False-positive iodine-131 whole-body scans due to cholecystitis and sebaceous cyst.

False-positive whole-body 131I scans are not frequent but have serious consequences in the management of patients with thyroid cancer. They can be classified in four main groups: elimination of iodine in body fluids, infection or inflammation, cysts or transudates and nonthyroid tumors. We report on two patients with false-positive post-therapy 131I scans. The first patient had uptake projected in the right pelvic area which was later proven to be a large gluteal sebaceous cyst. The second patient had uptake in the gallbladder area that did not disappear after 131I treatment; she underwent exploratory laparotomy which revealed extensive chronic cholecystitis. These cases illustrate two new causes of false-positive 131I whole-body scans (sebaceous cyst and cholecystitis), which highlights two mechanisms (elimination in body fluid and inflammation).