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

Abstract

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 ((131)I) treatment (1-4). In the TTFUS cohort, 23,020 patients were treated with (131)I, 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 (131)I treatments per patient was 1.7 ± 1.4 for the GD patients and 2.1 ± 2.1 for the TNG patients. The average total (131)I 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 (131)I to hyperthyroid patients. Based on (131)I data for a small, kinetically well-characterized sub-cohort of patients, multivariate regression equations were developed to relate the numbers of disintegrations of (131)I 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 (131)I disintegrations in the organs and tissues of all other hyperthyroid patients in the TTFUS who were treated with (131)I. 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 (131)I for hyperthyroidism.

Exposure to Medical Radiation during Fetal Life, Childhood and Adolescence and Risk of Brain Tumor in Young Age: Results from The MOBI-Kids Case-Control Study

“…We aimed to perform the risk analysis using as exposure metric the cumulative dose to the brain, expressed in mGy. Thus, for each examination, brain doses were estimated based on typical protocols by time period and age at exposure as follows [8,26].…”

“…Despite the limitations discussed, our work, presents 2 important original aspects. It is the first brain tumor casecontrol study of medical IR diagnostic exposure to use time-period based dose estimation; even if it comes with some uncertainty, this is a valid alternative to using merely number of examinations [8,26]. Indeed, using the number of examinations leads to exposure misclassification due to the wide range of doses for each procedure.…”

Joint statement from the Society for Endocrinology and the British Thyroid Association regarding ‘Association of Radioactive Iodine Treatment with cancer mortality in patients with hyperthyroidism’

“…urine. 4 The model was applied to each patient in the cohort using their Iodine-131 thyroid uptake, thyroid gland weight and administered radioiodine dose in multivariable regression equations to calculate the number of Iodine-131 disintegrations in source organs. Following this, reference standards were then used to estimate the mean absorbed organ dose from the Iodine-131 disintegrations.…”

“…Following this, reference standards were then used to estimate the mean absorbed organ dose from the Iodine-131 disintegrations. 4 However, the validity of the model has been questioned due to the wide error margins incurred in the estimates and the assumptions required of the thyroid gland weight and uptake measures. 3,5 These assumptions leave significant uncertainties in organ dose exposure, and whilst impressive, the model will need to be replicated and further validated.…”

Radioactive Iodine Treatment and Cancer Mortality in Hyperthyroid Patients: Questioning Standard Clinical Practice Requires Indisputable Scientific Data

“…Each organ-absorbed dose for an individual patient was calculated using a published biokinetic model, which was previously developed and calibrated using data from a group of 197 hyperthyroidism patients (5). How reliable is the extension of this model to the entire population of 18,805 patients with differing severities of hyperthyroidism?…”