Systems, methods, and related computer program products for image-guided radiation treatment (IGRT) are described. For one preferred embodiment, an IGRT apparatus is provided comprising a ring gantry having a central opening sufficiently large to accommodate a body of a patient positioned along a longitudinal axis and extending therethrough, and a gantry tilting mechanism configured to tilt the ring gantry to a plurality of different tilt angles relative to the longitudinal axis. A radiation treatment head is coupled to the ring gantry and is rotatable around said central opening in at least a 180 degree arc. The radiation treatment head is mechanically coupled to the ring gantry such that a distance by which the radiation treatment head extends inwardly toward the central opening relative to the ring gantry is dynamically controllable.