ESTRO 2025 - Abstract Book

S22

Invited Speaker

ESTRO 2025

organs targeted with increased precision. It will also inform the repurposing of PT to address other pathologies, such as arrythmic myocardial tissue ablation with high-precision proton beams.

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Speaker Abstracts Upright PT Ye Zhang Center for Proton Therapy, Paul Scherrer Institute, Villigen-PSI, Switzerland

Abstract:

Upright proton therapy is emerging as a transformative approach combining technical innovation with practical efficiency, leveraging a gantry-less design to cut costs and enhance treatment workflows. By rotating the patient instead of the beam, this approach eliminates the need for a massive (hundred-ton) rotating gantry, shrinking the facility size by up to 80% and dramatically reducing infrastructure and installation expenses. In fact, a specialized upright positioning system (e.g., a rotational chair) can cost on the order of only a few hundred thousand Euros, compared to the multi-million-Euro price tag of a conventional proton gantry. The absence of a heavy gantry not only lowers capital and maintenance costs, but also simplifies construction (requiring a smaller shielded bunker) and allows proton therapy suites to be more easily integrated or retrofitted into existing radiation oncology facilities. These economic and design advantages translate into a more accessible and scalable model for proton therapy, helping address the current reality that only ~1% of radiotherapy patients currently receive particle therapy. However, roughly 15% could potentially benefit. Moreover, the upright setup streamlines clinical workflow through improved patient access and potentially faster treatment setup; early clinical experience indicates that patient positioning and immobilization in an upright device (rotating chair system) can be accomplished within minutes with sub-millimetre reproducibility, supporting higher throughput and efficient use of resources. Beyond cost-efficiency, an upright orientation opens the door to novel radiotherapy concepts that are difficult to implement in conventional horizontal-beam rooms. The ample space around the patient’s upright position facilitates integrating advanced technologies such as Proton/Particle Arc Therapy and MR-guided proton therapy. Instead of rotating a gantry, the patient can be smoothly rotated to enable proton arc delivery, achieving dose distributions akin to photon VMAT while maintaining the dosimetric advantages of protons. Likewise, this gantry-free layout is amenable to combining proton beams with on-line MRI for real-time image guidance. The upright configuration is also compatible with emerging ultra-high-dose-rate FLASH radiotherapy (FLASH-RT), as delivering an entire fraction in a rapid burst is mechanically simpler without moving gantry parts, aligning with efforts to exploit the FLASH effect to spare normal tissues. Additionally, upright treatments naturally lend themselves to Adaptive Radiotherapy and modern on-board imaging (including both upright CT/CBCT or MRI systems) can facilitate daily replanning. Upright positioning may also improve motion management. Studies have shown that treating in a seated orientation can increase lung volumes and reduce respiratory motion amplitude, which could benefit tumours in the thorax and abdomen by reducing motion-related uncertainties. Despite the promise of upright proton therapy, several limitations must be addressed before widespread adoption. To date, only a handful of centres have implemented upright proton treatments clinically, underscoring the need for broader experience and validation. Further clinical outcome data are needed to confirm that upright treatments can achieve efficacy and safety comparable or superior to traditional supine treatments. Optimized patient positioning and immobilization techniques for various disease sites remain an active area of research, such as determining the best upright postures (fully seated, leaning, half-standing, etc.) for different indications and ensuring reproducible setups. There are also significant workflow integration challenges: transitioning to upright treatment affects every step of radiotherapy delivery – from imaging and treatment planning (e.g., re-training auto-segmentation algorithms