Sag measurement on common treatment couch for integrated CT/X-ray simulator system - Through patient model cases following actual irradiation sites and center of body gravity

The aim of this study is to analyze couch sag from the weight of a newly-installed Integrated Computed Tomography (CT) /X-ray simulator system. The system requires that the same sag characteristics be maintained regardless of any sliding and extended distance of a top plate, and/or any loading condition before and after rotational transfer of a treatment couch. This requirement is a dominating factor to ensure location accuracy of the integrated system. We measured sag on the top plate at isocenter in the case of treating pharynx, lung, and prostate. Under the conditions that the sliding and extended distances of the top plate were adjusted to each model, the weighted load was put on the center of body gravity. The displacements in both anterior-posterior and inferior-superior directions were measured between the CT and X-ray simulator. In addition, by reproducing evenly loaded condition in an acceptance test, we analyzed the sag at the isocenter, which was defined as a measurement benchmark. We also carried out a test to verify the sag depends on the sliding and extended distances of the top plate under evenly loaded condition in the anterior-posterior direction. Displacement caused by the weighted load mainly occurred in the posterior direction. The displacement value was greater in the prostate case because the center of gravity was placed in the superior position to the isocenter. The finding revealed that some sag characteristics depend on the sliding and extended distances of the top plate. The approximately 1mm sag occurred in the posterior direction at 10 cm of extension from the top plate. However, the sag characteristics by weighted load upon between the CT and the X-ray simulator showed strong correlation (r = 0.998 - 1.00. p < 0.01) and the differences of displacement value between the CT and the X-ray simulator were kept within 1 mm on every model and loading dose.