Malignant Tumors of Salivary Glands
Although most tumors of the parotid gland are benign, approximately 20% are malignant, most often primary malignancies and less often malignant transformations of mixed tumors, adenomas, and cysts.
Adenocarcinoma is rare in the oral cavity but is a primary tumor of the major salivary glands, particularly the parotid. In the oral cavity, it develops from embryonic epithelium associated with small mucous glands, arising chiefly in the palate. It begins as a deep-seated nodule beneath the mucosa, which may break through the surface and ulcerate at a late stage. Adenocarcinoma accounts for almost 20% of all salivary gland carcinomas, with up to 60% affecting the major glands, primarily the parotid.
Salivary duct carcinoma is an aggressive adenocarcinoma representing 9% of malignant salivary tumors that almost universally affect the major glands, particularly the parotid. Unlike most salivary gland tumors, there is a marked male predominance, with a wide age range from young adults to the elderly. As pictured here, it presents as a hard, nonencapsulated swelling, beginning in the uppermost part of the gland or in the retromandibular lobe and growing rapidly to distend the face. The intensity of pain may be severe from pressure on the sensory nerves. The adjacent tissues are infiltrated, and the mass appears, on palpation, to be implanted on the ramus of the mandible, with fixation of the overlying skin. Histologically there may be a reproduction of ducts or acini, or a composition of strands and groups of mucus-producing cells enclosing lumenlike structures (cylindroma) embedded in a mucoid or hyaline degenerative stroma. The tumor is solid, is white-gray or tan, and contains cystic, necrotic, and hemorrhagic components with suggestions of a preexisting pleomorphic adenoma. It presents as a rapidly growing mass with ulceration and frequent facial nerve paralysis, with associated regional lymph node and distant metastatic spread to lung and bone. As the most aggressive salivary gland tumor, the 5-year survival rate is less than 30%.
Polymorphous low-grade adenocarcinoma is almost exclusively seen in the minor glands and accounts for about 20% of intraoral malignant salivary tumors primarily involving the palate. There is a female predominance occurring most commonly in the sixth to eighth decade. Lobular, solid-nest and cribriform and ductlike architectural patterns are seen with the tumor. The prognosis is excellent, with a low risk of recurrence following surgical excision.
Carcinoma ex pleomorphic adenocarcinoma accounts for 4% of all salivary tumors and 12% of salivary malignancies. It is seen in the minor, major, and submandibular glands, with a predilection for the parotid. There is an equal gender distribution, and it occurs primarily in the sixth to seventh decade. The clinical presentation is of a long-standing mass, with rapid enlargement or recurrence of a previously resected pleomorphic adenoma. The tumors are of variable size but can become larger than 20 cm. The prognosis depends on the aggressiveness of the tumor grade.
Mucoepidermoid carcinoma is the most common salivary gland tumor (12% to 29% of tumors), with the majority occurring in the major gland, particularly the parotid. There is a female predominance, with most occurring in the fifth decade. Intraoral lesions are typically bluish red and fluctuant, mimicking a mucocele or vascular lesion. In the parotid, the lesions are typically circumscribed encapsulated cysts containing brown or gray mucous fluid and are lined by intermediate, mucous, or epidermoid cells. With increasing size there is a risk of rupture that can allow tumor cell contamination into surrounding structures. Low-grade tumors typically are cystic and have abundant mucocytes, minimal atypia, or low mitotic activity; the prognosis is excellent following surgical resection. High-grade tumors are much less common and show cytologic atypia, high mitotic activity, necrosis, and neural invasion. Despite surgical intervention, the mortality rate remains about 50%.
Acinic cell carcinoma accounts for about 6% of salivary gland tumors, with most occurring in the parotid gland.
It is a malignant epithelial tumor with serous acinar differentiation containing cytoplasmic zymogen secretory granules. Female predominance is seen, with a wide age spread from the twenties to the seventies. This is the second most common salivary gland malignancy in children (mucoepidermoid tumors are the most common). Its growth is variable and may take weeks to decades, with most tumors being slow growing. The tumor typically presents as a rubbery firm, circumscribed, oval, or round mass that is either mobile or fixed. Recurrences following surgical excision are seen in about a third of individuals.