S3022
Invited Speaker
ESTRO 2026
5379 RTT workflow based on CBCT dose calculations in H&N cancer patients Suzan Gerrets Radiotherapy, NKI-AVL, Amsterdam, Netherlands This presentation explores the evolution of adaptive radiotherapy workflows, focusing on the transition from traditional Traffic Light Protocols (TLP) to Action Protocols (AP) supported by CBCT dose recalculations. While TLP has traditionally guided clinical decision- making through visual assessment, the integration of dose recalculation enables more understanding of which anatomical changes truly require intervention, allowing the AP protocol to be redefined so that action is only taken when clinically meaningful. A secondary theme is the changing distribution of roles within ART workflows, particularly in relation to RTT-led dose recalculations, protocol-driven decision- making and decision support with regard to practical limitations. Using the head and neck cancer workflow as a clinical example, the presentation highlights the significant anatomical changes, how to register and how these changes inform adaptive strategies. From an RTT perspective, it addresses CBCT interpretation, workflow implementation, responsibility distribution and practical experiences. 5381 Modelling microbiome-driven variability in radiotherapy outcomes Tiziana Rancati Milano, Milano, Milano, Italy Radiotherapy outcomes show substantial inter-patient variability that cannot be fully explained by clinical and dosimetric factors alone. Growing evidence indicates that the human microbiome, particularly the gut and tumor-associated microbial ecosystems, modulates radiation response, influencing inflammation, immune activation, and tissue repair. Beyond direct modulation, the microbiota may also serve as an indirect surrogate for the tumor “immune–hypoxia temperature,” a conceptual axis reflecting the balance between hypoxic stress and immune activation within the tumor and its microenvironment. Specific microbial configurations have been linked to hypoxia- inducible signaling and T-cell infiltration patterns, suggesting that microbial signatures could encode systemic readouts of tumor states that are otherwise difficult to measure non-invasively. Technology makes it feasible to characterize the microbiota at the “bench side, and data on cohorts of patients receiving radiotherapy are becoming available to study and quantify microbiome-driven variability in
radiotherapy outcomes and to integrate microbial, clinical, and dosimetric data. Microbiome data are also fostering ways to describe complexity and derive features that can be used as biomarkers in the clinic, such as alpha/beta diversity indices, relative taxonomic abundance, and functional pathway enrichment scores. The presentation will cover the experience at the National Cancer Institute in Milan, with cohorts of prostate and head and neck cancers. We considered both associations with radio-induced side effects and with tumor control. 5383 Towards maximizing treatment success through microbiota modulation Anne E Kiltie Rowett Institute, University of Aberdeen, Aberdeen, United Kingdom Radiotherapy is a curative option for patients with pelvic cancers and is particularly useful in the elderly who cannot tolerate the necessary surgery, or sometimes even chemotherapy/chemoradiation. With an ageing population, it is important to find a means of improving cure rates with radiotherapy, without increasing side effects. One such approach would be to give patients a dietary fibre supplement, which the gut microbiota can ferment to produce beneficial metabolites including short chain fatty acids. Whilst attempts have been made to increase fibre in patients’ diets before radiotherapy, this is difficult to achieve at a time when patients are under a lot of stress. However, taking a supplement as a powder or capsule is more likely to be tolerated, as many patients are already taking medication, so this could be introduced as part of their routine. We have demonstrated in preclinical in vivo studies, that mice fed various dietary fibres have slower tumour growth with or without radiotherapy, but in mice where the lower abdomen is irradiated, those on high fibre diets have higher regenerating crypt numbers at 3.75 days than those fed a low fibre diet. So we are seeing a double effect on the therapeutic ratio, both improved tumour control and reduced side effects. We have also shown that it is feasible to approach patients at the start of radiotherapy and for them to complete food frequency questionnaires and diet diaries and to provide a faecal sample just before their radiotherapy starts. Patient and public involvement (PPI) sessions have also taught us that patients would be happy to take dietary supplements as part of a clinical trial. As a result of this work, we are about to embark on an eight-centre, randomised controlled clinical trial of 220
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