## Discriminating Feature: Myoepithelial Component **Key Point:** Pleomorphic adenoma is uniquely characterized by a prominent myoepithelial component admixed with ductal epithelium and mesenchymal (chondroid, myxoid, osseous) stroma. This mixed epithelial-mesenchymal architecture is pathognomonic. **High-Yield:** Mucoepidermoid carcinoma, by contrast, contains three cell types (mucous, intermediate, epidermoid) but lacks the stromal mesenchymal component and myoepithelial differentiation that defines pleomorphic adenoma. ### Comparative Histopathology | Feature | Pleomorphic Adenoma | Mucoepidermoid Carcinoma | |---------|---------------------|------------------------| | **Myoepithelial component** | Prominent, characteristic | Absent or minimal | | **Stromal element** | Abundant chondroid/myxoid/osseous | Fibrous, minimal mesenchymal | | **Cell types** | Mixed epithelial + mesenchymal | Mucous, intermediate, epidermoid | | **Growth pattern** | Well-circumscribed, pushing borders | Infiltrative, perineural invasion | | **Malignant potential** | Low (~2–5% carcinoma ex PA) | Intrinsically malignant | | **Clinical course** | Slow, painless, long history | Variable (grade-dependent); pain common | **Clinical Pearl:** The presence of chondroid or myxoid stroma with prominent myoepithelial cells around ducts is virtually diagnostic of pleomorphic adenoma and is absent in mucoepidermoid carcinoma, making this the single best discriminator. **Warning:** Do not confuse the three cell types of mucoepidermoid carcinoma (mucous, intermediate, epidermoid) with the epithelial-mesenchymal mixed pattern of pleomorphic adenoma — they are fundamentally different architectural principles. 
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