Lung and nodal hairy cell leukemia with concurrent infectious granulomatosis: a mimic of metastatic lung epithelial neoplasia.

Hairy cell leukemia (HCL) is a rare, indolent B-cell neoplasm, typically involving bone marrow, spleen, and peripheral blood, with extranodal sites rarely affected. Herein, we present the unique case of a 52-year-old man with lung and lymph node involvement by HCL concurrently with atypical mycobacteriosis. Initial imaging showed mediastinal lymphadenopathy and a pulmonary nodule, which raised suspicion for lung neoplasia. A minimally invasive biopsy of the mediastinal nodes and hilar lesion revealed a lymphoid proliferation mixed with necrotizing granulomatous inflammation, with an immunophenotype consistent with HCL and BRAF V600E mutation, confirmed by digital PCR. Notably, molecular analyses detected atypical mycobacteria in lymph nodes. This unusual co-occurrence of HCL with atypical mycobacterial infection in the lung and lymph nodes poses a complex diagnostic and therapeutic challenge, highlighting the importance of recognizing such presentations to optimize patient management.