MBq for GD patients and 640 ± 550 MBq for TNG patients. In this work, a biokinetic model for iodine was developed to derive organ residence times and to reconstruct the radiation-absorbed doses to the thyroid gland and to other organs resulting from administration of ¹³¹I to hyperthyroid patients. Based on ¹³¹I data for a small, kinetically well-characterized sub-cohort of patients, multivariate regression equations were developed to relate the numbers of disintegrations of ¹³¹I in more than 50 organs and tissues to anatomical (thyroid mass) and clinical (percentage thyroid uptake and pulse rate) parameters. These equations were then applied to estimate the numbers of ¹³¹I disintegrations in the organs and tissues of all other hyperthyroid patients in the TTFUS who were treated with ¹³¹I. The reference voxel phantoms adopted by the International Commission on Radiological Protection (ICRP) were then used to calculate the absorbed doses in more than 20 organs and tissues of the body. As expected, the absorbed doses were found to be highest in the thyroid (arithmetic means of 120 and 140 Gy for GD and TNG patients, respectively). Absorbed doses in organs other than the thyroid were much smaller, with arithmetic means of 1.6 Gy, 1.5 Gy and 0.65 Gy for esophagus, thymus and salivary glands, respectively. The arithmetic mean doses to all other organs and tissues were more than 100 times less than those to the thyroid gland. To our knowledge, this work represents the most comprehensive study to date of the doses received by persons treated with ¹³¹I for hyperthyroidism.