An apparatus for image guided radiation treatment (IGRT) of a body part, the apparatus comprising: a first rotatable gantry structure and a second rotatable gantry structure, the body part being positionable in an initial treatment position relative to the IGRT apparatus during a patient setup interval, said initial treatment position being along an axis of rotation of said first rotatable gantry structure and said second rotatable gantry structure; a radiation treatment head mounted to the first rotatable gantry structure; a treatment guidance imaging system including a first x-ray cone beam imaging source and a first imaging detector, the first x-ray cone beam imaging source being mounted to and rotatable with said second rotatable gantry structure, wherein the first x-ray cone beam imaging source is rotatable around the axis of rotation independently of the radiation treatment head; means for 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; means for rotating the second rotatable gantry structure through a range of gantry angles greater than 180 degrees plus a fan beam angle of the first x-ray cone beam imaging source; means for operating said first x-ray cone beam imaging source and said first imaging detector to acquire, subsequent to said patient setup interval, 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; means for processing the first population of x-ray cone beam projection images to compute therefrom a time sequence of sliding-window cone beam CT (CBCT) 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 means for 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 CBCT image volumes and said pre-acquired image data set. a first rotatable gantry structure and a second rotatable gantry structure, the body part being positionable in an initial treatment position relative to the IGRT apparatus during a patient setup interval, said initial treatment position being along an axis of rotation of said first rotatable gantry structure and said second rotatable gantry structure; a radiation treatment head mounted to the first rotatable gantry structure; a treatment guidance imaging system including a first x-ray cone beam imaging source and a first imaging detector, the first x-ray cone beam imaging source being mounted to and rotatable with said second rotatable gantry structure, wherein the first x-ray cone beam imaging source is rotatable around the axis of rotation independently of the radiation treatment head; means for 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; means for rotating the second rotatable gantry structure through a range of gantry angles greater than 180 degrees plus a fan beam angle of the first x-ray cone beam imaging source; means for operating said first x-ray cone beam imaging source and said first imaging detector to acquire, subsequent to said patient setup interval, 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; means for processing the first population of x-ray cone beam projection images to compute therefrom a time sequence of sliding-window cone beam CT (CBCT) 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 means for 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 CBCT image volumes and said pre-acquired image data set.