External-beam radiation therapy is a common treatment modality for cancers. e treatment course typically consists of 30-40 daily fractions of 180-200 cGy over several weeks. On-board imaging (OBI) guidance is oen designed to track shis in patient positioning and tumor location immediately before treatment sessions. Many clinicians believe that daily imaging followed by any required adjustments to the plan or patient positioning would make radiation treatments more precise, especially if decisions for plan revisions can be based on 3D volumetric imaging of so-tissue targets and nearby tissues.