ESTRO 2021 Abstract Book

S1091

ESTRO 2021

Conclusion A SABR schedule of 35 Gy with a boost dose to the DIL up to 42.5 Gy in 5 fractions resulted to be safe and feasible in low and intermediate-risk prostate cancer. The maximum tolerable dose has not yet been reached and the study is actually ongoing. PO-1329 Longterm Biochemical Recurrence-Free Survival after PSMA-PET/CT–Based Salvage Radiotherapy S. Adebahr 1 , C. Zamboglou 1 , J. Ruf 2 , C. Gratzke 3 , S. Kirste 1 , S. Spohn 1 , C. Stief 4 , P. Bartenstein 5 , C. Belka 6,7 , M. Li 6 , C. Trapp 6 , P. Rogowski 8 , A. Grosu 9,11 , A. Grosu 10 , N. Schmidt-Hegemann 12 1 Medical Center, Faculty of Medicine, University of Freiburg, Department of Radiation Oncology, Freiburg, Germany; 2 Medical Center, Faculty of Medicine, University of Freiburg, Department of Nuclear Medicine, Freiburg, Germany; 3 Medical Center, Faculty of Medicine, University of Freiburg, Department of Urology, Freiburg, Germany; 4 University Hospital 81377 LMU Munic, Department of Urology, Munic, Germany; 5 University Hospital 81377 LMU Munic, Department of Nuclear Medicine, Munic, Germany; 6 University Hospital 81377 LMU Munic, Department of Radiation Oncology, Munic, Germany; 7 German Cancer Consortium (DKTK), Partner Site Munic, Munic, Germany; 8 University Hospital 81377 LMU Munic, Deaprtment of Radiation Oncology, Munic, Germany; 9 Medical Center, Faculty of Medicine, University of Freiburg, Department of Radiation Oncology , Freiburg, Germany; 10 German Cancer Consortium (DKTK), Partner Site Freiburg, Freiburg, Germany; 11 German Cancer Research Center , (DKFZ), Heidelberg, Germany; 12 University Hospital 81377 LMU Munic,, Department of Radiation Oncology , Munic, Germany Purpose or Objective Prostate-specific membrane antigen (PSMA)-PET/CT detects prostate cancer recurrence at low levels of prostate-specific antigen (PSA) after primary surgery and PSMA-PET/CT- guided salvage radiotherapy (sRT) has been reported to be a safe local treatment option with effective targeting of PET-positive lesions. Here we present long-term biochemical recurrence-free survival (BRFS) after sRT and prognostic factors for this regimen. Materials and Methods In this biinstitutional retrospective study we analyzed long term BRFS in patients, who underwent PSMA- PET/CT guided sRT due to biochemical recurrence or PSA persistence after radical prostatectomy. Patients with metastases, time-gap between PSMA-PET/CT and RT > 3 months and FU < 3 years were excluded. Primary endpoint was BRFS, defined as PSA nadir + 0.2 ng/mL. All known prognostic factors for BRFS after non-PSMA- PET based sRT derived from the MSKCC nomogram (except surgical margin), as well as positive findings and positive lymph nodes (LN) in PSMA-PET were evaluated regarding their prognostic value for BFRS after PSMA- PET guided sRT in univariate cox analyses, factors with significant findings were re-analyzed in a multivariate cox analysis. Results In 165 eligible patients, PET-positive lesions were detected in 101 patients (61%), 31 and 55 of 101 patients hadfossa recurrences and pelvic LN only, respectively; 17 of 101 presented both, fossa recurrences and pelvic LN. PSA before RT was <0.2, 0.21-0.5, 0.51-1 or >1 ng/mL for 16, 61, 37 and 51 patients. 162 patients were treated with sRT of the prostatic fossa or boost to PET positive lesion in the fossa; 68 patients additionally, 3 patients solely received sRT of pelvic lymph nodes (LN), elective RT of LN was performed in 81 patients. After a median follow-up of 49 months, BRFS was 44.8%. No significant difference in BRFS between PET-positive patients (44.5%) and PET-negative patients (45.3%; p .92) was observed. In the multivariate analysis PSA value before sRT, PSA doubling time, Gleason score, pT and pN stage, hormone therapy and PET-positive LN were taken into account: only PSA before sRT (HR: 0.28, p 0,017), PSA doubling time (HR: 3.001, p 0.038) and pT stage (HR: 0.246, p 0.011) were significantly associated with BFRS. Conclusion PSMA-PET/CT-guided sRT is an effective treatment option with potential long term BRFS. No difference in BRFS between PET-positive and PET-negative patients was observed, indicating effective targeting of PET- positive lesions. Pre-sRT PSA and PSA doubling time are relevant for considering early sRT even in the PSMA- PET era. PO-1330 Radiotherapy toxicity in prostate cancer patients with bilateral hip prostheses A. Fischer 1 , P. Hoskin 2 1 Mount Vernon Cancer Centre, Physics, London, United Kingdom; 2 University of Manchester, Division of Cancer Sciences, Manchester, United Kingdom