Tumor‐to‐tumor metastasis to a thyroid follicular adenoma as the initial presentation of a colonic adenocarcinoma

The incidence of thyroid involvement by metastatic disease from distant organs ranges from an average of 3.1% in surgical series to 5.3% in autopsy series. However, the metastasis of one tumor into another (traditionally referred to as ‘tumor‐to‐tumor metastasis’) is distinctly uncommon. Typically, they are identified as new manifestations or necropsy findings of a known, pre‐existing donor tumor. Herein is described the case of a 59‐year‐old woman whose thyroid nodule (a follicular adenoma) was resected and found to contain foci of a well‐differentiated adenocarcinoma with a morphologic and immunohistochemical profile consistent with origination from the lower gastrointestinal tract. Subsequent diagnostic work‐up revealed a sigmoid colon tumor with metastases to the liver. This is, to the authors’ knowledge, the first reported example of a colon adenocarcinoma whose initial clinical manifestation was a metastasis to a thyroid neoplasm and only the third reported example of a colonic adenocarcinoma metastatic to a thyroid tumor. In a review of previously reported examples of tumor‐to‐tumor metastases involving a thyroid neoplasm as the recipient, the following features were present in the majority: (i) multifocality of the metastatic tumor aggregates; (ii) a total lack of, or only minimal amounts of reaction (desmoplastic, inflammatory or myxoid) of the recipient tumor to the metastatic deposits; and (iii) retention of the histopathologic characteristics of the donor tumor in the metastatic deposits. In general, strikingly divergent morphologic features in an otherwise typical thyroid neoplasm should elicit a differential diagnosis that takes into consideration the possibility of metastasis.