Benign Tumors of Salivary Glands
Pleomorphic adenoma, also referred to as a mixed salivary gland tumor, is the most frequently seen benign salivary gland neoplasm, accounting for 60% of such tumors. The most common site of origin is the parotid, followed by the minor salivary gland and submandibular gland. It has a female predominance in adults and a male predominance in children. The tumors are indolent, slow-growing, typically asymptomatic lesions. Most often, a tumor begins in the lower portion of the gland and gradually enlarges to present as an ovoid or rounded well-circumscribed mass that is typically encapsulated. When expanding into the depth of the parenchyma, the tumor tends to be hard and lobulated, causing thinning of the overlying skin. Facial nerve involvement may result from direct infiltration or through external pressure on the neural tissue. The tumor is composed of ductal epithelial and myoepithelial cells with morphologic features of spindle, plasmacytoid, epithelioid, stellate, or basaloid cells residing most often in a mucochondroidal mesenchymal stroma. Recurrent lesions following surgical excision are typically multinodular.
Basal cell adenoma is a result of a proliferation of basaloid cells in a solid, tubular, trabecular, or membranous pattern. The tumors are typically solitary, asymptomatic, and slow growing and arise from the parotid gland. Lesions are well circumscribed and can grow to 3 cm in diameter. The recurrence rate following surgical excision for all but the membranous variant is quite low.
Papillary cystadenoma lymphomatosum, typically referred to as a Warthin tumor, is the second most common salivary gland neoplasm, occurring primarily in the parotid gland. Warthin tumors, unlike other salivary gland lesions, have a strong association with tobacco use. These lesions are most often seen in white men in the sixth or seventh decade. The tumor is frequently multilobular and bilateral, and like other salivary tumors, it is slow growing, reaching 4 cm in diameter. It consists of a double layer of oncocytic epithelium within a dense lymphoid stroma arranged in a papillary and cystic pattern. The lesions likely develop from salivary tissue intertwined with lymph nodes draining the parotid gland. The recurrence rate is up to 25% with surgical excision. Although malignant transformation is rare, squamous cell carcinoma or B-cell lymphoma may develop from the tumors.
Canalicular adenoma accounts for 1% of benign salivary gland tumors, with a preference for the minor salivary gland, specifically, the upper lip. It is most commonly seen in African American women in their seventies. The lesions are typically firm, slow growing, and solitary, reaching up to 2 cm in diameter. On gross inspection, the lesions are well-circumscribed, solid, or cystic pink/tan nonencapsulated masses. Histologically, the tumor consists of long, single-layered strands or tubules of cuboidal to short columnar cells within a loose, lightly collagenous stroma. Treatment is surgical excision of the lesion. Recurrence is rare.