An image-guided radiation treatment (IGRT) system and method are described. An IGRT apparatus having a gantry frame including a ring member with the ring member being rotatable around a substantially horizontal, longitudinally extending central axis, and the ring member having first and second horizontally opposing ends. The IGRT apparatus further includes a radiation treatment head coupled to the ring member by an arm member with the arm member being connected to the ring member at an arm member base. 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.
