Pleomorphic adenomas constitute the most common tumor of the parotid glands in humans, representing up to 75% of all salivary gland neoplasms. Biologically, these are benign tumors that show a wide cytomorphologic and architectural diversity. These tumors are most commonly found in the superficial lobe of the parotid glands. Within the minor salivary glands, the site of occurrence is usually in the palate or the lips or the cheeks. Other rare sites include the throat, floor of the mouth, tongue, tonsil, pharynx, retromolar area and nasal cavity. Pleomorphic adenomas are chronic, slow-growing, painless masses, although some cases with rapid growth have also been reported. Individuals with the tumor in their parotid glands may show signs of facial nerve weakness. With minor salivary glands being affected, patients may present with a range of symptoms, including dysphagia, dyspnea, hoarseness, and epistaxis.
Histologically, the tumors in pleomorphic adenomas are composed of a variety of tissues consisting of epithelial cells arranged in a cord like cell pattern. Along with these are areas of squamous differentiation or with plasmacytoid appearance. Pleomorphic adenomas are most common in the third to sixth decade of life, and are found more often in females. The preferred form of management is surgical resection of the tumors. Most patients have an excellent prognosis after resection. However, although the tumor is considered benign, in about 2-7% of the cases it has the potential to transform into a malignant form known as carcinoma ex-pleomorphic adenoma that invades adjacent tissues and even metastasizes distantly. The tumor may also show a tendency to recur. Exposure to radiation, 15-20 years previously, has been implicated in the development of pleomorphic adenomas. A study has also suggested a role for Simian Virus 40 in the development of these tumors.
Most cases of pleomorphic adenomas show chromosome abnormalities with aberrations involving 8q12 and 12q15. In about 39% of cases, these aberrations involve rearrangement of chromosome band 8q12, which includes the Pleomorphic Adenoma Gene 1 (PLAG1). Rearrangement of chromosome band 12q13-15 is seen in about 8% of the cases. This region includes the high mobility group AT-hook 2 gene (HMGA2). Studies have also indicated over-expression of the Mucin 1 (MUC1) gene in cases where malignant transformation of pleomophic adenomas occurred.