Dedifferentiated adenoid cystic carcinoma ex pleomorphic adenoma of the parotid.

Carcinoma ex pleomorphic adenoma (Ca ex PA) is defined as a carcinoma arising from a primary (de novo) or recurrent benign pleomorphic adenoma. The most common histological subtype is adenocarcinoma, not otherwise specified, salivary duct carcinoma, mucoepidermoid carcinoma, and adenoid cystic carcinoma (ACC). Few case reports of de novo dedifferentiation in ACC are there in the literature, mostly involving minor salivary glands. The dedifferentiated adenoid cystic carcinoma arising in a pleomorphic adenoma of the parotid gland has not been reported earlier. The present case highlights this rare histological subtype seen in Ca ex PA and the role of extensive histopathological examination and immunohistochemistry.