Four-dimensional radiation therapy (4DRT) and adaptive radiation therapy (ART) are treatments to account for respiratory motion and anatomical changes during a course of treatment, respectively. Recent development of both imaging and delivery techniques made these radiation therapy possible. In these planning, dose distributions are calculated with multiple image sets for example each respiratory phase constituting 4-dimensional computed tomography and cone-beam computed tomography acquired on each treatment day. To evaluate overall dose distributions, we need to accumulate dose distributions calculated with multiple image sets. However, pixel-wise dose accumulation is impossible because it does not accumulate dose based on anatomy if image sets are different. Deformable image registration (DIR) is used to find corresponding anatomical points between image sets. Thus, dose accumulation based on DIR is a promising technology to enable us to evaluate overall dose distributions of 4DRT and ART. In this article, I will briefly introduce dose accumulation based on DIR and its application to 4DRT and ART.