Objective: To investigate the effect of onboard image (OBI) system-based\nimage guided radiotherapy (IGRT) on the precision of fractionated intensity\nmodulated radiotherapy (IMRT) for patients with locally advanced rectal\ncancer. Methods: The IGRT validation images of the 12 patients with rectal\ncancer were obtained after initial setup by the OBI system of Varian Novalis\nTX linear accelerator, and registered to the planning CT image system. Subsequently,\nthe setup deviations on three translational directions [ventral-dorsal\ndirection (VD), cranial-caudal direction (CD) and lateral direction (LD)] for\nthe three-validation phase including Pre-treatment (Pre-RT1), repositioning\n(Pre-RT2) and Post-treatment (Post-RT) were obtained and comparatively\nanalyzed. Results: The frequency of setup deviation of <--2.0 mm in the lateral,\ncephalocaudal and ventral direction was 83.01%, 65.71%, and 68.91%, respectively\nfor Pre-RT1; 100%, 98.72% and 100%, respectively for Pre-RT2; 100%,\n97.76%, and 99.68%, respectively for Post-RT. Compared with the Pre-RT1\nphase, the ranges of setup deviation on Pre-RT2 and Post-RT phases possessed\na significant contraction trend. The absolute values of setup deviations\non the three translation directions between the Pre-RT1 and Pre-RT2 or\nPost-RT were statistically significant (p < 0.05). Through positioning adjustment\nbased on IGRT based on the OBI system, the setup deviations on the\nthree translational directions decreased significantly. Conclusion: Application\nof OBI-based daily IGRT may help improve the precise delivery of fractionated\nIMRT by decreasing the inter- and intra-fractionated setup deviation\nin the ventral-dorsal direction, cranial-caudal direction and lateral direction\nfor patients with locally advanced rectal cancer.
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