Salivary Gland Neoplasms: Mucoepidermoid Carcinoma

Mucoepidermoid carcinoma is the most common salivary gland malignancy and makes up between 5 and 9% of all salivary gland neoplasms. It develops most commonly in the major salivary glands, most often the parotid (45-70%). The second most common site of occurrence is the palate (18%). This tumor displays a uniform age distribution between the ages of 20 and 70 years, with a slight peak in occurrence in the 5^th decade. Although it is rare before age 20, it is the most common salivary gland malignancy in the pediatric and adolescent populations. Mucoepidermoid carcinoma occurs more frequently in women than in men and in Caucasians than in African Americans.

The clinical presentation of a salivary gland malignancy can be very similar to that of a benign lesion. Often the only complaint is the presence of an enlarging but asymptomatic mass. Occasionally patients will report a rapid enlargement of a previously stable mass. Symptoms such as pain, fixation to the surrounding tissues or skin or facial paralysis are uncommon and should increase suspicion for a high-grade tumor. A mucoepidermoid carcinoma occurring in an intraoral minor salivary gland will often be mistaken for a benign or inflammatory process. It is not unusual for them to appear as a bluish or red-purple, soft and smooth growth. Others may present with a papillomatous appearance or as a hard submucosal mass, identical to a torus.

On gross inspection, some mucoepidermoid carcinomas appear well-circumscribed and may be partially encapsulated. Others are poorly defined and infiltrative. The cut surface of the tumor may contain solid areas, cystic areas or both. The cystic spaces contain viscous or mucoid material.

Microscopically, these tumors are characterized by the presence of two populations of cells—the mucus cells and the epidermoid cells, the proportion of which helps to define the grade of the tumor. Low-grade mucoepidermoid carcinoma is characterized by prominent cystic structures and mature cellular elements. This tumor contains proportionally more mucus cells, which may form gland-like structures, and fewer epidermoid cells. Intermediate-grade tumors display fewer and smaller cysts and occasional solid islands of epidermoid tumor cells. Although mucus cells are still present, there is an increasing proportion of epidermoid cells and occasional keratin pearl formation. The high-grade carcinomas are hypercellular, solid tumors with noticeable cellular atypia and frequent mitotic figures. These tumors will often be mistaken for a squamous cell carcinoma and the differentiation between the two can be quite difficult. Positive immunohistochemical staining for mucin indicates a high-grade mucoepidermoid carcinoma rather than a squamous cell carcinoma.

Appropriate therapy for mucoepidermoid carcinoma depends primarily upon the stage of disease, but is also influenced by tumor grade and location. Stage I and II disease can often be treated by surgical excision alone—parotidectomy with facial nerve preservation, submandibular gland excision or wide local excision of an involved minor salivary gland. Stage III and IV disease often require more radical excision and may warrant additional intervention such as a neck dissection or postoperative radiation therapy.

Category: Pathology Notes