Image-Guided Decubitus Craniospinal Irradiation Setup in a Fixed-Beam Proton Room: Accuracy and Efficiency.

Purpose: Recent developments have further demonstrated the feasibility of craniospinal irradiation (CSI) delivery using simplified beam arrangements, such as a single posterior brain beam. Extending these advances, CSI delivered in the lateral decubitus position within a fixed-beam proton room offers an opportunity to improve room utilization and treatment access but introduces unique challenges in patient setup and image guidance. Methods: This study compares 2 image-guided setup workflows for decubitus CSI to evaluate whether meaningful differences exist. We retrospectively analyzed 10 CSI patients treated in the decubitus position between 2024 and 2025 using the fixed-beam ProBeam system with the 3-iso technique. Two setup workflows were evaluated: (1) Continuous Full-Field Image Guidance (CFF-IG), in which all isocenters were aligned prior to treatment delivery, and (2) Selective Field-Based Image Guidance (SF-IG), in which full-field imaging was applied only at the initial fraction, followed by field-specific verification thereafter. Setup accuracy was quantified using translational and rotational deviations derived from oblique kV imaging, and dosimetric robustness was assessed through target coverage and junction dose gradient analysis. Setup efficiency was evaluated using treatment room time stamps. Results: Across all 3 isocenters, mean translational deviations were below 4 mm and mean angular deviations were below 1.5°. Differences in target coverage (ΔD95) and junction dose gradients were small between the 2 workflows. No statistically significant differences in setup accuracy or dosimetric quality were observed. SF-IG was associated with shorter average setup time, with the majority of treatment sessions completed within 30 minutes. Conclusions: Both CFF-IG and SF-IG achieved setup accuracy within institutional action levels and maintained dosimetric robustness for CSI delivered in the decubitus position using pencil beam scanning. The absence of significant differences suggests that either approach may be clinically feasible for decubitus CSI delivery. SF-IG offers a more time-efficient workflow without compromising treatment quality, supporting its use as the preferred default strategy, while CFF-IG may be reserved for select cases requiring enhanced global realignment. These findings support decubitus CSI as a practical extension of modern single-beam and simplified pencil beam scanning CSI techniques.