Diffuse large B-cell lymphoma mimicking musculoskeletal nontuberculous mycobacterial infection: A fatal diagnostic challenge.

BACKGROUND: Musculoskeletal infections caused by nontuberculous mycobacteria (NTM) can clinically and radiologically mimic malignant disorders. Accurate diagnosis requires correlation between microbiological, histopathological, and clinical findings. We report a case of diffuse large B-cell lymphoma (DLBCL) initially misdiagnosed as NTM infection based on polymerase chain reaction (PCR) positivity despite negative cultures. CASE REPORT: A 58-year-old man with diabetes mellitus and prior coronary artery bypass grafting presented with a progressively enlarging painful right flank mass. Magnetic resonance imaging (MRI) showed an infiltrative lesion involving multiple abdominal wall muscles and extension to the L2 vertebral body. Initial bone and soft tissue biopsies revealed chronic inflammation with necrosis. NTM PCR was positive; however, acid-fast bacillus (AFB) staining and culture were negative. Empirical antimycobacterial therapy was initiated.After three months, the lesion progressed, and repeat biopsy confirmed Epstein-Barr virus (EBV)-positive diffuse large B-cell lymphoma, stage IV. The patient received six cycles of R-CHOP chemotherapy with an initial metabolic response on 18F-fluorodeoxyglucose positron emission tomography/computed tomography (FDG-PET/CT), followed by early relapse despite salvage chemotherapy and palliative radiotherapy. He ultimately died of refractory lymphoma. CONCLUSIONS: This case underscores the importance of re-evaluating suspected NTM infection with negative cultures when the clinical course is atypical or unresponsive to therapy. PCR positivity alone, in the absence of microbiological culture confirmation and compatible clinicopathological findings, should not be considered sufficient to establish a diagnosis of NTM infection. Diagnostic anchoring to molecular test results without clinicopathological reassessment may lead to critical delays and death.