A system for radiotherapy includes a first imaging system and a second imaging system. The first imaging system generates projection images of an area of interest of an object, and the second imaging system generates portal images of the area of interest of the object synchronously with the generation of the projection images. The radiotherapy system further includes a processing system that receives data associated with the projection images and data associated with the portal images and reconstructs 3D and 4D portal images from the projection images and the portal images.
