Gemcitabine in Treating Patients With Newly Diagnosed, Relapsed, or Chemotherapy-Resistant Mantle Cell Lymphoma

DISEASE CHARACTERISTICS: * Histologically confirmed mantle cell lymphoma meeting 1 of the following criteria: * Newly diagnosed * Chemotherapy resistant * Relapsed disease after no more than 2 prior lines of chemotherapy * Measurable disease * At least 1 unidimensionally measurable lesion ≥ 11 mm by CT scan * No prior or current CNS lymphoma or lymphomatous meningosis PATIENT CHARACTERISTICS: Age * 18 and over Performance status * WHO 0-1 Life expectancy * Not specified Hematopoietic * Neutrophil count ≥ 1,500/mm\^3 (1,000/mm\^3 in case of bone marrow infiltration) * Platelet count ≥ 100,000/mm\^3 (75,000/mm\^3 in case of bone marrow infiltration) Hepatic * Bilirubin ≤ 2 times upper limit of normal (ULN) * Alkaline phosphatase ≤ 2 times ULN * AST and ALT ≤ 2 times ULN * No active hepatitis Renal * Creatinine clearance ≥ 50 mL/min Cardiovascular * No congestive heart failure * No New York Heart Association class III or IV heart disease * No unstable angina pectoris * No significant cardiac arrhythmia or arrhythmia requiring chronic treatment * No myocardial infarction within the past 3 months Immunologic * No active autoimmune disease * No ongoing infection (e.g., HIV) Other * Not pregnant or nursing * Negative pregnancy test * Fertile patients must use effective contraception during and for 12 months after study participation * No other malignancy within the past 5 years except nonmelanoma skin cancer or adequately treated carcinoma in situ of the cervix * No uncontrolled diabetes mellitus * No gastric ulcers * No other uncontrolled medical condition that would preclude study participation PRIOR CONCURRENT THERAPY: Biologic therapy * No concurrent thalidomide Chemotherapy * See Disease Characteristics Endocrine therapy * Not specified Radiotherapy * Prior radiotherapy allowed provided indicator lesions were not in the irradiated field * No concurrent radiotherapy to the lungs or mediastinum Surgery * Not specified Other * More than 30 days since prior systemic anticancer treatment * More than 30 days since prior clinical trial participation * No other concurrent anticancer drugs * No other concurrent experimental drugs