1. A method of 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; computing a latest member of said time sequence of sliding-window tomosynthesis reconstructed image volumes; comparing said latest member with said pre-acquired image data set; and operating the radiation treatment head to deliver treatment radiation to the body part based on said comparison of said latest member with said pre-acquired image data set and 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 wherein said latest member of said time sequence of sliding-window tomosynthesis reconstructed image volumes being associated with a latest gantry angle, wherein said computing said latest member comprises: receiving a first parameter indicative of a desired tomosynthesis reconstruction coverage arc; identifying from said first population of x-ray cone beam projection images a first subset thereof having corresponding gantry angles that are within said desired tomosynthesis reconstruction coverage arc of said latest gantry angle; and computing said latest member based on said first subset of x-ray cone beam projection images; and wherein said latest member of said time sequence of sliding-window tomosynthesis reconstructed image volumes being associated with a latest acquisition time, wherein said computing said latest member based on said first subset of x-ray cone beam images comprises: receiving a second parameter indicative of a desired data aging threshold; identifying from said first subset of x-ray cone beam projection images a second subset thereof having corresponding acquisition times that are within said desired data aging threshold of said latest acquisition time; and computing said latest member using only said second subset of x-ray cone beam projection images.