Introduction: Local recurrence or second lung primaries are common indications for radical thoracic re-irradiation (re-RT), affecting approximately 700 patients in the UK annually. Re-RT is usually the only suitable curative-intent treatment but prospective evidence on toxicity, dose constraints, and optimal treatment technique is lacking. We performed a Delphi process to identify areas of consensus in re-RT for NSCLC. Methods: An international panel of 15 radiation oncologists specialising in lung cancer participated in an initial survey on 23/09/2019 to capture their definition of re-RT, suitable patients, re-RT technique and dose constraints used. The most common responses to questions from the first survey were used to make statements which participants voted on in subsequent rounds using a 5-point Likert scale. Consensus was achieved once 75% of participants agreed with a statement. For the statements which did not reach consensus, respondents provided additional evidence/comments to refine them. In total, four surveys were performed using a web-based survey programme. Results: All respondents completed three rounds of the survey, with the final round currently in progress. Consensus was achieved within two rounds regarding re-RT indications, patient eligibility and work-up (Table 1). In addition, agreement was reached to use stereotactic ablative body radiotherapy (SABR) if possible for re-RT. Dose constraints, due to the lack of supportive data, required three rounds to develop agreement. Several volumetric lung constraints were suggested, but due to post-radiotherapy fibrosis, it was concluded that there was insufficient evidence to form recommendations (Table 2). Areas of controversy were how much overlap was significant when performing re-RT, what were the minimum lung function requirements and the minimum safe interval between treatments. [Formula presented] [Formula presented] Conclusion: This Delphi process with international experts has developed key recommendations on the criteria for suitable re-RT patients, dose constraints and preferred technique. These statements can be used to develop prospective trials to provide better evidence for re-RT.