The parotid glands are divided by the facial nerve into superficial and deep lobes. 

• The facial nerve is not normally visible on imaging so radiologists use the retromandibular vein as the demarcation. 

• Deep lobe extends through the stylomandibular tunnel into the prestyloid parapharyngeal space.

• Parotid tail: inferior projection of the superficial lobe overlying the angle of the mandible.

• Parotid glands are only salivary glands to contain internal LNs, mostly located in the superficial lobe along the retromandibular vein.

• Accessory parotid glands may be located along the parotid duct (stensen duct) superficial to masseter mm.

AKA benign mixed tumor; MC salivary gland tumor accounting for 70% of all salivary tumors and is most commonly in the parotid gland.

• Typical patient is a middle-aged female

• Composed of a mixture of variable histology containing both epithelial and myoepithelial tissues. They appear encapsulated and well-circumscribed however the pseudocapsule is delicate and incomplete w microscopic extensions reaching beyond it, accounting for the high risk of recurrence when they tumors are enucleated. 

• Location: 

  • 84% parotid gland, MC in superficial lobe

  • 8% submandibular gland

  • 6.5% minor salivary glands (nasal cavity, pharynx, larynx, trachea)

  • Also found in lacrimal glands where they account for approximately 50% of lacrimal gland tumors.

IMAGING

• CT: 

  • When small, have homogeneous attenuation and prominent enhancement. 

  • When larger, they can be heterogeneous w/ less prominent enhancement, foci of necrosis and possible delayed enhancement. Small regions of calcification are common.

• MRI: 

  • Well-defined mass w bosselated margins (small undulations)

  • Very high T2 signal

  • Strong contrast enhancement; usually homogeneous

  • Large pleomorphic adenomas can have heterogenous signal

• DSA: hypovascular

• US: hypoechoic

Although benign, there is a risk of malignant transformation. Thus, standard tx is superficial parotidectomy.

Carcinoma ex-pleomorphic adenoma: proportional to the time the lesion is in situe (1.5% in first 5 years, 9.5% after 15 years). RF for transformation are advanced age, large size, radiation therapy, recurrent tumors.