Background/Aim: It is important to identify radiation pneumonitis above Common Terminology Criteria for Adverse Events Grade 2 (G2) in order to safely continue durvalumab maintenance after chemoradiotherapy for advanced lung cancer. The aim of this study was to discover factors that predict pneumonitis above G2. Patients and Methods: A follow-up computed tomography (CT) image was superimposed on the planning CT image using deformable image registration (DIR). The pneumonitis area was contoured on follow-up CT after DIR and the dose-volume histogram parameters of the contoured pneumonitis area were calculated. Results: V5 (Percentage of total volume receiving ≥5 Gy) to V50 of pneumonitis were significantly lower in patients with G2 pneumonitis than in those with G1 pneumonitis. The pneumonitis V15 was the most significant. The group with pneumonitis V15 <87.10% had significantly more G2 pneumonitis than the group with pneumonitis V15 ≥87.10%. Conclusion: Pneumonitis V15 <87.10% was a risk factor for G2 pneumonitis.Radiation pneumonitis is a serious adverse event after radiotherapy for lung cancer. V30 (percentage of total volume receiving ≥30 Gy), V20, V5 of lungs and mean lung dose (MLD) have been established as risk factors for radiation pneumonitis development after radiotherapy for lung cancer (1-3).In recent years, durvalumab has been used after concurrent chemoradiotherapy (CCRT) as the standard treatment for non-small cell lung cancer. However, data are limited on radiation pneumonitis when combined with immune checkpoint inhibitors (ICI) such as durvalumab (4, 5). It is important to identify radiation pneumonitis above Grade 2 (G2) in order to safely continue durvalumab.In the simple radiotherapy with two opposed beams, radiation pneumonitis that spreads to the outside of the irradiated field is generally considered to have a poor prognosis (6). At present, multiport three-dimensional conformal radiation therapy (3D-CRT) and intensity-modulated radiation therapy are performed as radiotherapy for lung cancer (7), and several risk factors for radiation pneumonitis have been reported (3, 8). However, using modern radiotherapy techniques, the lowdose area is wide; therefore, the spread of pneumonitis out of the irradiated field cannot be used as a prognostic factor for radiation pneumonitis.Radiotherapy planning is performed using computed tomography (CT) under shallow breathing or fourdimensional computed tomography (4D-CT) (9). On the other hand, CT for diagnosing radiation pneumonitis is performed with deep inspiratory breath hold (10). As the lung volume is different between CT at rest or 4D-CT and CT in the deep inspiratory state, the positions do not match even if the images are superimposed. Therefore, it is not possible to confirm whether the isodose line on treatment planning CT and the area of pneumonitis on diagnostic CT match. However, deformable image registration (DIR) was recently developed and it has become possible to superimpose images of different respiratory phases (11).The purpose of this ...