Breath-hold curve and gating window. Breath-hold curve (black curve) tends to stabilize within a few seconds (typically 2 seconds). The CT imaging and treatment delivery were performed during a stable breath-hold period defined by upper and lower limits (blue and orange vertical lines, respectively). In this case the limits of ± 2.5 mm were chosen around the (average) reference breath-hold level based on several successive breath-holds. The numbers on the right show the location of the gating window with respect to baseline level (maximal exhale in free breathing).