S1446
Interdisciplinary - Other
ESTRO 2026
prostate cancer. Guidelines recommend ultrahypofractionated EBRT (35–36.25 Gy in 5 fractions of 7–7.25 Gy) for low- and intermediate-risk patients, emphasizing advanced technology and experienced teams. From a physics perspective, prostate SBRT poses two main challenges: mitigation of geometric uncertainties and delivery of highly conformal dose distributions to spare organs at risk (OARs).Minimizing setup and intrafraction motion is crucial to avoid large PTV margins.This study evaluated the relationship between prostate displacements measured on daily cone-beam CTs (CBCTs), acute genitourinary (GU) and gastrointestinal (GI) toxicity, and PSA reduction after prostate SBRT without fiducial markers or rectal spacers. Material/Methods: A retrospective analysis was conducted in 18 patients with localized prostate cancer treated with SBRT (180 CBCTs). All followed a standardized protocol: empty rectum, full bladder after 0.5 L of water, and urethral contrast catheter at simulation. The clinical target volume (CTV) included the prostate and seminal vesicle bases in favorable intermediate-risk cases, and the prostate only in low-risk cases, with a 5 mm isotropic margin, except 3 mm posteriorly, to generate the PTV. Intra- and interfraction prostate displacements were quantified along anteroposterior (AP), vertical, and longitudinal axes. Spearman’s correlation assessed associations between displacements and acute GU/GI toxicity (CTCAE v5.0), Pearson’s coefficient evaluated correlations with PSA reduction. Results: Median follow-up was 6 months (range, 2–13). Mean age was 73 years (62–82), prostate volume 39.3 cc (17– 60), and PSA at diagnosis 8.0 ng/mL (4–16). Half the patients were low-risk and half favorable intermediate- risk. Mean interfraction displacements were: AP 0.04 mm (–0.84 to 0.50), vertical –0.04 mm (–0.32 to 0.46), and longitudinal 0.22 mm (–0.66 to 0.52). Mean intrafraction displacements: AP 0.04 mm (–0.24 to 0.46), vertical –0.04 mm (–0.16 to 0.24), and longitudinal 0.04 mm (–0.20 to 0.20). No significant correlations were found between prostate motion and PSA reduction (p = 0.722), GU toxicity (p = 0.566), or GI toxicity (p = 0.800). The greatest variability occurred along the AP axis, likely related to rectal filling.
Conclusion: In this cohort, daily CBCT-based prostate motion showed no significant association with early toxicity or short-term PSA response. These findings reflect the small sample size and low event rate rather than a true absence of effect. Results support the feasibility, reproducibility, and safety of markerless, image- guided SBRT workflows, warranting validation in larger prospective studies with longer follow-up. Keywords: prostate SBRT, intrafraction, displacements The Human Dimensions of MR-LINAC Integration: Early Insights from a Longitudinal Evaluation of Staff and Patient Quality of Life Francois Fabi 1 , Damien Carignan 2 , Janelle Morrier 3 , Eric Poulin 3 , Laurence Bernier 1,2 , Isabelle Thibault 1,2 , Thomas Reynaud 2 , Elizabeth Guimond 1,2 , William Foster 1,2 1 Radiation Oncology, Centre Hospitalier Universitaire (CHU) de Québec-Université Laval, Québec, Canada. 2 CRCHUQ, Centre Hospitalier Universitaire (CHU) de Québec-Université Laval, Québec, Canada. 3 Service of Medical Physics and Radioprotection, Centre Hospitalier Universitaire (CHU) de Québec-Université Laval, Québec, Canada Purpose/Objective: The implementation of MR-guided linear accelerators (MR-LINACs) represents a major advance in precision radiotherapy[1]. While clinical and dosimetric benefits are increasingly documented, the broader human impact on professionals and patients remains largely underexplored. This work reports the early institutional experience of MR-LINAC integration in a Digital Poster 2435 Canadian academic center, within a pragmatic framework aimed at assessing the work-related quality of life (WRQoL)[2], satisfaction, and well-being of both healthcare workers and patients during implementation[3]. Material/Methods: Two complementary datasets were analyzed within a
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