Upright Immobilization for Proton Therapy: A Critical Review and Single-Institution Experience.

Upright positioning coupled with gantry-less proton therapy designs may reduce facility footprint and cost while introducing specific technical requirements for reproducible positioning, immobilization, volumetric image guidance, motion management, and treatment delivery. Historically, upright radiotherapy was limited to specialized applications and institution-specific systems, while recent advances in six-degree-of-freedom positioning and upright volumetric imaging have enabled broader image-guided clinical workflows. Published clinical evidence is most mature for head-and-neck applications, where thermoplastic immobilization, tailored patient support, and image-guided corrections have demonstrated clinically acceptable positioning reproducibility. Evidence for thoracic, breast, abdominal, and pelvic applications remains limited and is derived largely from small patient cohorts, healthy-volunteer studies, and early feasibility investigations. A recurring practical issue across sites is vertical position change related to fatigue, posture relaxation, or device compliance, reinforcing the need for robust immobilization design and consistent upright volumetric imaging. Current evidence supports the technical feasibility of upright proton therapy but remains insufficient to define standardized workflows across disease sites. Further prospective and multi-institutional studies are needed to establish site-specific positioning tolerances, validate motion-management strategies, and define standardized quality-assurance and clinical workflows.