1. A method for image guided radiation treatment (IGRT) of a body part, comprising: providing an IGRT apparatus including a rotatable gantry structure, a radiation treatment head, and a treatment guidance imaging system including a first x-ray cone beam imaging source mounted to and rotatable with said rotatable gantry structure, the treatment guidance imaging system further including a first imaging detector; receiving a pre-acquired image data set of the body part acquired in a reference frame generally independent of a reference frame of the IGRT apparatus; during a patient setup interval, positioning the body part into an initial treatment position relative to the IGRT apparatus, said initial treatment position being along an axis of rotation of said rotatable gantry structure; rotating the rotatable gantry structure through a range of gantry angles; subsequent to said patient setup interval, operating said first x-ray cone beam imaging source and said first imaging detector to acquire a first population of x-ray cone beam projection images of the body part for a respective first population of gantry angles and acquisition times; processing the first population of x-ray cone beam projection images to compute therefrom a time sequence of sliding-window tomosynthesis reconstructed image volumes characterized in that each subsequent member of said time sequence is computed using at least one same x-ray cone beam projection image as used in computing at least one previous member of said time sequence; and operating the radiation treatment head to deliver treatment radiation to the body part based at least in part on a comparison between each of said time sequence of sliding-window tomosynthesis reconstructed image volumes and said pre-acquired image data set.