Mitoxantrone versus daunorubicin in induction - consolidation chemotherapy - the value of low-dose cytarabine for maintenance of remission, and an assessment of prognostic factors in acute myeloid leukemia in the elderly : final report of the leukemia cooperative group of the European Organization for the Research and Treatment of Cancer and the Dutch-Belgian Hemato-Oncology Cooperative Hovon Group randomized phase III study AML-9