Matrix-Containing Tumors: Pleomorphic Adenoma and Adenoid Cystic Carcinoma

There are many tumors of the salivary gland that produce matrix, including pleomorphic adenoma, carcinoma ex pleomorphic adenoma, adenoid cystic carcinoma, basal cell adenoma and adenocarcinoma, epithelial-myoepithelial carcinoma, and polymorphous low-grade adenocarcinoma. By far, however, the most commonly encountered among these tumors is pleomorphic adenoma. And one of the more common and clinically important differential diagnoses that the cytopathologist faces is between pleomorphic adenoma and adenoid cystic carcinoma. Because the difference in clinical prognosis and management between these two neoplasms is dramatically different, it is imperative not to confuse these two lesions cytologically. In their most classic and frequently encountered forms, FNA can readily distinguish the two. However, in a subset of cases, both pleomorphic adenoma and adenoid cystic carcinoma can exhibit significant cytomorphologic variation and overlap microscopically. The differential diagnosis for these two lesions can encompass a variety of tumors. In view of this, both lesions will also be discussed for comparative purposes in other chapters, particularly with regard to basaloid tumors (Chapter 7) and spindle cell lesions (Chapter 11).

The term “pleomorphic” refers to the tumor’s admixture of epithelial and mesenchymal elements, and it is believed to be derived from a pluripotent reserve cell of the intercalated duct. A broad age range of individuals is affected – from children to adults. In adults, women are more commonly affected, and the mean age at presentation is approximately 46 years (4th to 6th decade). Patients with pleomorphic adenoma usually present with a painless, slow-growing mass which on examination is solitary, well-defined, firm, and somewhat mobile. In many instances, the tumor has been present for several years and can attain large size, making surgical excision more difficult. Clinical problems in the management of these tumors include recurrent pleomorphic adenoma, malignant transformation into carcinoma ex pleomorphic adenoma, and the rare metastasizing pleomorphic adenoma. Even more uncommon is the development of carcinosarcoma ex pleomorphic adenoma (aka true malignant mixed tumor).