Introduction
Background
The pleomorphic adenoma is by far the most common benign salivary gland tumor, accounting for as many as 80% of all such tumors. Although pleomorphic adenomas most commonly occur in the parotid gland (about 85%), this tumor may be encountered in the submandibular, lingual, and minor salivary glands as well. Although almost one half of tumors found in the minor salivary glands are malignant, the pleomorphic adenoma is still the most common tumor in these glands.
Pathophysiology
Pleomorphic adenomas, commonly called a benign mixed tumor, are histologically composed of 2 subtypes of cells: epithelial and mesenchymal. The tumors are typically well demarcated from the surrounding tissue by a fibrous capsule, which varies both in thickness and completeness.
These lesions have been reported to contain small protrusions (pseudopodia) that extend beyond the central mass, caused by variability in the growth rates of the various cell types. This factor contributes to recurrence rates as high as 50%, depending on the type of surgical intervention.
Hemorrhage, calcification, and necrosis are occasionally present. Most pleomorphic adenomas are confined to the superficial lobe of the parotid gland, but they can occasionally arise in the deep lobe in other salivary glands. Malignant degeneration, most commonly carcinoma ex pleomorphic adenoma, has been reported to occur in as many as 25% of untreated cases. In rare cases, pleomorphic adenomas degenerate into a true malignant mixed tumor and a metastasizing benign mixed tumor.
Frequency
United States
Pleomorphic adenomas account for as many as 80% of all benign salivary tumors.
Mortality/Morbidity
The morbidity and mortality of pleomorphic adenomas can be divided into 3 main categories: carcinoma, local recurrence after surgery, and facial nerve injury.
- The prevalence of carcinoma ex pleomorphic adenoma is of some debate, with reports of the prevalence in the range of 2-25%. The rate is certainly high enough that resection is typically warranted when the lesions are diagnosed.
- If the fibrous capsule can be completely removed, these tumors can be cured with surgery. Local recurrence can occur when portions of the capsule is left. The rate of successful cure after recurrence is less than 25%, and recurrence rates of up to 50% are reported with enucleation procedures. These tumors are now typically treated with partial parotidectomy. With appropriate surgery, the recurrence rate is 1-5%, and these recurrences may be due to capsular disruption during surgery.
- Facial nerve injury can occur during resection of these tumors, as the tumor may arise close to the nerve. One group reported that more than 50% of the tumors in their study contacted the facial nerve. The approximate risk of facial nerve injury is 1-2% during initial surgery, with an increased risk in operations on recurrent tumors.
Race
White persons have a slightly higher risk of pleomorphic adenomas than that of other races.
Sex
Women are predominantly affected, with a female-to-male ratio of 3:2.
Age
Pleomorphic adenoma typically appears in between the fourth and sixth decades of life and is rare in children. The average patient age at presentation is 43 years.
Anatomy
The parotid gland is the largest of the salivary glands. It arises as an epithelial invagination in the lining of the oral cavity at about 5 weeks' gestation. The parotid gland is located anteroinferior to the external acoustic meatus wedged between the ramus of the mandible anteriorly and the mastoid process posteriorly. Its apex is found posterior to the angle of the mandible, and its base is found slightly inferior to the zygomatic arch.
The gland is divided into a larger superficial lobe and smaller deep lobe by the facial nerve, which enters the posterior parotid, branches and then exits the gland anteriorly. Adjacent to but slightly deeper to the facial nerve is the retromandibular vein, followed by the external carotid artery, both of which have a number of branches within the gland. The retromandibular vein, which drains into the external jugular vein, roughly parallels the course of the facial nerve. Therefore, this vein can be used as guide to determine the location of the nerve on imaging studies.
The parotid gland itself is enclosed by a tough fascial capsule, the parotid sheath, which is derived from the deep cervical fascia. Innervation of the parotid gland includes both parasympathetic fibers from the glossopharyngeal nerve and sympathetic innervation via the external carotid nerve plexus.
Histologically, the gland is composed primarily of acinar cells, and upon stimulation, they produce a watery, serous solution, which is excreted by the parotid (Stensen) duct. This duct exits the anteromedial portion of the gland, crosses the masseter superficially, pierces the buccinator, and enters the oral cavity opposite the second maxillary molar. In addition to parotid tissue, parotid glands typically have lymph nodes that may or may not be visible on imaging.
Presentation
Parotid pleomorphic adenomas typically arise as a slow-growing, firm parotid mass that is slightly compressible. Almost all are asymptomatic, and they are usually brought to the attention of the physician when routine physical examination is performed or when the patient feels or sees a parotid mass. Typically, the signs and symptoms have been present for at least a year before patients seek medical attention.
Preferred Examination
CT or MRI depicts the mass, and the findings may be essentially diagnostic in routine cases with typical features. Some authors have reported excellent sensitivity with ultrasonography, though this study typically does not help the surgeon to understand the 3-dimensional (3D) relationship of the tumor to the parotid gland and the facial nerve.
Limitations of Techniques
MRI has the advantages of multiplanar imaging, and MRI results may suggest the tissue type on the basis of signal intensity characteristics. CT is often the first study ordered in patients with neck masses, and scans can show the mass and the retromandibular vein. Newer multisection CT scanners offer multiplanar capabilities rivaling that of MRI.
Disadvantages of CT include radiation exposure, the use of iodinated contrast material, and tissue distinction poorer than that of other studies. CT may also be problematic in cases of benign pleomorphic adenomas when the outer margin of the tumor appears indistinct and suggests malignant invasion of the surrounding tissue. MRIs show the well-defined outer borders in these instances. In many cases, however, CT may be the only study needed to guide the surgeon.
Differential Diagnoses
Other Problems to Be Considered
Warthin tumor
Salivary gland tumors (adenoid cystic carcinoma, mucoepidermoid carcinoma)
Sarcoidosis
Lymphoma
Lymphadenopathy
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References
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Further Reading
Keywords
benign mixed tumor
