Non-Hodgkin Lymphoma with Synovial Involvement of the Knee

Musculoskeletal involvement occurs in approximately 25% of patients diagnosed with non-Hodgkin lymphoma (NHL). Skeletal involvement is typically secondary, with primary lymphoma of the bone being much rarer. We describe a case in which a 52-year-old man initially presented features suggestive of Lyme arthritis in his left knee. A synovial biopsy performed as part of a synovectomy procedure revealed a proliferative synovium with dense lymphoplasmacytic B cell infiltrate, suggestive of NHL. An inguinal lymph node biopsy was also performed but did not produce results pathologically similar to those of the left knee synovial biopsy.