Scintigraphy and treatment of medullary carcinoma of the thyroid with iodine-131 metaiodobenzylguanidine.

We report our experience using [131I]metaiodobenzylguanidine (MIBG) to image nine patients with proven medullary carcinoma of the thyroid (MCT). Positive uptake was seen in four patients, equivocal uptake in one patient, and no uptake in four patients. Data is presented to demonstrate the pharmacokinetics of [131I]MIBG in three of the patients studied. Two patients, with diarrhea and severe pain from known bone metastases and positive uptake on [131I]MIBG diagnostic scanning, subsequently received therapeutic doses of [131I]MIBG, with marked improvement in both pain and diarrhea, but no evidence of significant biochemical response. Iodine-131 MIBG uptake in patients with MCT is variable, and gives a higher false-negative rate than is found when using [131I]MIBG to image other neuroectodermally derived tumors. The therapeutic potential of [131I]MIBG in patients with MCT warrants further evaluation, in view of the symptomatic relief experienced following therapy doses in two patients with extensive disease.