2.1. Phantom

The CIRS dynamic thorax motion phantom (CIRS Inc. Norfolk, VA) was used in this study. The phantom approximates an average human thorax in size and with the embedded structures (lung cavities, ribs, and spine). Two different spherical target inserts with diameters of 10 and 15 mm were used in this study. A lung equivalent cylinder containing the spherical target was inserted into the lung equivalent lobe of the phantom. Subsequently, the cos ⁴ waveform ²³ with amplitude = 30 mm and period = 5 s was applied to produce target motion in the superior‐inferior direction.