Low grade mucoepidermoid carcinoma ex pleomorphic adenoma. A diagnostic problem in fine needle aspiration biopsy.

We report a case of low grade mucoepidermoid carcinoma arising in a pleomorphic adenoma (ex pleomorphic adenoma) in the parotid salivary gland of a 32-year-old woman. Fine needle aspiration biopsy showed the typical biphasic pattern of pleomorphic adenoma: groups of benign-appearing epithelial cells and chondromyxoid stroma. In addition, features of low grade mucoepidermoid carcinoma were identified retrospectively, consisting of background mucin and rare mucin-containing cells. This case illustrates that the presence of background mucin and mucin-containing cells in an otherwise usual pleomorphic adenoma may indicate the presence of a well-differentiated mucoepidermoid carcinoma. In cases such as this, a definitive diagnosis should be postponed until the lesion is examined histologically.