ESTRO 2021 Abstract Book

S1125

ESTRO 2021

after primary radiotherapy. Patients were staged with 3T mp-MRI (recurrence suspicion on T2, dynamic contrast enhanced and/or diffusion weighted imaging) and PSMA PET-CT. Both imaging modalities were fused and used to delineate the gross tumor volume (GTV) on MRI. The GTV was transperineally biopsied using transrectal ultrasound fused with MRI. Treatment was performed using MRI-guidance of catheter positions. Results A total of 42 patients were identified from the database who had both mp-MRI and PSMA PET-CT positive recurrence in the prostate and underwent cognitive (n=7) targeted or MRI-TRUS fusion targeted (n=34) biopsies (n=1 unknown). A total of 37 (88%) patients had positive biopsies for cancer. Four patients were initially negative (all MRI-TRUS fusion targeted) and were re-biopsied after which recurrent prostate cancer was verified. Only one (2.4%) patient had negative biopsies and refused re-biopsy. We observed higher PSMA standardised uptake values (SUV) in patients with Gleason sum score 7-9 compared to 6: median 9.3 versus 4.9 (p=0.02). An example of the adopted imaging modalities is shown in figure 1.

Conclusion In patients with radiorecurrent prostate cancer, with a concordant lesion on mp-MRI and PSMA PET-CT, targeted biopsies have a minimal detection rate of 97.6%. Biopsies can therefore safely be withheld in these patients, thereby avoiding an unnecessary invasive and burdensome procedure. Higher PSMA SUV values might lead to higher detection rates. PO-1374 MR-guided SBRT for localized prostate cancer: the first results from the MOMENTUM study F. Teunissen 1 , T. Willigenburg 1 , A. Tree 2 , W. Hall 3 , S. Choi 4 , A. Choudhury 5,6 , J. Christodouleas 7,8 , J. de Boer 1 , E. de Groot-van Breugel 1 , L. Kerkmeijer 9,1 , F. Pos 10 , D. Vesprini 11 , H. Verkooijen 12 , J. van der Voort van Zyp 1 1 University Medical Center Utrecht, Radiation Oncology, Utrecht, The Netherlands; 2 The Royal Marsden NHS Foundation Trust and the Institute of Cancer Research, Urological Oncology, London, United Kingdom; 3 Medical College of Wisconsin, Radiation Oncology, Milwaukee, USA; 4 The University of Texas MD Anderson Cancer Center, Radiation Oncology, Houston, USA; 5 University of Manchester, Division of Cancer Sciences, Manchester, United Kingdom; 6 The Christie NHS Foundation Trust, Manchester Academic Health Sciences Centre, Clinical Oncology, Manchester, United Kingdom; 7 University of Pennsylvania, Radiation Oncology, Philadelphia, USA; 8 Elekta AB, Radiation Oncology, Stockholm, Sweden; 9 Radboud University Medical Center, Radiation Oncology, Nijmegen, The Netherlands; 10 Antoni van Leeuwenhoek Hospital and The Netherlands Cancer Institute, Radiation Oncology, Amsterdam, The Netherlands; 11 Sunnybrook Health Sciences Centre, Radiation Oncology, Toronto, Canada; 12 University Medical Center Utrecht, Imaging and Oncology Division, Utrecht, The Netherlands Purpose or Objective Magnetic resonance (MR) guided adaptive radiotherapy (MRgRT) is a new technique for treatment of localized prostate cancer (PCa). MR-guided linear accelerator (MR-Linac) systems have been implemented in radiotherapy departments around the world. However, the theoretical benefits of MRgRT still need to be confirmed in clinical practice. We report the short-term outcomes for the first PCa patients treated within an international consortium on a 1.5T MR-Linac system with ultrahypofractionated radiotherapy. Materials and Methods Patients treated with 5x7.25 Gray were identified within the registry. Prostate specific antigen (PSA), Common Terminology Criteria for Adverse events (CTCAE) and patient reported outcome (PRO) using the European Organisation for Research and Treatment of Cancer (EORTC) Quality of Life Questionnaire (QLQ)- PR25, EORTC QLQ-C30 and the EuroQol EQ5D-5L were prospectively recorded at baseline and at 3 and 6 months follow-up (FU). Descriptive and pairwise comparative statistics were conducted. Results One-hundred-and-fifty-six consecutive patients with localized PCa (13.2% low-, 77.2% intermediate-, and 9.6% high-risk [National Comprehensive Cancer Network risk groups]) were included. Thirty-one patients (19.9%) received neoadjuvant androgen deprivation therapy (ADT). A significant decline of PSA in non-ADT patients was observed between baseline (median: 7.8 ng/mL), 3 months FU (median: 2.7 ng/mL) and 6 months FU (median: 1.7 ng/mL) (p<0.001). No grade ≥3 genitourinary (GU) or gastrointestinal (GI) toxicity was reported (table). No significant deterioration of PRO scores were observed. The percentage of men reporting no difficulty getting or maintaining an erection remained constant throughout FU (44.4% at baseline, 40.0% at 3 months FU, and 42.9% at 6 months FU). Table: Clinician-reported toxicity using the CTCAE