ESTRO 2025 - Abstract Book

S2726

Physics - Dose prediction, optimisation and applications of photon and electron planning

ESTRO 2025

[2] Spruijt KH, Dahele M, Cuijpers JP, et al. Flattening Filter Free vs Flattened Beams for Breast Irradiation. International Journal of Radiation Oncology Biology Physics . 2013;85(2):506-513.

[3] Koivumäki T, Heikkilä J, Väänänen A, Koskela K, Sillanmäki S, Seppälä J. Flattening filter free technique in breath hold treatments of left-sided breast cancer: The effect on beam-on time and dose distributions. Radiotherapy and Oncology . 2016;118(1):194-198.

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Proffered Paper Evaluating ultra-hypofractionated focal boost to intraprostatic lesions for high-risk localized prostate cancer with histological reference Erik Nilsson 1 , Anneli Nilsson 2 , Joakim Jonsson 2 , Kristina Sandgren 2 , Josefine Grefve 2 , Jan Axelsson 2 , Angsana K Lindberg 2 , Karin Söderkvist 3 , Camilla T Karlsson 3 , Björn Zackrisson 3 , Sara Strandberg 4 , Katrine Riklund 4 , Anders Bergh 5 , Lars E Olsson 6 , Mathieu Moreau 7 , Adalsteinn Gunnlaugsson 7 , Tufve Nyholm 2 1 Department of Diagnostics and Intervention, Umeå University, Umeå, Sweden. 2 Department of Diagnostics and Intervention, Radiation Physics, Umeå University, Umeå, Sweden. 3 Department of Diagnostics and Intervention, Oncology, Umeå University, Umeå, Sweden. 4 Department of Diagnostics and Intervention, Diagnostic Radiology, Umeå University, Umeå, Sweden. 5 Department of Medical Biosciences, Pathology, Umeå University, Umeå, Sweden. 6 Department of Translational Medicine, Medical Radiation Physics, Lund University, Malmö, Sweden. 7 Department of Hematology, Oncology and Radiation Physics, Lund University, Lund, Sweden Purpose/Objective: Hypofractionated radiotherapy with focal boosts for prostate cancer (PCa) has been shown to increase five-year biochemical disease-free survival with acceptable toxicity levels 1 . In the present study, we evaluated properties of optimized dose distributions by using whole-slide histopathology as reference standard. Material/Methods: Fifteen high-risk (ISUP grade group (IGG) ≥ 4) radical prostatectomy patients with preoperative [ 68 Ga]PSMA PET/mpMRI and [ 11 C]Acetate-PET/CT were included. Prostate molds were 3D printed for each patient, based on the prostatic contour on T2-weighted (T2w) images. Ex vivo T2w were obtained for resected prostates within the molds, followed by histopathological preparation and evaluation. Prostates were sectioned into 5 mm blocks, from which 5 µm thick microtome sections were taken such that sections coincided with ex-vivo T2w image slices. Microtome sections were digitized, and Gleason graded, resulting in detailed annotations of lesions with IGGs. The graded microtome sections were registered to ex-vivo images, which in turn were registered to the in-vivo T2w. The in-vivo T2w also served as a common frame of reference for co-registering PSMA-PET/mpMRI and CT. Treatment planning was performed in Raystation (RaySearch Laboratories, Sweden) and image analysis in Hero (Hero Imaging AB, Sweden). We delineated organs at risk, seminal vesicles and lymph nodes. For each patient, four radiation oncologists contoured visible tumor on PSMA-PET/mpMRI as gross tumor volume (GTV). Ultra hypofractionated treatment plans were generated to deliver 42.7 Gy to the prostate gland in 7 fractions, with a simultaneously integrated focal boost up to 49 Gy to the GTV, 29.4 Gy to lymph nodes and 31.2 Gy to seminal vesicles. Results: The optimization resulted in 60 treatment plans, one for each patient and observer. The median fraction of histopathologically confirmed tumor also defined as GTV was estimated to be 0.61 (0.48-0.77) 2 . The average median dose to GTV was 48.8 (IQR: 48.5-48.8) Gy. Here, the median was taken across patients and the average over observers. The corresponding value for the highest-grade regions was 48.6 (IQR: 47.5-48.9) Gy, and for the lowest-grade regions 43.0 (IQR: 42.8-43.5) Gy. Dose-volume histograms for GTV and all grades are shown in Figure 1.