Chapter 37
Pleomorphic Adenoma (Benign Mixed Tumor)
Epidemiology
Pleomorphic adenoma is the most common salivary gland tumor and the most common tumor to arise within the parotid gland. These tumors constitute 60% of all parotid neoplasms and 75% of benign parotid tumors. Eighty-four percent of pleomorphic adenomas arise within the parotid gland, 8% occur in the submandibular gland, 0.5% occur in the sublingual gland, and 6.5% arise in the minor salivary glands of the respiratory and upper digestive tracts. Pleomorphic adenomas are more common in females. They may occur in any age group; however, they most often present between 40 and 50 years of age.
Clinical Findings
The typical history is a painless mass that may be stable or slowly growing. On physical examination, the mass is smooth, firm, and nontender. The facial nerve is usually functioning normally.
Pathology
On gross pathological examination, these tumors are encapsulated, smooth, round or oval masses. The terms “pleomorphic” and “benign mixed” refer to the histologic appearance of this neoplasm. The tumor consists of both epithelial and mesenchymal elements. The epithelial component consists of an inner layer of epithelial cells, whereas the outer layer consists of myoepithelial cells that constitute the mesenchymal component. Both elements must be present for the diagnosis of pleomorphic adenoma to be made. These tissue elements may be arranged in a variety of patterns and associated with a variable amount of stroma. The stroma can consist of variable amounts of mucoid, fibroid, chondroid, vascular, or myxochondroid elements. The mucoid stroma is thought to be responsible for the signal characteristics present on T2-weighted imaging.
There are several malignancies associated with pleomorphic adenomas. These include carcinoma-expleomorphic adenoma, carcinosarcoma, and metastasizing benign mixed tumor. These will be discussed in separate chapters. It is felt that the incidence of an associated malignancy is related to longstanding tumors and those that have had multiple recurrences.
Treatment
