objectives: Anatomic changes may occur during chemoradiation treatment for lung cancers, requiring adaptive replanning. Here we characterize these cases. Methods: We retrospectively studied lung cancer cases that underwent resimulation and adaptive replanning during 1/2016–3/2019. We compared first and second CT-simulation regarding tumor location, timing of change, tumor volume, anatomical alteration and change in simulation technique. We also compared dosimetric parameters between the plans, recorded local control, and overall survival outcomes. results: Out of 281 patients, 58 underwent replanning (20.6%). Histology included small cell (22.4%) and non-small cell (77.6%). Stage III was in 91.4%. Mean radiation dose of 59.4 Gray (Gy) (range 50-66Gy). Tumor location was peribronchial in 53.5%. Timing of replanning was in the first, second and final third of the treatment course in 26%, 43% and 31% respectively. Changes in gross tumor volume were observed in 74%; mean gross tumor volume was 276.7cc vs 192.7 cc (first vs second simulation, p = 0.001). Anatomical changes were identified in 35.4% including pleural fluid accumulation, atelectasis or pneumothorax alteration. Change in simulation technique was performed in 25.9%, including breath-hold or continuous positive airway pressure. Changes in dosimetric parameters when the same technique was used: lung V20Gy 26% (standard deviation, SD 7.6) vs 25.3% (SD 6.6) (p = 0.36), mean lung dose 15.1 Gy (SD 3.7) vs 14.7Gy (SD 3.3) (p = 0.23), heart V40Gy 10.2% (SD13) vs 7.2% (SD 9.8) (p = 0.037). When simulation technique changed: lung V20Gy 30.8% (SD 8.2) vs 27.3% (SD 8) (p = 0.012), mean lung dose 17.3 Gy (SD 4.4) vs 15.3 Gy (SD 3.8) (p = 0.007), heart V40Gy 11.1% (SD 14.7) vs 6.5% (SD 6.7) (p = 0.014). 2 year local control was 60.7% (95% confidence interval, 34.5–79.2%), and median overall survival was 19.7 months. Conclusion: Adaptive replanning of radiation was performed in a fifth of locally advanced lung cancer patients. In most cases tumor volume decreased, or atelectasis resolved, causing mediastinal shifts, which, if unidentified and left uncorrected, may have led to local failure and increased toxicity. The heart V40Gy was reduced significantly in all cases, but significant reduction in lung doses was evident only if simulation technique was altered. advances in knowledge: In locally advanced lung cancer image-guidance with cone beam CT can detect significant mediastinal shifts and gross tumor volume changes that raise the need for adaptive replanning. Image guidance-triggered adaptive replanning should be added to the armament of advanced radiation treatment planning in locally advanced lung cancer.