ESTRO 2021 Abstract Book

S391

ESTRO 2021

MRT in the case of radioresistant melanoma and glioblastoma, reaching the best reported treatment index of complete tumour remission in 50% of the tumours. Our results demonstrate that MRT administration can be adapted for clinical feasibility by irradiating with lower peak doses in multiple fractions of intersecting arrays or with combined treatment strategies. This provides alternatives to synchrotron sources where future MRT could be delivered by spatial fractionation of compact FLASH radiation sources. This modification of radiotherapy delivery seems to have the potential to overcome the inherit radioresistance of melanoma and glioblastoma.

Proffered papers: Proffered papers 30: Urology

OC-0508 ESTRO ACROP Guideline on Prostate Bed Delineation for Postoperative Radiotherapy in Prostate Cancer A. Dal Pra 1,2 , P. Dirix 3 , V. Khoo 4 , C. Carrie 5 , C. Cozzarini 6 , V. Fonteyne 7 , P. Ghadjar 8 , A. Gomez-Iturriaga 9 , V. Panebianco 10 , A. Zapatero 11 , A. Bossi 12 , T. Wiegel 13 1 University of Miami, Radiation Oncology, Miami, USA; 2 Bern University Hospital , Radiation Oncology, Bern , Switzerland; 3 Iridium Kankernetwerk, Radiation Oncology, Antwerp , Belgium; 4 The Institute of Cancer Research / Royal Marsden NHS Foundation Trust, Radiation Oncology , London, United Kingdom; 5 Leon Berard Cancer Center / University of Lyon, Radiation Oncology, Lyon, France; 6 San Raffaele Scientific Institute, Radiation Oncology, Milan , Italy; 7 Ghent University Hospital / Ghent University, Radiation Oncology, Ghent , Belgium; 8 Charité Universitätsmedizin , Radiation Oncology, Berlin, Germany; 9 Biocruces Health Research Institute / Cruces University Hospital, Radiation Oncology, Barakaldo, Spain; 10 Sapienza University / Policlinico Umberto I, Radiological Sciences, Oncology and Pathology, Rome, Italy; 11 Hospital Universitario de la Princesa , Radiation Oncology, Madrid, Spain; 12 Gustave Roussy Cancer Institute, Radiation Oncology, Villejuif, France; 13 Universitätsklinikum Ulm, Radiation Oncology, Ulm, Germany Purpose or Objective Radiotherapy to the prostate bed is a potentially curative treatment after radical prostatectomy. Although prostate bed contouring guidelines are available in the literature, important variabilities exist. The objective of this work is to provide a contemporary consensus guideline for prostate bed delineation for postoperative radiotherapy. Materials and Methods An ESTRO ACROP contouring consensus panel consisting of genitourinary radiation oncologists and a radiologist with known subspecialty expertise in prostate cancer was established. Participants were asked to delineate the prostate bed clinical target volumes (CTVs) in 3 separate clinically relevant scenarios: adjuvant radiation, salvage radiation with PSA progression, and salvage radiation with persistently elevated PSA. A case with visible local recurrence postprostatectomy was also assessed but will be presented in a separate analysis. The CT dataset was shared via FALCON platform and contours were performed using EduCase TM software. Contours were analyzed qualitatively using heatmaps which provided a visual assessment of controversial regions and quantitatively analyzed using Sorensen-Dice similarity coefficients. Participants also answered case-specific questionnaires addressing detailed recommendations on target delineation. Discussions via electronic mails and videoconferences for final editing and consensus were performed. Results Twelve participants’ contours were included. The mean CTV for the adjuvant case was 76 cc (40 - 118), salvage radiation with PSA progression was 51.80 cc (25 - 96), and salvage radiation with persistently elevated PSA 57.63 cc (31 - 98). Compared to the median, the mean Sorensen-Dice similarity coefficient for the adjuvant case was 0.60 (SD 0.10), salvage radiation with PSA progression was 0.58 (SD = 0.12), and salvage radiation with persistently elevated PSA 0.60 (SD = 0.11). A heatmap for each clinical scenario was generated. Several controversial areas of the prostate bed CTV were identified based on both heatmaps and questionnaires. This formed the basis for discussions via videoconference where the panel achieved consensus on the prostate bed CTV to be used as a novel guideline for postoperative prostate cancer radiotherapy. Conclusion Variability was observed in a group formed by experienced genitourinary radiation oncologists and a radiologist. A contemporary ESTRO ACROP consensus guideline was developed to address areas of dissonance and improve consistency in prostate bed delineation. OC-0509 10-year outcome of ultrahypofractionated stereotactic RT from two multicenter prostate cancer trials R. Meier 1 , I. Kaplan 2 , D. Bloch 3 , R. Chen 4 , B. Kane 5 , G. Henning 6 , S. Woodhouse 7 , T. Royce 8 , C. Cotrutz 9 , D. Fuller 10 1 Swedish Cancer Institute, Radiation Oncology, Seattle, USA; 2 Beth Israel Deaconess Medical Center, Radiation Oncology, Boston, USA; 3 Stanford University, Biomedical Data Science, Stanford, USA; 4 University of Kansas, Radiation Oncology, Lawrence, USA; 5 Community Medical, Radiation Oncology, Fresno, USA; 6 St. Joseph Mercy, Radiation Oncology, Ann Arbor, USA; 7 Carle Cancer Institute, Radiation Oncology, Normal, USA; 8 UNC Health, Radiation Oncology, Chapel Hill, USA; 9 Swedish Cancer Institute, Swedish Radiosurgery Center, Seattle, USA; 10 Genesis Healthcare Partners, Radiation Oncology, San Diego, USA Purpose or Objective We previously published 5-year outcomes in two large multicenter trials of ultrahypofractionated stereotactic radiotherapy for organ-confined prostate cancer. Randomized trials have subsequently demonstrated that, with similar follow up, ultrahypofractionated radiotherapy is a suitable alternative to conventional fractionation. Since data beyond five years are lacking, we now present combined 10-year survival and late toxicity outcomes from these two prospective trials. Materials and Methods