Evaluation of robust central nervous system treatment planning on a compact, gantry-less proton therapy system

OBJECTIVE: This study assessed the clinical feasibility of robust treatment planning for central nervous system (CNS) tumors using a gantry-less proton therapy configuration, with the goal of supporting adoption of compact proton systems that are more affordable and accessible. 
Approach: Treatment plans for CNS cases were created in RayStation with three configurations: a full-gantry with the patient in a supine position, and a fixed, horizontal beamline with the patient either in a supine or seated position. Robust optimization was used for each plan, accounting for ±2mm isotropic setup errors and ±3.5% range uncertainties. To simulate potential impact of slouch, anisotropic setup shifts (±2, ±3, ±4mm) in the superior-inferior direction were evaluated for treatments in the seated position. A radiation oncologist conducted a blind review of the gantry-based and gantry-less plans to determine the preferred option. Dose metrics and robustness were compared across the different configurations, and statistical analyses were performed to assess the significance of any differences. In total, there were 10 cases, 50 treatment plans, and 1,050 treatment course scenarios.
Main Results: Results showed that proton pencil beam scanning plans using a fixed horizontal beamline can produce comparable dosimetric outcomes to those utilizing a full-gantry. All three configurations achieved the required target coverage, with hotspots generally within 3-5% of the prescribed dose. In 8 out of 10 cases, the target homogeneity index was lowest in a gantry-less configuration. The gantry-based and gantry-less plans demonstrated similar robustness against setup errors and range uncertainties, though target coverage was slightly more robust in certain gantry or gantry-less plans, depending on the specific case and treatment configuration.
Significance: By integrating blinded radiation oncologist assessment with quantitative dosimetric analysis, this work shows that fixed, horizontal proton beamlines can deliver high-quality plans, supporting the initial clinical feasibility of compact proton therapy systems.