Ph II CHOP+Velcade in Mediastinal LBCL

* This is a single arm phase 2 study of CHOP/Rituxan plus VELCADE followed by involved field radiotherapy designed to evaluate the complete response to chemotherapy as determined by PET scan following six cycles of therapy. One cycle equals 21 days. * For each cycle, on Day 1, the patient will receive VELCADE intravenously, followed by rituxan, and then followed by CHOP chemotherapy. Before receiving these drugs, the patient will be given standard medications (Tylenol, Benadryl) to help minimize side effects. They will also continue to take prednisone by mouth on days 2, 3, 4, and 5. On Day 4, the patient will receive another dose of VELCADE. * Before the beginning of every cycle of study treatment, the following will be performed: Medical history; physical examination; and routine blood tests. After the 3rd and 6th cycle of study treatment, the patient will have tests to monitor the status of their disease. These include PET scan, CT scans, and standard blood tests. * After 6 cycles of chemotherapy, approximately 3 weeks of radiation therapy will begin. One month after completing radiation therapy, the patient will return to the clinic for a physical exam and blood tests. One month later, the following evaluations will occur: PET and CT scans; medical history; physical exam; routine blood tests. * There will be follow-up visits every 3 months for two years after the study treatment is completed.