Parotid tumors D.D

pleomorphic adenoma T1

warthin tumor,parotid gland

pleomorphic adenoma T2

Click on image to enlarge Hemangioma. Axial postcontrast CT image shows diffuse enhancement of a large neck mass that replaces the deep and superficial portions of the right parotid gland in an infant.

Parotid masses
Radiology Discussion:
Typical parotid masses can be separated into benign and malignant neoplasms. Benign masses include pleomorphic adenoma, Warthin’s tumor, oncocytoma, hemangioma, lipoma, schwannoma, and neurofibroma. Malignant masses include mucoepidermoid carcinoma, adenoid cystic carcinoma, squamous cell carcinoma, adenocarcinoma, acinic cell carcinoma,undifferentiated carcinoma and carcinoma ex pleomorphic adenoma.

Epithelial-myoepithelial carcinoma is extremely rare and is rarely included on the differential diagnosis. Other solid lesions include enlarged lymph nodes, lymphoma (primarily non-Hodgkins), lymphoepithelial lesions (cystic or solid, associated with HIV), and sarcoidosis (characterized by bilaterally enlarged glands and multifocal nodules).

Mass characteristics cannot distinguish histology or benign vs malignant because margins, architecture, CT density, T1W & T2W signal overlap. The role of radiology in diagnosis and treatment is to define location, e.g., superficial or deep parotid lobe. The facial nerve and retromandibular vein are the landmarks typically used to distinguish the location. This is important in helping the surgeons plan the resection and will help determine whether or not the facial nerve can be spared.

Pleomorphic adenoma is the most common benign tumor of salivary glands. They occur most often in middle aged patients and are more common among women than men. The typical locations are the tail or superficial lobe, although pleomorphic adenoma can appear in the deep lobe. These tumors tend to be well defined, solid, and round but may contain cystic degeneration and/or calcifications. Pleomorphic adenoma will undergo malignant transformation in a small number of cases, most commonly developing into adenocarcinoma. On CT the tumor is isodense to muscle and shows moderate enhancement. On MR, the mass is T1 hypointense (T2 hyperintense) to surrounding parotid fat and shows moderate enhancement. Pleomorphic adenoma follows the rule of 80’s:

80% of parotid tumors are benign
80% of parotid tumors are pleomorphic adenomas
80% of salivary gland pleomorphic adenomas occur in parotid
80% of parotid pleomorphic adenomas occur in the superficial lobe
80% of untreated pleomorphic adenomas remain benign

Warthin’s tumor (cystadenoma lymphomatosum) is the second most common benign tumor of parotid. It most often occurs around age 50 and is much more common among men than women. The tumor is bilateral in 10% of cases and favors the tail of the parotid gland. These masses may be cystic and do not undergo malignant transformation. On MR imaging, a Warthin’s tumor is T1 hypointense to the surrounding parotid fat but heterogeneous and variable on T2. Increased uptake of pertechnetate is helpful for equivocal FNA.

Oncocytoma is similar in appearance to pleomorphic adenoma and takes up TcO4-. Hemangioma is the most common salivary gland neoplasm in children. It is T2 hyperintense and enhances avidly on MR; it may or may not demonstrate phleboliths on CT. Lipoma is iso-dense and iso-intense to fat on imaging. Schwannoma and neurofibroma tend to occur along the facial nerve.

Mucoepidermoid carcinoma is the most common malignant tumor of the parotid gland. This tumor represents 30% of all salivary gland malignancies; 60% of them occur in the parotid. On CT, the mass is isodense to muscle. On MR, it is T1 hypointense to surrounding parotid fat but variable on T2; high-grade tumors are hypointense, while low-grade tumors may be hypointense on T2.

Adenoid cystic carcinoma is the second most common malignant tumor of parotid (and the most common in submandibular, sublingual and minor salivary glands). This slow-growing tumor has a propensity for perineural spread. On CT, it is isodense to muscle. On MR, it is T1 hypointense to surrounding parotid fat but has a variable appearance on T2-weighted images.
Squamous cell carcinoma is derived from metaplasia of ductal epithelium or intraparotid lymph node involvement of extra-parotid SCC. Adenocarcinoma typically arises from the glandular tissue of the parotid. Acinic cell carcinoma is the most common multifocal parotid malignancy. Undifferentiated carcinoma is very rare and is associated with a poor prognosis. Carcinoma ex pleomorphic adenoma is a pleomorphic adenoma that has undergone malignant transformation, typically to adenocarcinoma.

Epithelial-myoepithelial carcinoma is a rare, low-grade tumor occurring mainly in the parotid gland. It develops most often in elderly patients. The slow-growing mass is well defined, bulky and bosselated. The appellation "epi-myo-epi" refers to the distinctive epithelial tubules or ductules surrounded by neoplastic myoepithelial cells. On ultrasound, the mass is hypoechoic with a mildly heterogeneous echotexture. On CT, it is isodense to muscle and demonstrates moderate homogeneous enhancement with contrast. On MR, it is hypointense on T1 and displays dense, homogenous enhancement. The appearance on T2-weighted images is variable.