A Case of Ectopic Thyroid Cancer in the Abdominal Cavity.

Ectopic thyroid glands occur in the head and neck, typically at the base of the tongue but rarely in the abdominal cavity. Malignant tumors can also arise from ectopic thyroid glands. Here, we present an extremely rare case of intraperitoneal ectopic thyroid cancer. A 61-year-old woman was diagnosed with a mass in the left para-aortic region based on computed tomography (CT) at another hospital for a preoperative diagnosis of rectal cancer. Positron emission tomography-computed tomography revealed mild fluorodeoxyglucose accumulation in the mass, and rectal cancer metastasis was suspected. The mass was resected laparoscopically along with the primary tumor. Postoperative pathology revealed a follicular papillary carcinoma of the thyroid gland. The patient had a history of an ovarian cyst; therefore, we considered teratoma as a differential diagnosis, in addition to lymph node metastasis of thyroid cancer into the abdominal cavity. However, based on the histological findings, we considered primary intra-abdominal thyroid cancer to be the most likely diagnosis. The mechanism of intra-abdominal thyroid tissue development remains unknown because of the small number of cases, and future case reports are warranted.