Clinical Comparison of Beam-Specific Planning Target Volume Single-Field Uniform Dose and Robust Intensity-Modulated Proton Therapy in Ultra-Hypofractionation for Prostate Cancer

Purpose: This study aimed to compare the robustness of beam-specific planning target volume (bs-PTV)-based single-field uniform dose (SFUD) and robustly optimized intensity-modulated proton therapy (IMPT) plans against setup, range, and anatomical uncertainties in ultra-hypofractionated (UHF) proton therapy for localized prostate cancer. Methods: SFUD and IMPT plans were generated and compared using imaging datasets from ten consecutive patients who underwent proton therapy for prostate cancer at our institution. SFUD plans using a bs-PTV and IMPT plans using robust optimization were generated with 36.25 Gy equivalent (GyE) to the clinical target volume (CTV) in 5 fractions. To evaluate robustness against setup and range uncertainties, 20 error scenarios were applied, including combined ±3 mm isocenter shifts and ±3.5% Hounsfield Unit (HU) variations. Additionally, the SFUD and IMPT plans were recalculated using virtual CT (vCT) derived from weekly cone-beam CT (wCBCT) to estimate accumulated doses under anatomical changes. The worst-case and accumulated doses to the CTV, rectum, and bladder were compared. Results: Both SFUD and IMPT plans demonstrated stable target coverage under setup, range, and anatomical uncertainties. Variations in CTV D99 remained within commonly accepted robustness criteria for all patients. Although a statistically significant difference in worst-case CTV D99 was observed between SFUD and IMPT plans, the magnitude of this difference was small. Dose variations under uncertainties were smaller for selected organs at risk (OAR) dose metrics in IMPT. Across the nominal, worst-case, and accumulated dose evaluations, IMPT plans achieved lower low- and intermediate-dose metrics for the rectum and bladder. Conclusion: Both bs-PTV-based SFUD and robustly optimized IMPT plans maintained adequate target coverage under uncertainties in UHF proton therapy for prostate cancer. The lower OAR doses and slightly improved OAR robustness observed with IMPT likely reflect the combined effects of differences in planning techniques and uncertainty management strategies, suggesting a modest dosimetric benefit.