The main purpose of this study is to begin to collect information and try to learn whether or not VELCADE, when added to standard chemotherapy with CHOP/Rituxan, works in treating patients mediastinal large B-cell lymphoma. Recent research has shown that this type of lymphoma shares features with Hodgkin's lymphoma, including the importance of a particular pathway in the tumor cells called the NF-kB pathway. VELCADE works in part by blocking this pathway.