Salivary Gland Pathology
Core Concepts
Salivary neoplasms are organized by site and cell type. Benign: pleomorphic adenoma (most common, PAX3/PLAG1), Warthin tumor (parotid tail, smokers), oncocytoma, and basal cell adenoma. Malignant: mucoepidermoid carcinoma (MAML2 rearrangement), adenoid cystic carcinoma (MYB-NFIB), acinic cell carcinoma, salivary duct carcinoma, polymorphous adenocarcinoma, and carcinoma ex pleomorphic adenoma. Know the grading systems and the significance of perineural invasion in adenoid cystic carcinoma.
Common Traps
- Calling polymorphous adenocarcinoma a pleomorphic adenoma; PAC lacks chondromyxoid stroma and has uniform cytology with targetoid perineural growth.
- Missing carcinoma ex pleomorphic adenoma when a long-standing mass changes; look for a hyaline scar and malignant component.
- Underestimating adenoid cystic carcinoma because of bland cytology; perineural invasion and late metastases are typical.
- Confusing acinic cell carcinoma with normal acini; look for zymogen granules and DOG1 positivity.
Rule of Thumb
Use the "stroma and cytology" approach: abundant chondromyxoid stroma with ductal cells favors pleomorphic adenoma; cribriform architecture with basaloid cells favors adenoid cystic carcinoma; oncocytic cells with lymphoid stroma favor Warthin. A worked example: a parotid mass with cribriform nests and perineural invasion - diagnose adenoid cystic carcinoma and request MYB testing if morphology is ambiguous; report perineural invasion because it predicts recurrence.
High-Yield IHC
PLAG1 for pleomorphic adenoma, DOG1 for acinic cell carcinoma, and MYB for adenoid cystic carcinoma.