Primary ALK-Negative Anaplastic Large Cell Lymphoma Mimicking Septic Arthritis in a Diabetic Patient: Diagnostic and...

Anaplastic lymphoma kinase (ALK)-negative anaplastic large cell lymphoma (ALCL) is a rare, aggressive subtype of peripheral T-cell lymphoma that frequently presents with atypical features, creating substantial diagnostic challenges. Primary ALK-negative ALCL arising from the knee joint is exceptionally uncommon. This report describes a patient who was initially misdiagnosed with septic arthritis of the knee due to localized pain, swelling, and erythema. The delayed diagnosis illustrates the difficulty of distinguishing hematologic malignancies from common infectious conditions, particularly in immunocompromised individuals. At the time of diagnosis, the disease was at an advanced stage IVB and complicated by septicemia, septic shock, and pneumonia, limiting tolerance for intensive chemotherapy. A carefully staged treatment plan was employed, beginning with corticosteroids and etoposide, followed by liposomal mitoxantrone and the CD30-targeted antibody-drug conjugate brentuximab vedotin. This multidisciplinary approach led to clinical stabilization, recovery from critical illness, and eventual discharge, with ongoing hematologic follow-up. The clinical course highlights the necessity of maintaining a high index of suspicion for lymphoma in refractory joint infections and demonstrates that individualized, stepwise therapy can achieve remission even in critically ill patients with aggressive ALCL. These findings provide valuable insights for improving the diagnosis and management of lymphoma cases that mimic infectious diseases.