Setup variability affects the appropriate delivery of radiation and informs the setup margin required to treat radiation patients. Twenty-four veterinary patients with head and neck cancers were enrolled in this prospective, cross-sectional study to determine the accuracy of an indexed board immobilization device for positioning. Couch position values were defined at the first treatment based on setup films. At subsequent treatments, patients were moved to the previously defined couch location, orthogonal films were acquired, table position was modified, and displacement was recorded. The mean systematic displacement, random displacement, overall displacement, and mean displacement values of the three-dimensional (3D) vector were calculated. Three hundred thirty-two pairs of orthogonal setup films were analyzed for displacement in cranial-caudal, lateral, and dorsal-ventral directions. The mean systematic displacements were 0.5, 0.8, and 0.5 mm, respectively. The mean random displacements were 1.0, 1.1, and 0.7 mm, respectively. The overall displacements were 1.1, 1.4, and 0.9 mm, respectively. The mean 3D vector value was 1.6 mm with a standard deviation of 1.2 mm. Ninety-five percent of the vectors were <3.6 mm. These values were compared to data obtained with a previously used immobilization device. A t-test was used to compare the two devices. The 3D vector, random displacement in all directions, and overall displacement in the cranial-caudal and dorsal-ventral directions were significantly smaller than displacements with the previous device. The precision and accuracy of the indexed board device was superior to the historical head and neck device.
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