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Thyroid Carcinoma with High Levels of Function: Treatment with ¹³¹I

Division of Nuclear Medicine, Departments of Internal Medicine and Radiology, University of Michigan Health System, Ann Arbor, Michigan

In some patients with well-differentiated thyroid carcinoma, dosimetry is necessary to avoid toxicity from therapy and to guide prescription of the administered activity of radioiodine. Methods: The presentations and courses of 2 patients exemplify the points. In the second patient, the clues to the need for dosimetry were the large size of the tumor and high circulating levels of thyroxine in the absence of exogenous hormone. The other patient manifested hyperthyroidism from stimulation of the tumors by thyroid-stimulating immunoglobulin. Dosimetry was performed by published methods. Results: Dosimetry of radioactivity in the body and blood warned of increased irradiation per gigabecquerel of administered ¹³¹I. In each patient, the tumors sequestered a substantial amount of administered ¹³¹I and secreted ¹³¹I-labeled hormones that circulated for days. In 1 patient, the blood time–activity curve was complex, making a broad range of predictions for irradiation to blood and bone marrow. Still, dosimetry gave information that helped to avoid severe toxicity. At, respectively, 1.85 and 2.2 GBq ¹³¹I, initial treatments were relatively low. There was a modest escalation in subsequent administered activities. Leukopenia with neutropenia developed in each patient, and one had moderate thrombocytopenia and anemia, but toxicity appeared to be transient. Each patient had a marked increase in well-being and evidence of reduced tumor function and volume. Conclusion: Two patients with advanced, well-differentiated thyroid carcinoma illustrate the need for dosimetry to help prevent toxicity to normal tissues from therapeutic radioiodine. Conversion of radioiodide to circulating radiothyroxine by functioning carcinomas increases the absorbed radiation in normal tissues. Yet, dosimetric data acquired for 4 d or more may be insufficient for accurate calculations of absorbed radiation in blood. Guidelines suggested for avoiding toxicity are based on the circulating thyroxine concentrations, the presence of thyroid stimulators, the amount of radioactivity retained in the body at 48 h, and the general status of the patient.

Key Words: thyroid carcinoma • hyperthyroidism • radioiodine • dosimetry • bone marrow toxicity • thyroid-stimulating immunoglobulin

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