Answer: Adenoid cystic carcinoma

Histology: The tumor is highly invasive. Perineural and intraneural invasion is a prominent finding. Most of the tumor is comprised of dark basoloid cells forming large irregular sheets and solid nests. These solid areas show zones of necrosis, and the tumor cells within the solid areas are pleomorphic with a high mitotic rate. A minor component of the tumor is characterized by a more cribriform pattern of growth where the tumor cells are arranged around sharply punched-out holes, imparting a “swiss cheese”-like appearance. In these cribriform areas, the cells have uniform round-to-angulated basophilic nuclei, and they do not demonstrate appreciable pleomorphism or mitotic activity.

Discussion: Adenoid cystic carcinoma is the 4th most common salivary gland carcinoma. Most adenoid cystic carcinomas arise in the parotid and submandibular glands, but they can also originate from seromucinous glands anywhere along the upper respiratory tract. Once considered the most common malignant salivary gland tumor to involve the palate, they are now outnumbered at this site by polymorphous low-grade adenocarcinoma.

The cribriform pattern is the hallmark morphologic finding 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. In the solid pattern, the tumor cells form large irregular nests and sheets. The formation of cysts and ducts is not a conspicuous finding in these solid areas, and the cells may exhibit appreciable pleomorphism, mitotic activity, and even necrosis.

Although tumor stage is probably the most reliable indicator of overall outcome, some studies have emphasized the importance of histologic subtyping. In particular, the solid variant portends a more explosive course, and has been associated with early metastases and poor survival even at five years.
The solid variant of ACC may cause some diagnostic confusion, especially in its distinction from other basaloid neoplasms composed of dark basaloid cells growing in solid nests. A thorough microscopic examination of the tumor can usually establish the diagnosis without the need for ancillary diagnostic studies such as immunohistochemistry. The solid variant of ACC seldom occurs as a pure variant. Instead, thorough tumor sampling will usually disclose a component of classic cribriform growth.