Scenario-Based Robustness of Proton Arc vs IMPT for Prostate SBRT Under Translational, Rotational, and Range Uncertainties

Document Type

Conference Proceeding

Publication Date

9-2026

Publication Title

International Journal of Particle Therapy

Abstract

Background and Aims: To quantify and compare scenario-based robustness of proton arc versus two-field IMPT for prostate SBRT under translational, rotational, and range uncertainties. Methods: Four prostate patients were retrospectively planned in RayStation (research version 2024A) using (1) DynamicARC delivered as a single partial arc (gantry: 325⁰ – 180⁰) and (2) IMPT with two parallel-opposed lateral fields. Nominal plans for both techniques were robustly optimized using ±5 mm setup (translational) and ±3.5% range uncertainties for total dose of 36.25 Gy(RBE) in 5 fractions. Robustness evaluation comprised 37 scenarios/plan: nominal (1); translational x/y/z ±5 mm (6); rotation-only perturbations ±3° yaw/pitch/roll (6); and range-coupled combinations with ρ ±3.5% for translational×range (12) and rotation×range (12). Target endpoints included CTV D95%, D98%, D99%, and D0.03cc. OAR robustness used RTOG 0938 dose–volume limits as evaluation thresholds across all scenarios, summarized by pooled and endpoint-level exceedance rates. Results: Nominal Target coverage was similar in both nominal DynamicARC and IMPT planning. Both of them met RTOG 0938 OAR constraints; any OAR failures occurred only during scenario-based robustness evaluation. Under uncertainties, worst-case CTV D95% was 35.41±0.28 Gy(RBE) (DynamicARC) and 35.38±0.68 Gy(RBE) (IMPT), meeting the robustness criterion (D95%≥95%). Worst-case CTV D99% was 34.34±0.33 Gy(RBE) for DynamicARC versus 33.68±1.20 Gy(RBE) for IMPT, consistent with greater degradation and variability for IMPT at D98%–D99%. Worst-case target coverage was driven by different perturbation types: for DynamicARC plan, worst-case CTV D95%–D99% occurred under rotation×range scenarios, whereas for IMPT it occurred under translational×range scenarios (across patients and coverage endpoints). IMPT reduced the worst-case target hotspot relative to DynamicARC (39.92±0.88 vs 40.50±0.48 Gy(RBE)). Across all OAR checks in robustness evaluation, pooled per-check failure rates were 4.64% (DynamicARC) and 6.71% (IMPT). OAR failures clustered in range-coupled perturbations—particularly translational×- range—and were driven primarily by rectum dose–volume constraints for IMPT, whereas DynamicARC failures were more frequently associated with bladder high-dose constraints. Conclusions: In this pilot robustness study, DynamicARC showed less degradation in worst-case D98%–D99% target coverage and lower overall OAR failure rates under robustness evaluation, while IMPT demonstrated lower worst-case target hotspot. The observed technique-dependent patterns of worst-case coverage suggest technique-specific robustness evaluation and warrant confirmation in larger cohorts.

Volume

21

Issue

Suppl

First Page

101698

Last Page

101698

Comments

PTCOG 64 Particle Therapy Co-Operative Group 64th Annual Conference, June 8-13, 2026, Deauville, France

DOI

10.1016/j.ijpt.2026.101698

Link to Full Text

Share

COinS