Benign Tumors of Minor Salivary Glands

Updated: Nov 28, 2017
  • Author: Vijay A Patel, MD; Chief Editor: John Geibel, MD, DSc, MSc, AGAF  more...
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Overview

Background

Benign salivary gland tumors represent a diverse group of neoplasms with varied clinical behaviors. Successful management of these neoplasms relies on accurate clinical and diagnostic assessment, as well as appropriate therapeutic intervention. A strong understanding of the biologic behavior of the various types of lesions allows the development of an appropriate treatment plan tailored to each individual patient affected.

Tumors of the salivary glands account for only 2-4% of head and neck neoplasms. Most (~70%) salivary gland tumors originate in the parotid gland, with the bulk of the remainder arising in the submandibular gland (~8%) and in the minor salivary glands (~22%). [1, 2] Although 75% of parotid gland tumors are benign, slightly more than 50% of tumors of the submandibular gland and 60-80% of minor salivary gland tumors are found to be malignant. [3]

Minor salivary gland tumors thus are relatively uncommon neoplastic entities. Palatal minor salivary glands are the most common anatomic subsite affected by these tumors; most series identify a nearly equal distribution between benign and malignant tumors. [4]

The ubiquitous deposition of the minor salivary glands complicates the diagnosis and management of salivary gland tumors. The approach to 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 [FNA] cytology) often provide useful diagnostic information prior to definitive surgical therapy.

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Anatomy

The minor salivary glands consist of 600-1000 smaller, unnamed glands distributed throughout the upper aerodigestive tract (ie, the palate, lip, pharynx, nasopharynx, larynx, and parapharyngeal space). The majority (70-90%) are located in the oral cavity and oropharynx, including the lateral tongue, lips, buccal mucosa, palate, and retromolar pad. The greatest glandular densities are located within the hard (250) and soft (150) palates. The remaining glands are located in the nose, paranasal sinuses, pharynx, and larynx.

The minor salivary glands are commonly classified according to their anatomic location—for example, labial glands (upper and lower lips), buccal glands, and so forth.

As compared with the major salivary glands, the minor salivary glands are more numerous, are reduced in volume with regard to tissue size, have an abbreviated duct system, and have a paucity of capsular tissue. Overall, they contribute about 8-10% of the volume of unstimulated and stimulated whole saliva.

Most benign salivary gland tumors (95%) occur in adults, with the clear majority pathologically identified as pleomorphic adenomas. In children, the most common benign tumors of mesenchymal origin are hemangiomas, and the most common benign epithelial tumors are pleomorphic adenomas. [5]

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Pathophysiology

Two major theories of salivary gland neoplastic pathogenesis have emerged: the multicellular theory and the bicellular (reserve cell) theory. [6]  The histogenesis of salivary gland tumors is based on the salivary gland unit.

According to the multicellular theory, the origins of these lesions are as follows:

  • Mixed tumors 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
  • Mucoepidermoid and squamous cell tumors originate from the excretory duct cells

According to the bicellular theory, basal cells of the excretory duct and intercalated duct are stem cells from which the mature salivary gland unit arises. Tumors arise from one of these two stem cell populations. Warthin tumors, mixed tumors, oncocytomas, acinic cell carcinomas, adenoid cystic carcinomas, and oncocytic carcinomas arise from intercalated duct stem cells. Squamous cell carcinomas and mucoepidermoid carcinomas arise from stem cells of the excretory duct. [5]

Histologic architecture of the salivary gland unit Histologic architecture of the salivary gland unit.
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Etiology

Etiologic factors for the development of benign salivary gland neoplasms are not well understood; however, both environmental and genetic factors have been proposed as potential causes.

Low-dose radiation has been implicated in the development of benign salivary gland tumors. The strongest evidence for this association is derived from analysis of the incidence of salivary gland neoplasms in atomic bomb survivors. [7]  Supporting evidence is also provided by analysis of the relative risk conferred by exposure to early therapeutic external beam irradiation.

Exposure to diagnostic dental radiographs has also been found to confer an increased risk for salivary gland neoplasms, but this may be attributable to the greater radiation exposure inherent in older technology as compared with the reduced exposure characteristic of current low-dose diagnostic radiographs. [8]

The latency period for the development of a radiation-induced salivary gland tumor appears to be in the range of 15-20 years.

Although the molecular genetic basis for the development of benign salivary gland neoplasms is not well delineated, some preliminary observations have nonetheless been made. Initial studies demonstrated a high incidence of allelic loss of chromosomal arm 12q in pleomorphic adenoma. [9]  Additional alterations, including cytogenetic abnormalities such as an 8q12 translocation involving PLAG1 (a zinc finger protein), have also been described. [10]

Finally, Epstein-Barr virus (EBV) may be a factor in the development of lymphoepithelial tumors of the salivary glands.

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Epidemiology

Between 60% and 80% of all minor salivary gland tumors are malignant. Overall, adenoid cystic carcinoma is the most common malignant tumor of all minor salivary glands. [11]  In a study of 485 cases of minor salivary gland tumors 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. [12]

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Prognosis

With appropriate treatment of benign salivary gland tumors, the outcome is excellent, and the recurrence rate is very low.

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