1. A method of modifying treatment parameters for a patient undergoing radiotherapy, the method comprising the steps of: (a) establishing a set of baseline treatment parameters; (b) during a treatment session obtaining, using different imaging modalities, at least two registered images of an anatomical region to be treated wherein at least some of the images are portal images from at least one beam direction, and at least one of the images is a non-x-ray-based 3D image; (c) registering the at least two images to a coordinate system associated with a treatment device; (d) segmenting the non-x-ray-based 3D image to form a 3D surface; (e) projecting the surface onto the plane of each portal image; (f) enhancing the portal images with data from the projected surface; and (g) modifying the baseline treatment parameters based on the registered images.