Patients with rheumatoid arthritis (RA) have an increased risk of developing malignant lymphomas, especially non-Hodgkin’s lymphoma (NHL). However, primary lymphoma in a joint is rare. Here we report a case of a 68-year-old man with a background of RA who presented with a 1-year history of pain and swelling in his right ankle. Initial imaging results comprised of X-ray and magnetic resonance imaging were inconclusive. Ultrasound scan of the right foot revealed a very large vascular mass with grossly eroded tarsal bones, and a biopsy confirmed the diagnosis of diffuse large B cell lymphoma. Involvement of lymphoma of ankle/foot joints is very rare: to our knowledge this is the first case of a primary diffuse large B cell lymphoma of the joints of the mid and hind foot with underlying bone destruction in a patient with a background of RA. This case is important because it highlights that malignancy should be suspected in every patient with a background of RA and unusual characteristics before assuming a diagnosis of flare of RA. This is important because early diagnosis of NHL can contribute to improved outcome.
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