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Pleomorphic adenomas (benign mixed tumors)

Pleomorphic adenomas are the most common salivary gland tumors, typically presenting as a smooth, painless, enlarging mass in middle-aged patients, with a 2:1 female predominance.

Key Images

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Look For First
  • Well-circumscribed, rounded mass with bosselated/polylobulated borders (many small undulations)
  • Parotid gland location, especially superficial lobe
  • Very high T2 signal intensity on MRI (especially myxoid type with rim of decreased T2 signal representing fibrous capsule)
  • Hypoechoic appearance on ultrasound with homogeneous enhancement on contrast-enhanced MRI
Key Image Findings
  • On ultrasound: typically hypoechoic with possible posterior acoustic enhancement and well-defined borders.
  • On CT: small tumors show homogeneous attenuation with prominent enhancement; larger tumors may be heterogeneous with foci of necrosis, less prominent enhancement, and possible delayed enhancement; small regions of calcification are common.
  • On MRI T1-weighted images: low signal intensity with homogeneous enhancement after gadolinium administration in small tumors.
  • On MRI T2-weighted images: characteristically very high signal intensity especially in myxoid type, with a rim of decreased signal intensity representing the surrounding fibrous capsule; larger tumors may show heterogeneous signal.
  • Rounded masses with well-defined, bosselated or polylobulated borders (many small undulations, not truly lobulated), distinguishing them from truly lobulated masses.
  • When arising from deep lobe of parotid: may appear entirely extraparotid in the prestyloid parapharyngeal space without fat plane to parotid and widening the stylomandibular tunnel.
  • Pleomorphic adenomas can arise from salivary rest cells in the parapharyngeal space itself without connection to the parotid gland.
  • On FDG-PET: may show variable uptake with SUV>3 in approximately 25% of cases.
Differential Diagnosis
  • Warthin tumor: typically presents with heterogeneous appearance and may be bilateral in up to 10% of cases, whereas pleomorphic adenomas are usually solitary and homogeneous when small.
  • Küttner tumor (chronic sialadenitis): shows diffuse enlargement of salivary gland with heterogeneous signal, contrasting with the well-circumscribed mass of pleomorphic adenoma.
  • Mucoepidermoid carcinoma: often shows more aggressive imaging features, may show infiltration or destruction, and typically occurs in younger patients.
  • Adenoid cystic carcinoma: may show perineural spread along facial nerve and tends to be more homogeneous and infiltrative compared to the well-circumscribed pleomorphic adenoma.
  • Myoepithelioma: generally smaller than pleomorphic adenoma and may show different imaging characteristics based on cellular composition.
  • Parotid nodal metastasis or non-Hodgkin lymphoma: typically multiple lesions with different imaging characteristics; nodes may have heterogeneous enhancement patterns.
Discussion

Pleomorphic adenomas account for 70-80% of benign salivary gland tumors and comprise the majority of parotid gland tumors (84% of all pleomorphic adenomas arise in parotid, with superficial lobe being more common).

These tumors are composed of epithelial and myoepithelial tissues with mixed histology; the pseudocapsule is delicate and incomplete with microscopic extensions, accounting for high recurrence risk (20-45%) with simple enucleation.

Three histological subtypes exist: myxoid (hypocellular, most common with highest recurrence risk), cellular, and classic; myxoid type characteristically shows very high T2 signal on MRI.

Imaging appearance often belies true pathology: despite appearing well-encapsulated on imaging, the poorly demarcated pseudocapsule can lead to significant recurrence rates if inadequate surgical margins obtained.

Modern surgical approach using partial or total parotidectomy with preservation of facial nerve has dramatically reduced recurrence rates to 1-4% compared to historical enucleation rates of 20-45%.

Malignant transformation to carcinoma ex-pleomorphic adenoma is uncommon (1-10%) but more likely in larger tumors and older patients; recurrent tumors and radiation therapy are additional risk factors for malignancy.

Reporting Pearls

Describe pleomorphic adenomas as a well-circumscribed, homogeneous mass with characteristic bosselated borders and very high T2 signal (with or without a T2 hypointense fibrous capsule rim), located in the superficial lobe of the parotid gland; note the absence of perineural spread or aggressive features to distinguish from malignant mimics.

Pitfalls
  • Assuming a well-circumscribed imaging appearance guarantees complete encapsulation: the pseudocapsule is delicate and incomplete with microscopic extensions, requiring adequate surgical margins to prevent recurrence.
  • Overestimating aggressiveness based on size or heterogeneity: larger tumors can show heterogeneous signal and necrosis but remain benign and are not indicative of malignant transformation without other concerning features.
  • Confusing myoepithelial origin with myxoid composition: myxoid type is a histological subtype (hypocellular) characterized by very high T2 signal and highest recurrence risk, distinct from myoepithelioma.
  • Neglecting to assess for deep lobe origin: pleomorphic adenomas of the deep lobe can appear entirely extraparotid in the parapharyngeal space, potentially leading to misclassification if parotid origin is not considered.
Source: Reference source

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