Rituximab Combined With Chemotherapy in Burkitt's Lymphoma

The purpose of this clinical trial is to prove the efficacy of the following new regimen treatment: * Administration of anti-CD20 (Rituximab) combined with chemotherapy. * Combined treatment with high doses of methotrexate and high doses of cytarabine with conventional cytostatics (block C) * Prophylactic administration of G-CSF after all chemotherapy cycles * local irradiation after 6 cycle if CNS was affected or if there are residual tumour