ESTRO 2025 - Abstract Book

S1930

Clinical - Urology

ESTRO 2025

School of Medicine, Isehara, Japan. 7 Radiation Oncology Department, Seirei Hamamatsu General Hospital, Hamamatsu, Japan. 8 Department of Radiology, Nagoya City University Graduate School of Medical Sciences, Nagoya, Japan. 9 Department of Radiation Oncology, Yokohama Municipal Citizen's Hospital, Yokohama, Japan. 10 Division of Radiation Safety and Quality Assurance, National Cancer Center Hospital, Tokyo, Japan Purpose/Objective: Dose escalation in prostate cancer treatment has been shown to improve PSA control. This study aimed to compare two dose-escalation methods: focal dose escalation to MRI-identified lesions versus peripheral dose escalation to the prostate parenchyma, sparing the urethra, bladder, and rectum. Material/Methods: Prostate cancer patients with unfavorable intermediate risk, high risk, or very high risk received stereotactic body radiotherapy (SBRT) of 36.25 Gy in 5 fractions. Group A received dose escalation up to 55 Gy targeting MRI-identified lesions. Group B received dose escalation up to 55 Gy to the prostate parenchyma excluding areas within 3 mm of the bladder, urethra, and rectum. The primary endpoint was a Grade 2 or higher gastrointestinal or genitourinary adverse event (AE) related to radiotherapy. This was designed as a randomized phase II trial. Results: A total of 28 patients in Group A and 37 in Group B were enrolled, with 27 in Group A and 37 in Group B receiving the protocol treatment. The median age was 75 years in Group A and 76 years in Group B. The median PSA was 9.7 ng/mL (range: 6.1-41.3) in Group A, 10.9 ng/mL (range: 5.6-45.2) in Group B. Group A had 9 patients and Group B had 15 with unfavorable intermediate risk; 14 and 15 patients, respectively, were classified as high risk, and 4 and 7 patients as very high risk. Hormone therapy was administered to 26 patients in Group A and 35 in Group B. At a median follow-up of 12 months, the incidence of Grade 2 or higher gastrointestinal or genitourinary AEs related to radiotherapy was 7.4% in Group A and 13.5% in Group B (p=0.68) (Acute Grade 2 AEs: 3.7% in Group A and 13.5% in Group B, Late Grade 2 AEs: 3.7% in Group A and 0% in Group B). Conclusion: There was no significant difference in Grade 2 or higher radiotherapy-related AEs between Groups A and B. A trend toward a slightly higher rate of acute AEs was observed in Group B, but no differences were seen in the late AEs. Dose escalation to the prostate parenchyma sparing the urethra, bladder, and rectum (Group B) has advantages over focal dose escalation to MRI-identified lesions (Group A) in terms of the variability in MRI target delineation and the ability to treat lesions not delineated by MRI. Based on these findings, Group B was selected as the preferred treatment strategy for the upcoming RCT.

Keywords: SBRT, prostate, dose escalation

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Digital Poster Influences of the intestinal microbiome on radiogenic side effects after pelvic radiotherapy for recurrent or incompletely resected prostate cancer Sarah D Riedl 1 , Jonathan N Möller 1 , Oliver C Holmes 1 , Mohammadali K Mirzaei 2 , Marco M E Vogel 1 , Kai Borm 1 , Klaus Peter Janssen 3 , Stephanie E Combs 1,4 , Li Deng 2 , Julius C Fischer 1 1 Department of Radiation Oncology, Technical University of Munich, Klinikum rechts der Isar, Munich, Germany. 2 Institute of Virology, Helmholtz Zentrum München, Neuherberg, Germany. 3 Department of Surgery, Technical University of Munich, Klinikum rechts der Isar, Munich, Germany. 4 Institute for Radiation Medicine (IRM), Helmholtz Zentrum München, Neuherberg, Germany