Warthin's Tumor Involved by Monoclonal B-Cell Lymphocytosis: A Case Report.

Warthin's tumor is a benign salivary gland neoplasm characterized by oncocytic epithelium and lymphoid stroma. We describe the incidental discovery of a CD5-positive monoclonal B-cell population within a classic Warthin's tumor of the parotid gland in a 72-year-old woman who presented with a painless, slow-growing left parotid mass. Core needle biopsy and subsequent parotidectomy demonstrated typical histologic features of Warthin's tumor along with a low-level (8%-9%) CD5- and CD23-positive B-cell population consistent with chronic lymphocytic leukemia (CLL)-type monoclonal B-cell lymphocytosis (MBL). No morphologic evidence of overt lymphoma was present, and both CLL fluorescence in situ hybridization (FISH) testing and PET-CT imaging were unremarkable. The patient remains asymptomatic and is currently monitored without therapy. This case illustrates the occurrence of CLL-type MBL within the lymphoid stroma of a Warthin's tumor, emphasizing the importance of careful histologic and immunophenotypic evaluation and the need for clearly defined diagnostic criteria for extranodal tissue-based MBL.