Report on the ESTRO Physics Workshop
23-24 October 2025, Toulouse, France
Chairs: Dr Heng Li (USA) and Professor Yolanda Prezado (Spain)
The ESTRO Physics Workshop 2025 brought together 28 participants from Europe, the United States, China, and Australia for two days of focused dialogue on spatially fractionated radiotherapy (SFRT). With representation across physics, radiation oncology, radiobiology, and radiation therapy, the workshop reflected the growing global momentum behind SFRT and the shared goal of advancing it safely and consistently into clinical practice.
The workshop opened with an overview of the current state of spatial fractionation. The speakers highlighted encouraging early clinical experiences in patients with bulky, hypoxic, or radioresistant tumours. Presenters underscored that although enthusiasm about the technique is rising, the field faces significant challenges to bridge the gap between preclinical concepts and clinical implementation. This set the tone for the workshop: SFRT holds clear potential, but coordinated physics leadership is needed to establish the foundations that are required for broad clinical adoption.
A major theme that weaved throughout the discussions was the importance of physics standardisation. Participants highlighted the need for accurate beam modelling and validated small-field dosimetry data, both of which are essential to capture the intentional heterogeneity of SFRT dose distributions. Because participating centres currently rely on locally developed configurations of treatment planning systems (TPSs) and commissioning strategies, experiences vary widely. Many attendees identified the lack of common definitions for peak and valley doses, spatial frequency, and TPS model parameters as an obstacle that hinders the comparison of clinical outcomes. It was recognised that reference datasets and validation pathways in the style of American Association of Physicists in Medicine (AAPM) task groups 119 or 350 must be established as a priority step toward consistency.
The workshop also devoted significant attention to practical implementation and quality assurance (QA). Many current commercial QA tools lack the spatial resolution needed for SFRT verification, so centres are left to rely on films, custom setups, or adapted workflows. Participants discussed the need for clear end-to-end testing strategies, practical commissioning guidance, and standardised documentation. Motion management emerged as an important consideration, particularly for thoracic and abdominal treatments, in which maintaining spatial integrity can be challenging. Examples such as setting surface-guided gating thresholds around 3mm offered potential approaches, but participants agreed that more structured guidance was needed.
On the biological and clinical side, discussions highlighted how much remains unknown. SFRT’s therapeutic promise appears to stem from a combination of vascular preservation, enhanced tumour immunogenicity, and selective damage to resistant subregions. However, the field lacks validated radiobiological models that can translate these mechanisms into dose-effect predictions for planning or trial design. Participants discussed typical peak and valley dose ranges that are shown in the literature, but agreed that future progress would require stronger integration of biology, physics, and clinical data than has occurred so far. This need reinforced the importance of multi-institutional collaboration and the sharing of datasets.
Breakout sessions allowed delegates to highlight challenges and propose solutions in a structured format. Across groups, several requirements emerged consistently: standardised planning and reporting metrics, guidance for TPS setup and QA, templates for the incorporation of SFRT into clinical trials, motion-management frameworks, and a shared data infrastructure. Delegates strongly supported the idea of creating a multi-institutional registry that would capture planning parameters, delivery characteristics, toxicity profiles, and treatment outcomes. Such a registry would support evidence generation, harmonised reporting, and the development of future guidelines.
On the second day, the discussion shifted toward defining a practical roadmap. Participants emphasised the harmonisation of dose metrics and reporting conventions across SFRT modalities, including photons, protons, GRID, lattice, minibeam, microbeam, and pathy-style approaches. Although these modalities differ in mechanics and applications, the consensus was that the field would benefit from a common framework that improved comparability across centres and supported trial integration. International alignment across ESTRO, the AAPM, the European Federation of Organisations for Medical Physics, the Particle Therapy Co-operative Group and NRG Oncology was viewed as essential in order to create coherent, widely applicable, and sustainable guidance.
The workshop concluded with a set of concrete action items. These included forming expert consultation groups to support centres that were initiating SFRT programmes; establishing task forces to define minimum QA and registry standards; launching an educational webinar series; and conducting a medical physics survey to capture current practice patterns. A guidance white paper that summarises needs, priorities, and practical steps is planned as the next major deliverable.
Overall, the workshop demonstrated strong international commitment to the advancement of SFRT through harmonisation, structured guidance, and coordinated data efforts. The discussions reinforced the central role of physics in ensuring safe and reproducible implementation of SFRT and highlighted the opportunity for global collaboration to shape its future. Interested colleagues are invited to participate in upcoming working groups and to follow the development of the white paper as this community effort continues to move forward.
Yolanda Prezado
Oportunius Research Professor
Center for research in molecular medicine and chronic diseases
University of Santiago de Compostela
Spain
Heng Li, PhD, DABR, FAAPM,
Associate Professor, Chief Proton Physicist
Vice Chief of Physics/Clinical-NCR
Department of Radiation Oncology and Molecular Sciences
Johns Hopkins University School of Medicine
USA