Salivary gland neoplasms are remarkable for their histologic diversity. These neoplasms include benign and malignant tumors of epithelial, mesenchymal, and lymphoid origin. Salivary gland tumors pose a particular challenge to the surgical pathologist. Differentiating benign from malignant tumors may be difficult, primarily because of the complexity of the classification and the rarity of several entities, which may exhibit a broad spectrum of morphologic diversity in individual lesions. In some cases, hybrid lesions may be seen. The key guiding principle to establish the malignant nature of a salivary gland tumor is the demonstration of an infiltrative margin.
The following cellular classification scheme draws heavily from a scheme published by the Armed Forces Institute of Pathology (AFIP). Malignant nonepithelial neoplasms are included in the scheme because these neoplasms comprise a significant proportion of salivary gland neoplasms seen in the clinical setting. For completeness, malignant secondary tumors are also included in the scheme.
Where AFIP statistics regarding the incidence, or relative frequency, of particular histopathologies are cited, some bias may exist because of the AFIP methods of case accrual as a pathology reference service. When possible, other sources are cited for incidence data. Notwithstanding the AFIP data, the incidence of a particular histopathology has been found to vary considerably depending upon the study cited. This variability in reporting may be partially caused by the rare incidence of many salivary gland neoplasms.
The clinician should be aware that several benign epithelial salivary gland neoplasms have malignant counterparts, which are shown below:
- Pleomorphic adenoma (i.e., mixed tumor) (see carcinoma ex pleomorphic adenoma).
- Warthin tumor, also known as papillary cystadenoma lymphomatosum.
- Monomorphic adenomas:
- Basal cell adenoma (see basal cell adenocarcinoma).
- Canalicular adenoma.
- Oncocytoma (see oncocytic carcinoma).
- Sebaceous adenoma.
- Sebaceous lymphadenoma (see sebaceous lymphadenocarcinoma).
- Myoepithelioma (see myoepithelial carcinoma).
- Cystadenoma (see cystadenocarcinoma).
- Ductal papillomas.
Histologic grading of salivary gland carcinomas is important to determine the proper treatment approach, though it is not an independent indicator of the clinical course and must be considered in the context of the clinical stage. Clinical stage, particularly tumor size, may be the critical factor to determine the outcome of salivary gland cancer and may be more important than histologic grade. For example, stage I intermediate-grade or high-grade mucoepidermoid carcinomas can be successfully treated, whereas low-grade mucoepidermoid carcinomas that present as stage III disease may have a very aggressive clinical course.