Pleomorphic adenoma

Pleomorphic adenoma (also known as benign mixed tumor) is the most common tumor of the major salivary glands, and the most common benign tumor of the parotid glands (~75% of all benign salivary gland tumors). Prior head and neck irradiation is a risk factor. They are typically solitary, slowly growing, and asymptomatic. They are typically diagnosed after palpation by the patient or incidentally on imaging studies. Surgical resection is advised due to risk of malignant transformation.

On MRI, pleomorphic adenomas, have polylobulated margins a rim of low T2 signal corresponding to a fibrous capsule. They have heterogeneous low-to-intermediate signal intensity on T1-weighted images. High T2 signal and avid, solid enhancement are considered relatively specific features, especially when present in a younger patient (< 57 years). Dynamic contrast-enhanced (DCE) MRI shows gradual enhancement. On diffusion-weighted imaging (DWI), pleomorphic adenomas tend to have very high ADC values; however, DWI is not able to differentiate between benign and malignant parotid gland tumors.

On ultrasound, pleomorphic adenomas are typically hypoechogenic. They can have mild to moderate uptake on FDG PET.

Differential considerations include:

• Warthin tumor: 10–15% are bilateral. Can have proteinaceous cystic components with high T1 signal ranging from a few millimeters to 1–2 cm. Solid components have rapid enhancement and washout.
• Adenoid cystic carcinoma: Small, low-grade lesions can be mistaken for pleomorphic adenomas. Variable signal intensity on T2-weighted depending on type. Low-grade tumors have high T2 signal in the solid parts. Large tumors can have cystic areas of hemorrhagic necrosis.
• Myoepithelial adenoma: Also tend to have very high ADC values
• Basal cell adenoma: More commonly in the superficial lobe of the parotid gland. Tend to be round and well-circumscribed tumors. Have heterogeneous enhancement on CT.
• Carcinoma in pre-existing pleomorphic adenoma (carcinoma ex pleomorphic adenoma): Typically less well-circumscribed than benign pleomorphic adenoma. Tends to occur after 10–15 years of an existing pleomorphic adenoma, with sudden rapid growth (3–6 months) in patients in the sixth-to-eighth decades of life.
• Lymphoma: Tend to be multiple. Can have well-defined and lobulated margins. Tend to have low T2 signal and slight enhancement.
• Sarcoid: Can be placed in any differential, including this one.

References

• Christe A, Waldherr C, Hallett R, Zbaeren P, Thoeny H. MR imaging of parotid tumors: typical lesion characteristics in MR imaging improve discrimination between benign and malignant disease. AJNR Am J Neuroradiol. 2011 Aug;32(7):1202-7.
• Habermann CR, Arndt C, Graessner J, Diestel L, Petersen KU, Reitmeier F, Ussmueller JO, Adam G, Jaehne M. Diffusion-weighted echo-planar MR imaging of primary parotid gland tumors: is a prediction of different histologic subtypes possible? AJNR Am J Neuroradiol. 2009 Mar;30(3):591-6.
• Heaton CM, Chazen JL, van Zante A, Glastonbury CM, Kezirian EJ, Eisele DW. Pleomorphic adenoma of the major salivary glands: diagnostic utility of FNAB and MRI. Laryngoscope. 2013 Dec;123(12):3056-60.
• Kato H, Kanematsu M, Watanabe H, Kajita K, Mizuta K, Aoki M, Okuaki T. Perfusion imaging of parotid gland tumours: usefulness of arterial spin labeling for differentiating Warthin's tumours. Eur Radiol. 2015 Nov;25(11):3247-54.
• Kato H, Kanematsu M, Mizuta K, Ito Y, Hirose Y. Carcinoma ex pleomorphic adenoma of the parotid gland: radiologic-pathologic correlation with MR imaging including diffusion-weighted imaging. AJNR Am J Neuroradiol. 2008 May;29(5):865-7.
• Okahara M, Kiyosue H, Hori Y, Matsumoto A, Mori H, Yokoyama S. Parotid tumors: MR imaging with pathological correlation. Eur Radiol. 2003 Dec;13 Suppl 4:L25-33.