PURPOSE: Radiation pneumonitis (RP) is a common and potentially life-threatening toxicity from lung cancer radiotherapy. Data sets reporting RP rates after post-operative radiation therapy (PORT) have historically been small and with predominantly outdated field designs and radiation techniques. We examined a large cohort of patients in this context to assess the incidence and causes of RP in the modern era. MATERIALS AND METHODS: We reviewed 285 patients with non-small cell lung cancer (NSCLC) treated with PORT at our institution from 5/2004 to 1/2017. Complete dosimetric data and clinical records were reviewed and analyzed with grade 2 or higher RP as the endpoint (RP2+) (CTCAE v4.0). Patients were a median of 67 yo (range 28-87), and most had pathologic stage III NSCLC (91%) and received trimodality therapy (90%). Systematic dosimetric analyses using Dx increments of 5% and Vx increments of 2Gy were performed to robustly evaluate dosimetric variables. Lung V(5) was also evaluated. RESULTS: The incidence of RP2+ after PORT was 12.6%. Dosimetric factors most associated with RP2+ were total lungV(4) (HR 1.04, p