Answer: Polymorphous low grade adenocarcinoma, metastatic

Histology: A lymph node is partially replaced by an epithelial tumor. The tumor demonstrates mixed architectural patterns including tubular formations, papillary structures, and cyst formations. There is a discrepancy between the tumor’s behavior (metastatic) and its cytologic appearance: the tumor cells are bland and isomorphic, and mitotic figures are not appreciated. Immunohistochemical stains for TTF-1 and thyroglobulin are negative.

Discussion: Poymorphous low grade adenocarcinoma (PLGA) is the current consensus terminology for a tumor arising almost exclusively from the intraoral minor salivary glands. The name, although somewhat cumbersome, aptly describes the distinguishing attributes of this tumor; namely, its remarkable morphologic variability and its limited but definite capacity for local and regional spread.

The palate is the most common location of PLGA, but they can also arise from other intraoral sites including the cheek, tongue, lips, and floor of mouth. Clinically they typically present as palatal masses that are painless, slow-growing, and easily ignored.

At the microscopic level, PLGA is cytologically monotonous but architecturally diverse. The cells have uniformly round, ovoid, or spindled nuclei; and they are organized into any combination of solid, tubular, papillary, cribriform, or fascicular patterns. In the present case, these features are preserved in the lymph node metastasis.

The benign appearance of the tumor cells may cause confusion with a pleomorphic adenoma. Avoidance of this diagnostic pitfall requires careful inspection of the tumor periphery for evidence of invasion into the surrounding tissues, a defining feature of PLGA that is not seen in pleomorphic adenoma. Indeed, in the present case, the original palatal tumor was retrieved, re-reviewed, and re-classified as a PLGA base on the presence of tumor invasion at the periphery of the tumor. Diagnostic problems arise when the tumor’s interface with the surrounding tissues is not evaluated, as when a tumor is enucleated, removed in a piecemeal fashion, or aspirated. Even though it is not always easy, there is good reason to distinguish PLGA from the harmless pleomorphic adenoma. The behavior of PLGA is that of a low grade malignancy. Approximately 17 percent of tumors recur, and 10 percent metastasize to regional lymph nodes. With only a few exceptions, distant metastases and tumor-related deaths do not occur. Wide but conservative excision is appropriate. The benefit of post-operative radiation or chemotherapy has not been established. Neck dissection is reserved for those cases where lymph node metastases are clinically apparent.