Cure rates for primary mediastinal large B-cell lymphoma (PMBCL) have improved with the integration of rituximab. However, the type of primary therapy and role of radiotherapy (RT) remains ill-defined. Herein, we evaluated the outcome of PMBCL primarily treated with R-CHOP and the impact of an end-of-treatment (EOT) FDG-PET scan to guide consolidative RT. Patients ≥18 years of age with PMBCL treated with curative intent R-chemotherapy were identified. Prior to 2005, patients were recommended to receive R-CHOP +RT (RT era). Beginning in 2005, EOT PET was used to guide RT and only those with a PET-positive scan received RT (PET era). In total, 159 patients were identified, 94% were treated with R-CHOP and 44% received RT - 78% in RT era, 28% in PET era. The 5-year time to progression (TTP) and OS for the entire cohort were 80% and 89%, respectively, similar across treatment eras. Overall, 10% had refractory disease. In total, 113 patients had an EOT PET scan: 63% negative and 37% positive with a 5-year TTP of 90% vs 71% and 5-year OS of 97% vs 88%, respectively. For those with Deauville (D) scored PET scans (n=103), the 5-year TTP for PET-negative cases by Deauville criteria (D1-D3,DX) was 91%, with inferior outcomes for D5 vs D4 (5-year TTP 33% vs 87%, p=0.0002). Outcomes for R-CHOP treated PMBCL patients treated with R-CHOP are favorable and use of a PET-adapted approach reduces RT in the majority of patients. A small proportion have refractory disease and may benefit from an alternate treatment.