S1181
Clinical - Urology
ESTRO 2026
4 Department of Radiation Oncology, AZ Sint-Marteen, Mechelen, Belgium. 5 Department of Radiation Oncology, The Netherlands Cancer Institute, Amsterdam, Netherlands. 6 Department of Radiation Oncology, Medical Center – University of Freiburg, Freiburg, Germany. 7 German Oncology Center, European Universty of Cyprus, Limassol, Cyprus. 8 Department of Radiation Oncology, CHRU Tours, Tours, France. 9 Department of Radiation Oncology, University Hospital LMU Munich, Munich, Germany. 10 Department of Clinical Sciences, Lund University, Lund, Sweden. 11 Department of Haematology, Oncology and Radiation Physics, Skåne University Hospital, Lund, Sweden. 12 Department of Radiation Oncology, Radboud University Medical Center, Nijmegen, Netherlands. 13 Department of Radiation Oncology, The Royal Marsden Hospital, London, United Kingdom. 14 Department of Radiation Oncology, University Hospitals Leuven, Leuven, Belgium. 15 Radiation Oncology, Fondazione IRCCS San Gerardo dei Tintori, Monza, Italy Purpose/Objective: The FLAME trial (NCT01168479) was the first phase III trial to demonstrate that iso-toxic focal boosting of the macroscopic visible tumour with external beam radiotherapy improves biochemical disease-free survival in patients with prostate cancer. In the trial, based on conventionally fractionated regimens, higher near-minimum GTV focal boost doses were associated with lower biochemical failure rates, and the association was later demonstrated also for ultra- hypofractionation (hypo-FLAME (NCT02853110)). In iso-toxic boosting, normal tissue dose constraints – mainly for the rectum, bladder, and urethra – are prioritised over focal boost dose. This study investigated whether interobserver variability in MRI- based urethra contouring impacts the near-minimum GTV dose. Material/Methods: Eight radiation oncology professionals independently delineated the urethral contour on T2-weighted MRI scans of 20 patients enrolled in the ABRUPT trial (NCT04831983). For each case, three treatment plans were generated using the urethral contours closest to, farthest from, and at an intermediate distance from the GTV, based on mean distance-to-agreement (MDA). Plans were optimised to achieve the highest possible near-minimum (D99%) GTV dose while adhering to the near-maximum urethral hypo-FLAME dose constraint (D0.1cc <42 Gy), incorporating a 2 mm PRV margin around the urethra. Median difference between maximum and minimum GTV D99% ( Δ D99%) across the three plans was reported, along with proportion of patient cases in which GTV Δ D99% exceeded 1 Gy and 2 Gy. For each urethral contour, D0.1cc were extracted from the three plans, and both
the frequency and magnitude of dose constraint violations were recorded. Results: All plans met the hypo-FLAME target objectives, with absolute GTV D99% ranging from 40.2 Gy to 50.2 Gy. Median MDA difference between the closest and farthest urethra contour was 4.3 mm (range: 2.7– 8.8mm). Median GTV Δ D99% was 4.4 Gy (range: 0.6– 8.9Gy). Dose differences exceeding 1 Gy and 2 Gy due to urethral delineation variability were observed in 90% and 75% of the cases, respectively. Urethral constraint violations occurred in 33% of cases when optimising on a different urethra contour. Among these, the median D0.1cc was 46.1 Gy (range: 42.1– 51.3Gy), representing a median constraint exceeding of 4.1 Gy (range: 0.1–9.3Gy).
Conclusion: Interobserver variability in MRI-based urethral delineation resulted in clinically relevant variations in GTV dose coverage and may lead to urethral dose violations in focal boost prostate radiotherapy. These findings underscore the need for standardised urethral contouring protocols and robust planning strategies to ensure treatment safety and consistency, while enabling the full therapeutic potential of iso- toxic focal boosting. Keywords: Prostate Cancer, Focal Boost, SBRT
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