Radiation pneumonitis (RP) is a dose-limiting toxicity for lung cancer patients undergoing radiotherapy (RT) and systemic therapy. As the optimal practice for diagnosis, management and follow-up for RP is variable, we sought to establish expert consensus recommendations.International leaders in multidisciplinary thoracic oncology were invited to participate (n = 31) in this delphi consensus-building process. Following literature review on RP, open-ended questions related to knowledge, risk-reduction, diagnosis and treatment were generated for Round 1. Responses were used to generate 37 statements regarding RP diagnosis and management for Round 2. In this round, participants rated their agreement/disagreement with statements using a 5-point Likert scale, with oncologists receiving a survey with 2 additional items focused on planning technicalities. Consensus was achieved once ≥75% of respondents agreed with a statement. A final round to establish consensus in unresolved areas is planned.Round 1 had a 74% response rate (n = 23; 12 radiation oncologists, 5 clinical oncologists, 6 respirologists), and Round 2, a 61% response rate (n = 19; 15 oncologists, 4 respirologists). Of the 37 Round 2 statements, 36 received ≥75% agreement. By the end of Round 2, there was consensus opinion with agreement on the following: (1) risk stratification and mitigation should include patient factors (exposures, ILD, autoimmune and genetic conditions, COPD, emphysema, previous RT, and demographics); (2) minimizing RP risk through treatment planning (tight PTV margins, limiting dose to normal lung, dose/fractionation, IV contrast, motion management, IMRT/VMAT, daily imaging, and meeting constraints for V20 and MLD) should be utilized when possible; (3) diagnosis should be based on symptoms, exam, temporal relationship to treatment, imaging, and common toxicity grading scales; (4) Treatment of RP should be multidisciplinary, with oncologists and respiratory physicians, and should involve administration of oral steroids with gastroprotection, starting with 60 mg PO prednisolone or equivalent, for a duration of 2 weeks, with a taper of 10 mg in the daily dose per week, or for severe pneumonitis, IV methylprednisolone for 3 days before PO. Treatment adjuncts may include oxygen, inhalers, and antibiotics; (5) in differentiating drug-related pneumonitis vs. RP, standard review of patients receiving immunotherapy, per ASCO/ESMO guidelines, is insufficient to identify early pneumonitis, and thus it would be helpful to develop guidelines which recognize the additive nature of toxicities.Responses from international thoracic oncology experts highlight areas lacking consensus in the diagnosis and management of RP. These data will inform the development of final consensus statements to provide practical guidance on diagnosis and management of RP.