An unusual Case of p63/p40 Negative Myoepithelial cell-Rich Parotid Pleomorphic Adenoma.

Abstract

Pleomorphic adenoma is the most common benign salivary gland neoplasm; however, myoepithelial-rich variants with unusual immunophenotypes are rare and pose diagnostic challenges. We report a myoepithelial-rich pleomorphic adenoma of the parotid gland in a 23-year-old male presenting with a slowly enlarging, painless infraauricular swelling. Grossly, the lesion was well-circumscribed and encapsulated. Histopathology revealed predominantly spindle, oval, and plasmacytoid myoepithelial cells embedded in densely hyalinized fibromyxoid stroma, with sparse ductal structures. The paucity of ducts and prominent myoepithelial proliferation closely mimicked myoepithelioma. Immunohistochemically, tumor cells showed diffuse expression of S100, pan-cytokeratin, and high-molecular-weight cytokeratin, supporting epithelial and myoepithelial differentiation. EMA and CD117 selectively highlighted the sparse ductal structures, providing a crucial diagnostic clue. Notably, the tumor was completely negative for p40, p63, and smooth muscle actin, representing an unusual immunophenotype for pleomorphic adenoma. Despite the absence of conventional myoepithelial markers, integration of the subtle residual ductal morphology and complementary immunoprofile established the diagnosis of myoepithelial-rich pleomorphic adenoma, excluding myoepithelioma and other salivary gland neoplasms. The patient recovered uneventfully after surgery and remained recurrence-free at six months. This case expands the immunohistochemical spectrum of pleomorphic adenoma and highlights that the absence of p40 and p63 expression should not preclude its diagnosis. Comprehensive immunohistochemical assessment, interpreted alongside meticulous histomorphological evaluation, is essential to avoid diagnostic pitfalls in these uncommon tumors.