Answer: Adenoid cystic carcinoma

Histology: The tumor infiltrating the thyroid parenchyma demonstrates a prominent cribriform pattern of growth. The tumor cells are arranged around sharply punched-out holes. These luminal structures are filled with matrix ground substance that takes on a distinctive bluish-gray coloration. The cellular features of the tumor are deceptively non-threatening. The cells have uniform round-to-angulated basophilic nuclei, and they do not demonstrate appreciable pleomorphism or mitotic activity. Despite the bland cytology, the tumor is highly infiltrative.

Discussion: Not all tumors that involve the thyroid parenchyma are primary to that site. The thyroid gland can also be involved secondarily, either by metastatic implantation or direct invasion from a contiguous site. Of those carcinomas that seem to have a propensity to target the thyroid gland by metastatic spread, renal cell carcinoma and malignant melanoma are the most common. As for those tumors that can involve the thyroid be direct extension, one must always consider the possibility of a squamous cell carcinoma of laryngeal origin, paraganglioma arising from the laryngeal paraganglia, parathyroid carcinoma, thymic neoplasms, salivary gland carcinomas arising from the seromucinous glands of the larynx and trachea.

This case represents on example of secondary involvement of the thyroid by an adenoid cystic carcinoma of tracheal origin. The cribriform pattern quite distinctive and represents the hallmark morphologic featrue of adenoid cystic carcinoma. The tumor cells are arranged around sharply punched-out holes, imparting a “swiss cheese”-like appearance. These holes, or cysts, are not true glandular spaces. Instead, they are lined by myoepithelial cells and filled with matrix ground substance. Duct epithelial cells are also present, and they form true glandular spaces. Although there is a rare variant of papillary thyroid carcinoma known as the cribriform morular variant, its cribriforming structures tend to be lined by tall columnar cells with papillary nuclei. Anaplastic thyroid carcinomas do not grow in a cribriform pattern, and the tumor cells demonstrate high grade malignant features.