R-CHOP Alone vs. R-CHOP Plus Radiotherapy for Localized CD20+ DLBCL

Inclusion Criteria: * Written informed consent * Histologically confirmed CD20-positive Diffuse Large B-cell Lymphoma (DLBCL), NOS * Immunostains for CD20, CD5, CD3, CD10, MUM-1, BCL-6, BCL-2 and Ki-67 and the in situ hybridization for Epstein-Barr virus are recommended for diagnosis of DLBCL, NOS. * No prior treatment for DLBCL * Stage I or contiguous II disease (Involvement of two adjacent lymph node regions or organ involvement with regional lymph nodes) * Performance status: ECOG 0-2. * Age ≥ 18 years * Cardiac ejection fraction ≥ 45% as measured by MUGA or 2D ECHO without clinically significant abnormalities * Adequate renal function: serum creatinine level \< 2 mg/dL * Adequate liver functions * Adequate hematological function: hemoglobin ≥ 9.0 g/dL absolute neutrophil count (ANC) ≥ 1,500/μL and platelet count ≥ 75,000/μL, unless abnormalities are due to bone marrow involvement by lymphoma * Life expectancy \>= 6 months * A negative serum or urine pregnancy test prior to treatment must be available both for pre menopausal women and for women who are \< 1 years after the onset of menopause. * Female patients of child bearing potential must use an effective method of birth control (i.e. hormonal contraceptive, intrauterine device,diaphragm with spermicide, condom with spermicide or abstinence) during treatment period and 1 month thereafter; Males must use an effective method of birth control during treatment period and 3 months thereafter. Exclusion Criteria: * Other subtypes NHL than CD20 (+) DLBCL, NOS * Transformed DLBCL from follicular lymphoma or other indolent lymphomas * Bulky disease ( longest diameter \>=10 cm) * Previous treatment for DLBCL with immunotherapy or chemotherapy except for short-term corticosteroids (duration of ≤ 8 days) before inclusion * CNS involvement by lymphoma or any evidence of spinal cord compression. * Primary Central Nervous System (CNS) DLBCL * Primary testicular lymphoma * Primary breast lymphoma * Patients with a known history of HIV seropositivity or HCV (+). (Patients who have HBV (+) are eligible. However, primary prophylaxis using antiviral agents (i.e. lamivudine, etc) is recommended for HBV carrier to prevent HBV reactivation during whole treatment period.) * Prior history of malignancies other than lymphoma (except for basal cell or squamous cell carcinoma of the skin, early gastric cancer or carcinoma in situ of the cervix or breast or untreated prostatic cancer without any plan for a treatment) unless the patient has been free of the disease for ≥ 3 years * Pregnant or lactating women * Men who are not surgically sterile and women of childbearing potential not employing adequate contraception * Other serious illness or medical conditions * Unstable cardiac disease despite treatment, myocardial infarction within 6 months prior to study entry * History of significant neurological or psychiatric disorders including dementia or seizures * Active uncontrolled infection (viral, bacterial or fungal infection) * Other serious medical illnesses * Known hypersensitivity to any of the study drugs or its ingredients * Concomitant administration of any other experimental drug under investigation, or concomitant chemotherapy, hormonal therapy, or immunotherapy.