Orbital lymphoma of mucosa-associated lymphoid tissue in a patient with rheumatoid arthritis.

A 75-year-old male with rheumatoid arthritis complained of proptosis of the left eye. On examination, 6-mm left proptosis was seen. A hyperemic and swollen optic disc was visible in the left fundus. Laboratory test results showed positive rheumatoid factor. Computed tomography and magnetic resonance imaging showed a mass lesion in the left orbit. Histopathologic study of the excised specimen revealed centrocyte-like cells and lymphoplasmocytoid cells. Immunohistochemical study showed positive staining of CD 20 (B cells) and lambda light chain (immunoglobulin). The combination of rheumatoid arthritis and lymphoma of mucosa-associated lymphoid tissue in the orbit, as demonstrated in our patient, is rare.