Hairy Cell Leukaemia with an Uncommon Pulmonary Presentation: A Case Report and Review of the Literature.

Introduction: Hairy cell leukaemia (HCL) is a rare B-cell neoplasm and pulmonary involvement in HCL is exceedingly rare, often leading to diagnostic challenges. Non-resolving pulmonary infiltrates have a broad differential, including infections, malignancies, and autoimmune diseases, requiring a multidisciplinary approach for diagnosis. Case report: We report the case of a 61-year-old man who presented with fever, dyspnoea, weight loss, and bicytopenia. Initial chest imaging showed bilateral lower lung opacities, and empirical antibiotic therapy was administered. Despite treatment, symptoms persisted, prompting further investigations. High-resolution computed tomography scan revealed consolidations and ground-glass opacities. Extensive infectious workup was negative. Bronchoscopy with endobronchial ultrasound biopsy showed organizing pneumonia, and a bone marrow biopsy confirmed HCL with pulmonary involvement. The patient declined a surgical lung biopsy, but his pulmonary infiltrates and bicytopenia improved with corticosteroid and cladribine chemotherapy. Conclusion: This case highlights the importance of considering haematologic malignancies in non-resolving pulmonary infiltrates and persistent bicytopenia. A stepwise diagnostic approach, including bone marrow biopsy and bronchoscopy, was crucial in establishing the diagnosis. Successful treatment with cladribine reinforces its efficacy in HCL and suggests similar cases may benefit from this approach. Further research is needed to optimize diagnostic and treatment strategies for HCL with pulmonary involvement. LEARNING POINTS: Hairy cell leukaemia with initial pulmonary manifestation is very uncommon.The timing for bone marrow examination to persisted bicytopenia is uncertain.