Effect of DIR uncertainty on prostate passive-scattering proton therapy dose accumulation

Deformable image registration (DIR) is important in dose accumulation. Currently, the impact of DIR-algorithm-associated uncertainties in proton therapy is unclear. Here, we quantify the effect of DIR uncertainties on prostate passive-scattering proton therapy (PSPT) dose accumulation. Ten patients with an intermediate risk for prostate cancer formerly treated by PSPT (PTV D₉₅ = 78 GyE) were studied. Dose distributions from all verification CT images (five images per patient) were warped and accumulated in the planning CT geometries with DIR. The dose-volume histogram parameters (D_mean, V₄₀, and V₇₀) for rectum and bladder were calculated. Two commercially available DIR software packages were employed: Velocity AI (Varian Medical Systems) and RayStation (RaySearch Laboratories). The dice similarity coefficient (DSC) and surface distance, which were calculated between planning CT contours and deformed contours, were used for DIR validation, with the relationship between the dose parameter and DIR uncertainty ultimately investigated. On average, when using RayStation, the DSC increased by 0.14 and surface distance decreased by 6.4 mm, as compared to Velocity. For D_mean, V₄₀, and V₇₀ to the rectum, the average differences between the RayStation and Velocity were 3.9 GyE, 5.5%, and 3.2%, respectively. For the bladder, the differences were 5.2 GyE, 5.8%, and 5.4%, respectively. The maximum differences in V₄₀ between RayStation and Velocity were 14.4% and 22.8% for the rectum and bladder, respectively, when the average DSC and surface distance differences were more than 0.14 and 6.4 mm, respectively. Such results suggest that DIR uncertainties might significantly affect prostate PSPT dose accumulations.