Daily imaging (or use of a nonimage-based system such as Calypso) is necessary to locate the tumor, and the location must be correlated with measurements from a system that tracks breathing motion during treatment delivery. Given the state of treatment planning software, it is not possible to preplan whether a specific patient would benefit from one or another of these methods. 4D CT has been instrumental in most of these approaches. Approaches to management of this motion may involve motion-inclusive planning, gating or tracking. These may cause delivery misalignments because the tumor is not in the same average location at each treatment. Motion during the regular breathing cycling is important, but day-to-day breathing variations, as may be caused by changes in residual tidal volume, can cause systematic shifts in tumor position. Most often a result of respiration, this motion can cause dose delivery errors that are clinically significant when unmanaged, as demonstrated in many recent investigations. Recently developed 4D CT imaging technologies have shown that significant organ motion can occur within radiotherapy fields during treatment.
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