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Uptake of 1-131 by an Abdominal Neurilemoma Mimicking Metastatic Thyroid Carcinoma

1-131 whole-body imaging may show characteristic findings in functioning, metastatic differentiated thyroid carcinoma. Nonthyroidal tumor uptake of 1-131 is rare, but may mislead the physician to inappropriate treatment. A case is reported of a 59-year-old woman with papillary thyroid carcinoma who demonstrated strong uptake of 1-131 in an abdominal cystic neurilemoma. Her serum thyroglobulin concentration at that time was low, at 3.35 ng/ml. Although the concentrating mechanism of 1-131 was not clear, several clues pointed to the possibilities of nonthyroidal tumor uptake: 1) faster clearing of radioactivity than usual thyroid tissue, 2) persistent low serum thyroglobulin concentration, and 3) the presence of a cystic component in the nonthyroidal tumor.