Outcome and toxicity associated with maximum safe dose radioiodine treatment of metastatic thyroid cancer

PURPOSE: This study was designed to determine the outcomes of patients referred to the authors' nuclear medicine service for dosimetric evaluation and treatment of metastatic thyroid cancer with radioiodine I-131. In addition, the incidence and severity of hematologic toxicity associated with treatment were determined. METHODS: A retrospective review of patients referred for dosimetric evaluation was undertaken for the years 1984 to 2000. Chart reviews were performed for 20 patients treated with at least 80% of the calculated maximum safe dose. Outcomes were evaluated for 15 patients for whom sufficient follow-up information was available. The incidence and severity of hematologic toxicity were determined for 12 of the 20 patients. RESULTS: Outcomes were defined as good, fair, or poor. One third of the patients had good, one third had fair, and one third had poor outcomes. Three patients experienced anemia, but no life-threatening hematologic toxicity was observed in any of the patients. Serum thyroglobulin measurements proved to be an efficacious way to predict outcome. The use of recombinant human thyrotropin appears to have increased the sensitivity of the thyroglobulin measurements. CONCLUSIONS: Radioiodine I-131 treatments produce variable outcomes in patients with advanced disease. Hematologic toxicity is well controlled in patients if dosimetric guidelines are followed. I-131 treatment appeared to retard the progression of metastatic thyroid cancer in 50% of the patients who were not cured. Radioiodine doses may be increased to improve efficacy without associated irreversible toxicity.