Robust optimization of VMAT for prostate cancer accounting for geometric uncertainty

The aim of this study was to propose optimal robust planning by comparing the robustness with setup error with the robustness of a conventional planning target volume (PTV)-based plan and to compare the robust plan to the PTV-based plan for the target and organ at risk (OAR). Data from 13 patients with intermediate-to-high-risk localized prostate cancer who did not have T3b disease were analyzed. The dose distribution under multiple setup error scenarios was assessed using a conventional PTV-based plan. The clinical target volume (CTV) and OAR dose in moving coordinates were used for the dose constraint with the robust plan. The hybrid robust plan added the dose constraint of the PTV-rectum to the static coordinate system. When the isocenter was shifted by 10 mm in the superior-inferior direction and 8 mm in the right-left and anterior directions, the doses to the CTV, bladder, and rectum of the PTV-based plan, robust plan, and hybrid robust plan were compared. For the CTV D-99% in the PTV-based plan and hybrid robust plan, over 95% of the prescribed dose was secured in all directions, except in the inferior direction. There was no significant difference between the PTV-based plan and the hybrid robust plan for rectum V-70Gy, V-60Gy, and V-40Gy. This study proposed an optimization method for patients with prostate cancer. When the setup error occurred within the PTV margin, the dose robustness of the CTV for the hybrid robust plan was higher than that of the PTV-based plan, while maintaining the equivalent OAR dose.