Evaluation of the offset couch parameter between kilovoltage on-board imaging and cone-beam computed tomography in patients with prostate cancer

Background: External Beam Radiation Therapy (EBRT) is a curative therapy technique for prostate cancer. Since the prostate is unstable and surrounded by the bladder and rectum, precision of the target location is critical. Image Guided Radiation Therapy (IGRT) can improve treatment precision. The bladder and rectum may alter volume during IGRT, shifting the prostate’s position and resulting in missed target volume doses and extra organs at risk (OARs) doses. Objective: To assess setup error and residual error during patient positioning, as well as the current IGRT protocol efficiency, in prostate cancer patients while recommending a planning target volume (PTV) margin. Materials and methods: The offset couch parameter of on-board imaging (OBI) and cone-beam computed tomography (CBCT) was computed to determine the error distribution, magnitude, and error difference between treatment phases. The systematic and random errors were calculated using the van Herk equation to determine the planning target volume (PTV) margin. Results: The setup error was -0.86 to 0.25 mm, and the residual error was -0.15 to 0.32 mm. The couch displacement percentage for OBI was 29.44% to 58.89%, and for CBCT was 8.10% to 34.12%. The systematic error was 1.65 to 3.21. The random error was 1.78 to 3.29. The setup error was greatest in the longitudinal (Lng) direction, residual error was greatest in the vertical (Vrt) direction, and systematic and random error were greatest in the Vrt and lateral (Lat) direction, respectively. The PTV margin was greatest in the Vrt direction, while the Lng direction was the narrowest margin for every treatment phase. Conclusion: The highest setup error occurs in the Lng direction for all treatment phases. For the 46 Gy and 60 Gy phases, the highest residual error is in the Vrt direction. However, in the 78 Gy phase, the error is relatively close to 0.01mm in every direction. The current IGRT protocol is effective in detecting setup and residual errors. The 78 Gy phase has the greatest PTV margin, whereas the 46 Gy phase shows the narrowest margins in all directions.