What is a Pleomorphic Adenoma?
Pleomorphic adenoma is the most common benign salivary gland tumor, characterized by a mixed composition of epithelial, myoepithelial, and mesenchymal components enclosed by a fibrous capsule. 1, 2
Epidemiology and Clinical Presentation
- Occurs most frequently in the 4th through 6th decades of life, with a female predominance (60% female, 40% male) 2
- The parotid gland is the primary site in 70-76% of cases, followed by the submandibular gland (13-17%), minor salivary glands of the palate (7%), and rarely the lacrimal gland, lips, or nasal cavity 1, 2
- Presents as a slowly progressive, painless swelling in 85% of cases, typically well-delineated and not involving the facial nerve 1, 2
- When parotid tumors occur, 73% develop on the right side 1
Histopathological Features
- Contains three distinct components: epithelial cells, myoepithelial cells, and mesenchymal stroma (myxochondroid matrix), separated from surrounding tissue by a fibrous capsule 2
- Exhibits remarkable cytomorphological and architectural diversity, which gives the tumor its "pleomorphic" designation 3, 4
- Signal intensity on imaging varies according to cellular density, proportion of epithelial versus stromal components, and stromal type 4
Diagnostic Approach
- Fine-needle aspiration biopsy (FNAB) using the Milan System for Reporting Salivary Gland Cytopathology demonstrates 98.5% sensitivity and 99% specificity for detecting malignant parotid tumors, helping distinguish PA from malignancy 5
- MRI with and without intravenous contrast is the preferred imaging modality when detailed surgical planning is required, showing suspicious features, or demonstrating growth 5
- On T2-weighted MRI, typical pleomorphic adenomas show marked hyperintensity reflecting abundant myxochondroid stroma, with a hypointense rim indicating the fibrous capsule 4
- CT with IV contrast serves as an alternative when MRI is contraindicated, though it provides inferior soft-tissue detail 5
Diagnostic Pitfalls and Challenging Features
Pseudoinvasive Features (22% of cases)
- Satellite nodules (most common at 43.5%), capsular penetration, irregular growth patterns, pseudopodia, lipomatous changes, and vascular permeation can mimic malignant invasion 3
Pseudomalignant Cytomorphology (93% of cases)
- Increased cellularity, cellular atypia, heightened proliferative activity, oncocytic metaplasia, and necrosis may suggest malignancy on histology 3
Mimicry of Defined Malignancies (29% of cases)
- Can resemble myoepithelial carcinoma, adenoid cystic carcinoma, and polymorphous adenocarcinoma, requiring meticulous examination with immunohistochemical and molecular analyses 3, 6
Secondary Histological Changes
- Fibrosis, lipometaplasia, ossification, cystic degeneration, and infarction occur rarely but complicate differential diagnosis 4
Immunohistochemical Profile
- Positive markers include: calponin, CD9, GFAP, Mcl-2, NM23, p63, S-100, smooth muscle actin (SMA), and SOX10 2
- PLAG1 (pleomorphic adenoma gene 1) is the diagnostic marker most frequently employed as it is specific for pleomorphic adenoma 2
Clinical Behavior and Malignant Potential
- Despite being histologically benign, pleomorphic adenoma has a propensity to recur if incompletely excised and can undergo malignant transformation to carcinoma ex-pleomorphic adenoma 1, 2
- Metastatic pleomorphic adenoma (MPA) is a rare paradoxical entity where histologically benign tumor spreads to regional and distant sites (bone 21.88%, cervical lymph nodes 20.31%), typically after multiple recurrences with an average time to metastasis of 198 months 7
- Among MPA patients with follow-up, 52.7% were alive and 8.4% died from disease 7
- Carcinoma ex-pleomorphic adenoma occurs in high-grade malignancies with over 20% rate of clinically positive nodal involvement at presentation 8
Treatment Principles
- Surgical excision with adequate margins is the preferred treatment 1
- For benign or low-grade tumors in the superficial lobe, partial superficial parotidectomy with facial nerve preservation is appropriate per ASCO guidelines 5
- Less extensive surgery (partial and superficial parotidectomies) has increased significantly from 1976 to 2022, while total parotidectomies have decreased 9
- Facial nerve preservation is recommended when preoperative function is intact and a dissection plane can be created between tumor and nerve 5
- Complete excision with adequate free margins is critical, as narrow margins (≤5mm) show excellent disease control in absence of adverse features for low/intermediate-grade tumors 5