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 gantry frame including a ring member, the ring member being rotatable around a substantially horizontal, longitudinally extending central axis, the ring member having first and second horizontally opposing ends. The IGRT apparatus further comprises a radiation treatment head coupled to the ring member by an arm member, the arm member being connected to the ring member at an arm member base. Preferably, the IGRT apparatus is further characterized in that the arm member extends outwardly from the first end of the ring member in a direction away from the second end and is supported only by the arm member base, and the radiation treatment head is dynamically movable in at least a longitudinal direction toward and away from the first end of the ring member.