We used a combination of radioiodine scanning and quantitative radiation dosimetry to evaluate responses to therapeutic irradiation with ¹³¹l in 76 patients with thyroid adenocarcinoma.

Fifty patients received ¹³¹l treatment for ablation of residual thyroid tissue after surgical thyroidectomy, and 26 had ¹³¹l treatment for metastatic thyroid cancer. Successful ablation was observed in patients receiving higher radiation doses to the thyroid — about 4.4 times those in patients whose lesions were not ablated — largely because of a longer effective half-life of ¹³¹l in residual thyroid tissue in the patients with ablated lesions. Patients with metastases that persisted after ¹³¹l therapy tended to have more advanced disease and received significantly lower radiation doses per millicurie of administered ¹³¹l than did persons whose lesions responded to treatment.

Initial ¹³¹l treatment resulting in radiation doses of at least 30,000 rad to thyroid remnants and 8000 rad to metastases was associated with a significant increase in the rate of response to therapy. (N Engl J Med 1983; 309:937–41.)