Pulmonary Tuberculosis Mimicking Refractory Primary Pulmonary Diffuse Large B-Cell Lymphoma on 18F-FDG PET/CT: A Case Report and Diagnostic Pitfall

Primary pulmonary lymphoma is rare, and pulmonary tuberculosis (TB) can closely resemble lymphoma on 18F-fluorodeoxyglucose positron emission tomography/computed tomography (18F-FDG PET/CT). We report the case of a 90-year-old man with biopsy-proven primary pulmonary diffuse large B-cell lymphoma, germinal center B-cell type. Baseline PET/CT demonstrated hypermetabolic pulmonary nodules with a maximum standardized uptake value of 6.4. After six cycles of R-CHOP (rituximab, cyclophosphamide, doxorubicin, vincristine, and prednisolone), restaging PET/CT showed increased lesion size and FDG uptake, corresponding to a Deauville score of 5 and initially raising concern for refractory lymphoma. However, repeat biopsy of a metabolically active pulmonary lesion revealed necrotizing granulomatous inflammation with acid-fast bacilli, supporting active pulmonary TB. Anti-TB treatment was initiated. This case illustrates an important diagnostic pitfall: progressive FDG uptake after chemotherapy should not automatically be interpreted as treatment failure, particularly in TB-endemic settings or immunosuppressed patients. When imaging findings are discordant with the clinical context, repeat tissue confirmation should be strongly considered before escalating lymphoma therapy.