Pleomorphic Adenoma With Disrupted Biphasic Ductal Structures Attributed to Lymphoid Infiltration: A Case Report

Pleomorphic adenoma (PA) is a benign salivary gland tumor with diverse cytomorphological and architectural features, typically presenting biphasic ductal structures within a chondromyxoid matrix. We report a unique case of PA where dense lymphoid infiltration disrupted these structures, resulting in duct-like slit structures lined with a single layer of spindle cells, lacking the biphasic pattern. These spindle cells demonstrated myoepithelial nature, confirmed by positive immunostaining for pan-cytokeratin, S100 protein, and calponin. Fluorescence in situ hybridization analysis revealed HMGA2 rearrangement, supporting the diagnosis of PA. Unlike typical tumor-associated lymphoid proliferation (TALP), our case showed severe lymphocytic infiltration predominantly composed of CD8+ T cells, which eliminated mainly glandular epithelial cells. This case emphasizes highlights an unusual immune response in PA, broadening our understanding of its pathological spectrum.