High CR Rate and Prolonged EFS in aaIPI 2 – 3 Diffuse Large B-Cell Lymphoma Following High-Dose Sequential Chemotherapy and In Vivo Rituximab-Purged Stem Cell Autografting (R-HDS Regimen): Results of a Prospective Phase II Multicenter Study Sponsored by GITIL (Gruppo Italiano Terapie Innovative nei Linfomi)