Analysis of the minimal detected error and residual error for kilovoltage cone-beam CT guided radiotherapy system

Objective: To assess the minimal detected error and residual error for the kilovoltage (kV) cone-beam CT guided radiotherapy system. Methods: The ball-bearing phantom was used for analyzing the minimal detected error of the system. Different groups of real shift were intentionally set up and the true shift and the detected shift of the system were compared. The minimal real shift, which can be resolved by the system, is considered as the minimal detected error of the system. CIRS Model 002LFC thoracic phantom was used to analyze the residual error of the system. The intended set-up error was detected by the alignment of the kVCBCT and the planning CT. The phantom set-up error was then corrected by moving the couch of the accelerator accordingly. A second CBCT scan was acquired immediately after the correction to evaluate the residual set-up error. Results: The minimal detected error of the system was 0.5 mm. The result of the phantom measurement performed within 2 weeks showed that the average residual errors (±SD) in the online image guided process were (0.2±0.4), (0.1±0.4) and (-0.4±0.3) mm in the lateral, longitudinal, and vertical directions, respectively. Conclusions: This study developed the methods of analyzing the minimal detected error and residual error for kilovoltage cone-beam CT guided radiotherapy system. The minimal detected error of the system was 0.5 mm. The residual error of the system is about 1 mm. (authors)