Non-Hodgkin's lymphoma of the knee diagnosed by arthroscopy.

We report a case of non-Hodgkin's lymphoma of the knee diagnosed by arthroscopy in an otherwise healthy, active 31-year-old woman. Plain radiographs were unremarkable and magnetic resonance imaging findings indicated the presence of a posterior horn medial meniscal tear. The patient underwent arthroscopy for a planned partial medial meniscectomy. However, on joint visualization, hyperemic hypertrophic synovitis was found. Tissue samples were collected and sent for histopathologic testing that revealed diffuse large B cells. The patient was referred to the oncology department where 2 lesions on her lung were also found. Malignant disease should be in the mindset of the orthopaedic surgeon when a patient presents with constant pain, an unusual history, atypical response to conventional treatment, or a diagnostic dilemma.