The clinical diagnosis of primary thyroid cancer is uncommon, constituting 1.5% of all cancers in the United States. Clinically diagnosed metastatic cancer to the thyroid gland is rare. Colon cancer is one of the most common cancers in the United States, with a high propensity to metastasize; 30% to 40% of patients have metastatic disease at the initial diagnosis. The most common sites of metastasis from colon cancer are the regional lymph nodes, the liver, the lung, and the peritoneum. Colon cancer metastasis to the thyroid gland is rare, with only a few reported cases, mainly in the pathology literature. These cases describe metastasis from colon cancer to the thyroid gland that became apparent years after the initial diagnosis of colon cancer and were usually associated with dissemination to the liver, the lung, or both. We report a case of colonic adenocarcinoma metastatic to the thyroid gland and lung without involvement of the liver. A review of the literature is also included.
Tumor cells form large sheets and display a picket-fence arrangement. Necrosis is present in the background (direct smear, Diff-Quick, original magnification ×200).
Ancillary immunocytochemical studies support the diagnosis of metastatic colon adenocarcinoma. A, Tumor cells are strongly immunoreactive for cytokeratin 20 (CK-20), a marker for colon cancer (ThinPrep, CK-20 immunoperoxidase, original magnification ×200). B, Tumor cells show no reactivity for thyroid transcription factor 1 (TTF-1), a marker for thyroid malignancy (ThinPrep, TTF-1 immunoperoxidase, original magnification ×200).
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