Parotid Tumors

1

Describe the location and characteristics of the parotid gland

The paired parotid glands are the largest of the three major salivary glands. The parotid gland is triangular in shape bounded superiorly by the zygomatic arch; posteriorly by the external auditory canal; inferiorly by the styloid process, the styloid muscle, and the jugular and internal carotid vessels; and anteriorly by the masseter muscle. The tail of the parotid gland may extend inferior-posterior to the level of the sternocleidomastoid muscle and mastoid process. Its main histologic feature consists of clusters of acinar cells that are mainly serous secreting.

2

What is the salivary gland unit?

The parotid salivary gland units consist of the acinar cells of the parotid gland and a transport system. The transport system consists of the following: the intercalated ducts, the striated ducts, and the excretory ducts. These ducts are all interconnecting, emptying into the oral cavity via Stenson’s duct. Contractile myoepithelial cells surrounding both the acinous cell units and the intercalated ducts force the watery secretions through the duct system into the oral cavity.

3

What is the relationship of the facial nerve to the parotid gland?

The facial nerve divides the parotid gland into a superficial and deep lobe. The facial nerve enters the temporal bone at the anterior superior portion of the internal auditory canal. The nerve then travels through the mastoid bone in the fallopian canal exiting the skull base at the stylomastoid foramen. The nerve lies lateral to the styloid process and posterior belly of the digastric muscle and medial to the mastoid tip. As the nerve exits the stylomastoid foramen, it immediately gives off three motor branches: one to the stylohyoid muscle, one to the posterior belly of the digastric muscle, and the third to the three postauricular muscles of the pinna. The nerve then proceeds anteriorly for a short distance and at the pes anserinus divides into two major divisions—the temporofacial and cervicofacial divisions. After dividing, the nerve will then turn laterally to enter the posterior aspect of the parotid gland. The temporofacial division subsequently divides into the temporal, zygomatic, and buccal branches. The cervicofacial division divides into the marginal mandibular and cervical branches. The deep parotid lobe lies between the temporofacial and cervicofacial divisions. There are numerous variations in the division of the nerve, and careful identification of each division and branch must occur to avoid injury to the nerve.

4

What branches of the facial nerve are at major risk of injury during parotid gland surgery?

The temporal and marginal mandibular branches of the facial nerve are at major risk of injury because of their small size and lack of anastomotic connections. Careful identification and dissection is extremely important.

5

What is the significance of the salivary gland unit in tumor development?

There are currently two theories of tumor development based on the salivary gland unit.

• a.

  Bicellular theory: Tumors arise from stem cells. The intercalated duct reserve cells give rise to the pleomorphic adenoma, oncocytomas, adenoid cystic carcinomas, adenocarcinomas, and acinic cell carcinomas. The excretory duct reserve cells will give rise to squamous cell and mucoepidermoid carcinomas.

• b.

  Multicellular theory: Each tumor type is associated with a specific differentiated cell of origin within the salivary gland unit. Therefore, excretory duct cells give rise to squamous cell carcinomas, intercalated duct cells give rise to pleomorphic adenomas, striated ducts give rise to oncocytomas, and acinar cells give rise to acinic cell carcinomas.

6

What are the four most common benign tumors of the salivary gland origin and their characteristics?

• a.

  The pleomorphic adenoma (mixed tumor) accounts for approximately 80% of all benign parotid tumors. They are slow growing and are not well encapsulated. The recurrence rate is 1%–5% with appropriate excision. Malignant degeneration may occur in approximately 2%–10% of cases.

• b.

  Warthin’s tumor (papillary cystadenoma lymphomatosum or adenolymphoma). This tumor occurs later in life. It is the second most common tumor representing approximately 5% of all benign tumors. There is a male predominance. Approximately 12% of Warthin’s tumors occur bilaterally.

• c.

  Oncytoma occur in the sixth decade of life and are composed of large oxyphilic cells. Oncocytes found in these tumors and Warthin’s tumors are responsible for concentration of technetium 99m pertechnetate.

• d.

  Monomorphic adenoma includes the following: Basal cell adenoma, clear cell adenoma, glycogen-rich adenoma. The most common of the three is the basal cell adenoma. These tumors are well circumscribed and encapsulated.

7

What is the treatment for benign tumors of the parotid gland?

The treatment is a superficial parotidectomy with preservation of the facial nerve. After the gland is removed and sent to the frozen section lab, it should be properly oriented and tagged for the pathologist. If there is a close margin, then patients should be observed for recurrence, particularly in the case of a pleomorphic adenoma.

8

Describe the five most common malignant parotid tumors and their characteristics

• a.

  Mucoepidermoid carcinoma is the most common malignant tumor of the parotid gland, accounting for 30% of all parotid malignancies. They are classified as either low-grade or high-grade malignancies. The low-grade form has a higher ratio of mucous cells to epidermoid cells and behaves like benign tumors. In the case of high-grade tumors, there is a higher portion of epidermoid cells resembling a squamous cell carcinoma. The latter have a high propensity for metastasis.

• b.

  Adenocarcinomas represent approximately 15% of parotid gland tumors. The neoplasms present as firm or hard masses attached to the surrounding tissue. Adenocarcinomas lack keratin and therefore are easily differentiated from mucoepidermoid carcinomas.

• c.

  Adenoid cystic carcinoma (cylindromas) account for 6% of all salivary gland neoplasms. It is the most common malignancy of the submandibular and minor salivary glands. Adenoid cystic carcinomas are unpredictable and may remain quiescent for years. These tumors grow along perineural planes and have a high incidence of distant metastasis, particularly to the lungs. There are three histologic types—cribriform, solid, and tubular. The solid form has the worst prognosis, and the cribriform is considered the most benign of the group.

• d.

  Malignant mixed tumors (carcinoma expleomorphic adenoma) are believed to develop from a preexisting pleomorphic adenoma. It appears to represent 2%–5% of parotid malignancies.

• e.

  Lymphomas of the parotid gland most commonly occur in elderly males. They account for 0.6%–5% of parotid tumors. The entire parotid gland is enlarged as well as regional lymph nodes. Fine-needle aspiration (FNA) with flow cytometry may assist in diagnosing this condition because the treatment consists of chemotherapy followed by radiation therapy.