Small cell undifferentiated carcinoma
Small cell undifferentiated carcinoma, also known as extrapulmonary oat cell carcinoma, is a rare, primary malignant tumor that, with conventional light microscopy, is composed of undifferentiated cells and, with ultrastructural or immunohistochemical studies, does not demonstrate neuroendocrine differentiation. This is the undifferentiated counterpart of anaplastic small cell carcinoma (Refer to the anaplastic small cell carcinoma section of this summary.)
In an AFIP review of case files, small cell carcinoma represented 1.8% of all major salivary gland malignancies; the mean age of patients was 56 years. In 50% of the cases, patients present with an asymptomatic parotid mass of 3 months' or less duration.[68,69,70] This is a high-grade neoplasm. In a retrospective review of 12 cases, a tumor size of more than 4 cm was found to be the most important predictor of behavior. In another small retrospective series, estimated survival rates at 2 and 5 years were 70% and 46%, respectively.
Large cell undifferentiated carcinoma
Large cell undifferentiated carcinoma is a tumor in which features of acinar, ductal, epidermoid, or myoepithelial differentiation are absent under light microscopy, though occasionally, poorly formed duct–like structures are found. This neoplasm accounts for approximately 1% of all epithelial salivary gland neoplasms.[3,53,71,72] Most of these tumors occur in the parotid gland.[70,72] In AFIP data, the peak incidence is in the seventh to eighth decades of life.
Rapid growth of a parotid swelling is a common clinical presentation. This is a high-grade neoplasm that frequently metastasizes and has a poor prognosis. Neoplasms 4 cm or larger may have a particularly poor outcome.[70,72]
Lymphoepithelial carcinoma, also known as undifferentiated carcinoma with lymphoid stroma and carcinoma ex lymphoepithelial lesion, is an undifferentiated tumor that is associated with a dense lymphoid stroma. An exceptionally high incidence of this tumor is found in the Eskimo and Inuit populations.[3,73] This neoplasm has been associated with Epstein-Barr virus infection.[74,75] Of the occurrences, 80% are in the parotid gland.