A Study Comparing Rituximab/Bendamustin(RB) Alternating With Rituximab/Bendamustin/Cytarabin(RBAC) With RB Therapy in Elderly Patients With Mentle Cell Lymphoma

Inclusion Criteria: 1. newly diagnosed, previously untreated, histologically confirmed CD20+ mantle cell lymphoma, confirmed by WHO classification criteria 2. age ≥70 years or 60-69 years if the patients are ineligible for high-dose therapy with autologous stem cell transplantation. 3. ECOG performance status 2 or less 4. Adequate organ functions * adequate heart function: LVEF ≥50% by echocardiography or MUGA * adequate renal function: serum creatinine ≤ 2.0mg/dL or CrCl ≥40 mL/min based on the Cockcroft-Gault method * adequate hepatic function: ≤2.5 times the upper limit of ALT (≤5 times the upper limit of ALT if the elevation is attributed by underlying lymphoma) and ≤2 times the upper limit of ALT (≤3 times the upper limit of total bilirubin if the elevation is attributed by underlying lymphoma) * adequate hematologic function: absolute neutrophil counts (ANC) ≥ 1,500/mL, platelet counts ≥ 100,000/mL (any ANC and platelet counts are allowed, if they were related to bone marrow involvement) 5. Written informed consent Exclusion Criteria: 1. In-situ mantle cell lymphoma 2. Ann Arbor stage 1 disease 3. Prior treatment for Hodgkin lymphoma or non-Hodgkin lymphoma within the last 5 years. 4. Active malignancy within the past 3 years except for localized non-melanoma skin cancer, papillary thyroid cancer, cervical carcinoma in situ, breast cancer in situ, or localized prostate cancer that has been definitely treated, 5. Central nervous system involvement 6. HBsAg (+) or anti-HBc Ab (+) (patients will be eligible if they receive appropriate prophylactic antiviral therapy using entecavir, tenofovir, and so on) 7. History of prior hepatitis C infection (patients positive for HCV IgG will be eligible if they are negative for HCV-RNA) 8. Known history of human immunodeficiency virus (HIV) infection 9. any serious illness or medical conditions that are unstable or could jeopardize the safety of the patient and his/her compliance in the study * Congestive heart failure ≥ NYHA class 3 * Acute coronary syndrome within 6 months (unstable angina or new-onset angina, myocardial infarct, or ventricular arrhythmia) * History of significant neurological or psychological disorder including dementia and seizure disorder * Severe chronic obstructive pulmonary disease with hypoxemia * Cerebrovascular disease including transient ischemic attack within the past 6 months * Non-healing wound, ulcer, or bone fracture * Active uncontrolled bacterial, viral, or fungal infection requiring systemic therapy 10. concomitant administration of any other experimental drugs under investigation 11. Known hypersensitivity to bendamustine, rituximab, cytarabine, or mannitol 12. major surgical procedure or significant trauma within 28 days before start of study treatment, open biopsy within 7 days before start of study treatment 13. If the patient's partner is a woman who could possibly get pregnant, men who didn't have a vasectomy must agree to use medically recommended methods for adequate contraception (tubal ligation, intrauterine devices, or barriers \[diaphragm, cervical cap\] in the patient's partner and the use of condoms in men) when sexually active.