Newly diagnosed DLCL patients will have baselne FDG-PET as part of initial staging (PET-1). Patients will then undergo 2 cycles of chemotherapy with R/CHOP. 14 to 21 days following cycle 2 of R/CHOP, patients will undergo repeat FDG-PET scan (PET-2). They will then complete therapy as planned. Following completion of therapy, standard response assessment will be performed, including CT scans of the chest, abdomen and pelvis and FDG-PET scan (PET-3).