Benign Tumors of Minor Salivary Glands

Updated: Jun 19, 2015
  • Author: Fadi Chahin, MD; Chief Editor: John Geibel, MD, DSc, MSc, AGAF  more...
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Overview

Background

Salivary gland surgery dates back to the 16th century. The anatomy of the parotid gland and the role of the main ducts were described in the mid-17th century. The earliest references to "para-auricular swellings," as the Greeks called them, described the findings associated with calculi and inflammation.

Between 1650 and 1750, salivary gland surgery was limited to the treatment of ranulas and oral calculi. The concept of surgical excision of a parotid tumor has been attributed to Bertrandi in 1802. The initial applications of this surgery included an extensive approach, causing serious disfiguration and disability.

In about the 1850s, the focus shifted toward dissection and the intimate relationship between the facial nerve and the parotid gland. Attempts were made to perform the surgery with nerve preservation. John C Warren, MD, was the first to use ether inhalation anesthesia during his resection of a parotid tumor in Boston in 1846. In 1892, Codreanu (a Romanian native) performed the first total parotidectomy with facial nerve preservation. Grafting of the facial nerve after resection was attempted in the early 1950s.

In 1958, Beahrs and Adson eloquently described the relevant anatomy and surgical technique of current parotid gland surgery. [1] They stressed surgical landmarks for avoiding injury to the facial nerve and advocated complete removal of the superficial portion of the parotid gland for noninvasive lesions confined to that portion of the gland. [1]

The glands are divided into major and minor salivary gland categories. The major salivary glands are the parotid, the submandibular, and the sublingual glands. The minor glands are dispersed throughout the upper aerodigestive submucosa (ie, palate, lip, pharynx, nasopharynx, larynx, parapharyngeal space).

Tumors of the salivary glands are uncommon and represent 2-4% of head and neck neoplasms. They may be broadly categorized into benign neoplasms, tumorlike conditions, and malignant neoplasms.

Most (70%) salivary gland tumors (SGTs) originate in the parotid gland. [2] The remaining tumors arise in the submandibular gland (8%) and in the minor salivary glands (22%). Although 75% of parotid gland tumors are benign, slightly more than 50% of tumors of the submandibular gland and 60-80% of minor SGTs are found to be malignant. [3]  Pleomorphic adenomas (benign mixed tumors) are the most common benign SGTs, comprising 85% of all salivary gland neoplasms.

The ubiquitous deposition of the minor salivary glands complicates the diagnosis and management of SGTs. The approach for a suspected tumor of the minor salivary glands begins with a thorough history and a physical examination. Radiographic imaging (computed tomography [CT] with or without magnetic resonance imaging [MRI]) and a histopathologic diagnosis (obtained via fine-needle aspiration biopsy [FNAB]) often provide useful information prior to definitive surgical therapy.

For patient education resources, see the Cancer Center, as well as Cancer of the Mouth and Throat.

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Anatomy

The parotid gland is situated in the musculoskeletal recess formed by portions of the temporal bone, atlas and mandible, and their related muscles. The gland has a superficial and deep lobe, between which runs the extratemporal portion of the facial nerve. The deep lobe is in contact with the parapharyngeal space. The deep cervical fascia surrounds the parotid gland. This fascia has an anteroinferior portion that becomes the stylomandibular ligament, separating the parotid gland from the submandibular gland.

The facial nerve exits the stylomastoid foramen just posterior to the base of the styloid, gives off small branches to the postauricular and posterior belly of the digastric muscles, and then turns anterolaterally. The main trunk then becomes embedded in parotid tissue and divides into temporofacial and cervicofacial branches just superficial to the retromandibular vein and external carotid artery. Beyond this point, the nerve anatomy varies some; however, five general peripheral nerve branches exist: frontal, zygomatic, buccal, marginal mandibular, and cervical. Surgical landmarks for the main trunk of the facial nerve include the tragal pointer and the tympanomastoid suture line.

The submandibular gland encompasses most of the submandibular or digastric triangle. Similar to the parotid gland, the submandibular gland can be divided into a superficial and deep lobe on the basis of the relation to the mylohyoid. The marginal mandibular branch of the facial nerve courses between the deep surface of the platysma and the superficial aspect of the fascia that lies over the submandibular gland. The facial artery and vein are located just deep to this nerve, and ligation and superior traction of these vascular structures can prevent nerve injury.

Along the posterior border of the mylohyoid are located the lingual nerve and submandibular duct (Wharton duct). The hypoglossal nerve courses deep to the tendon of the digastric and then lies medial to the deep cervical fascia.

The sublingual gland occupies the same anatomic space as the submandibular gland, located between the mylohyoid and the hyoglossus. The gland can often be palpated in the floor of mouth, as it is rather superficial, covered by only a thin layer of oral mucosa.

The minor salivary glands are widely dispersed throughout the upper respiratory tract, including the palate, lip, pharynx, nasopharynx, larynx, and parapharyngeal space. The greatest densities of glands are located in the hard (250 glands) and soft (150 glands) palates.

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Pathophysiology

The histogenesis of SGTs is based on the salivary gland unit (see the image below). According to the multicellular theory of SGTs, pleomorphic adenomas originate from the intercalated duct cells and myoepithelial cells; oncocytic tumors originate from the striated duct cells; acinic cell tumors originate from the acinar cells; and mucoepidermoid and squamous cell tumors originate from the excretory duct cells.

Histologic architecture of the salivary glands. Histologic architecture of the salivary glands.
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Etiology

Although the etiology of SGTs is unknown, the involvement of environmental or genetic factors has been suggested.

Radiation exposure has been linked to the development of the benign Warthin tumor and to the malignant mucoepidermoid carcinoma. Epstein-Barr virus may be a factor in the development of lymphoepithelial tumors of the salivary glands. Genetic alterations, such as allelic loss, monosomy and polysomy, and structural rearrangement, have all been studied as factors in the development of SGTs.

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Epidemiology

Tumors of the parotid gland are the most common SGTs and are five times more common than tumors of the minor salivary glands. The latter are almost twice as common as neoplasms that develop in the submandibular gland. The incidence of salivary gland neoplasms peaks in the fifth decade of life. The most common benign tumor is the benign mixed tumor, or pleomorphic adenoma.

Between 60% and 80% of all minor SGTs are malignant. Overall, adenoid cystic carcinoma is the most common malignant tumor of all minor salivary glands. The submandibular gland has a high incidence of malignant tumors (65% malignant vs 35% benign). [4]

In a study of 485 cases of minor SGTs from northeastern China, pleomorphic adenoma was found to be the most common type of benign tumor, and adenoid cystic carcinoma was the most common type of malignant tumor. [5] The sites most frequently affected were the palate (64.74%), the buccal mucosa (7.63%), and the tongue (5.98%).

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Prognosis

With the appropriate treatment of benign SGTs (ie, complete excision, superficial parotidectomy), the outcome is excellent and the recurrence rate is very low.

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