Thyroid carcinoma with high levels of function: Treatment with I-131

In some patients with well-differentiated thyroid carcinoma, dosimetry is n ecessary to avoid toxicity from therapy and to guide prescription of the ad ministered 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 l evels of thyroxine in the absence of exogenous hormone. The other patient m anifested hyperthyroidism from stimulation of the tumors by thyroid-stimula ting immunoglobulin. Dosimetry was performed by published methods. Results: Dosimetry of radioactivity in the body and blood warned of increased irrad iation per gigabecquerel of administered I-131. In each patient, the tumors sequestered a substantial amount of administered I-131 and secreted I-131- labeled hormones that circulated for days. In 1 patient, the blood time-act ivity curve was complex, making a broad range of predictions for irradiatio n to blood and bone marrow. Still, dosimetry gave information that helped t o avoid severe toxicity. At, respectively, 1.85 and 2.2 GBq I-131, initial treatments were relatively tow. There was a modest escalation in subsequent administered activities. Leukopenia with neutropenia developed in each pat ient, and one had moderate thrombocytopenia and anemia, but toxicity appear ed to be transient. Each patient had a marked increase in well-being and ev idence of reduced tumor function and volume. Conclusion: Two patients with advanced, well-differentiated thyroid carcinoma illustrate the need for dos imetry to help prevent toxicity to normal tissues from therapeutic radioiod ine. Conversion of radioiodide to circulating radiothyroxine by functioning carcinomas increases the absorbed radiation in normal tissues. Yet, dosime tric data acquired for 4 d or more may be insufficient for accurate calcula tions of absorbed radiation in blood. Guidelines suggested for avoiding tox icity are based on the circulating thyroxine concentrations, the presence o f thyroid stimulators, the amount of radioactivity retained in the body at 48 h, and the general status of the patient.