We report the case of a patient affected by an extra-nodal non-Hodgkin lymphoma presenting as a unique, large retroperitoneal mass with an unusual clinical presentation mimicking gastric peptic or neoplastic disease. The patient was successfully treated with a first generation therapy, CHOP modified regimen (cyclophosphamide 600 mg/m2 intravenously on d 1, epirubicin 55 mg/m2 intravenously on d 1, vincristine 1.2 mg/m2 intravenously on d 1, prednisone 60 mg/m2 on d 1-5), and a complete response was achieved. The (18)F-fluorodeoxyglucose positron emission tomography was used to assess the therapy outcome. A brief review of literature is provided.