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and PET/CT lesion detection outside the S-RT delivery site, as defined before 68Ga-PSMA PET/CT image analysis. Results 119 PCa patients were enrolled (mean age 66 years old; range 44-78) and mean PSA value before 68Ga-PSMA- PET/CT was 0.33 ng/mL (median 0.32; SD ±0,09). Nine/119 (7.6%) were on ADT at the time of scan. Thirty- eight/119 (31.9%) already received adjuvant radiotherapy after RP. 68Ga-PSMA-PET/CT was positive in 41/119, thus resulting in an overall detection rate of 34.4%. 68Ga-PSMA uptake was observed in prostatic fossa (3/119; 2.5%), in pelvic lymph nodes (21/119; 17.6%), in distant lymph nodes (retroperitoneal and common iliac chain: 4/119; 3.4%) and in bone (21/119; 17.6%). Multiple bone lesions (≥2 bone lesions) were observed in 8/119 patients (6.7%). Regarding ITT, blinded to 68Ga-PSMA- PET/CT results, 81/119 patients (68.1%) were candidates for S-RT in prostatic fossa, 0/111 (0%) for SBRT and PLND, while 35/119 patients (29.4%) were eligible for ADT alone. Considering 68Ga-PSMA-PET/CT results, S-RT was suggested in 70/119 (58.8%), SBRT in 29/119 (24.4%), PLND in 2/119 (1.7%) and ADT alone in 21/119 (17.6%) of patients. When 68Ga-PSMA-PET/CT was positive, RT planning was modified in 87.8% of cases (36/41 positive scans). Conclusion Patients undergoing RP with BCR and PSA lower than 0.5 ng/mL can benefit from 68Ga-PSMA-PET/CT. It shows encouraging sensibility and give the possibility to better define the therapeutic choice. Furthermore, based on its results, most patients had a modified RT treatment planning. Further analyses are warranted to confirm these results. PV-0623 Toxicity and Quality of Life of Salvage Pelvic Irradiation of Prostatic Cancer Node Relapse L. Vaugier 1 , C. Palpacuer 2 , E. Rio 1 , V. Pacteau 3 , M.H. Mauboussin 3 , L. Campion 2 , F. Pein 3 , S. Supiot 1,4 1 ICO René Gauducheau, Radiation Oncology, Saint Herblain, France 2 ICO René Gauducheau, Biostatistics, Saint Herblain, France 3 ICO René Gauducheau, Research department, Saint Herblain, France 4 Centre de recherche en cancérologie Nantes-Angers, UMR 892 Inserm, Nantes, France Purpose or Objective Pelvic lymph nodes-only (PLN) relapses of prostatic adenocarcinoma – as pointed out by Fluorocholine (FCH)- based PET – could be a paradigm of a so called oligometastatic disease. The OLIGOPELVIS-GETUG P07 trial investigated high-dose image-guided intensity modulated salvage pelvic irradiation with an additional dose to the FCH-PET- positive PLN (30x1.8 Gy = 54 Gy; up to 33x2=66 Gy onto the pathological PLN) combined to 6 months androgen deprivation therapy (ADT). Our purpose is to report on the acute and 6-months toxicity of such salvage strategy. Material and Methods OLIGOPELVIS-GETUG P07 was a prospective multicenter phase 2 trial. The primary endpoint was the 2-year relapse-free survival. Medical examination was achieved weekly during RT, at 1 month (M1) and 6 months (M6) after the end of RT, and every 6 months thereafter. Acute toxicity was defined until M1. Quality of life of the patients was considered through patient-based EORTC questionnaires (QLQ-C30 and QLQ- PR25). Results 74 patients were included along 15 French medical centers between August 2014 and July 2016. 67 patients were analysed. 35/67 patients (52%) had received prior

prostatic irradiation (29 salvage prostatic bed RT after radical prostatectomy; 3 prostate RT; 3 prostate brachytherapy). Acute and M6 urinary toxicity was mainly grade ≤ 2 affecting respectively 49/67 (73%) and 32/67 (48%) patients. It was dominated by urinary urgencies. Only 1 patient experienced a grade 3 urinary incontinence at M6 that required intravesical injections. Acute and M6 digestive toxicity was mainly grade ≤ 2 affecting respectively 57/67 (85%) and 18/67 (27%) patients, mostly moderate diarrhea or bowel incomfort. The patients with prior prostate or prostatic bed irradiation did not exhibit any increased urinary or digestive toxicity. 12/67 (18%) grade 3 ADT-related hypertension, thus requiring an adaptation of medical drugs, was observed at M1 and was cured at M6. Only dyspnea and sexual functioning, but not bladder or rectum-related symptoms, significantly worsened according to the EORTC questionnaires. Conclusion The profile of acute and 6-months toxicity of OLIGOPELVIS-GETUG P07 is highly satisfactory, even in patients with a past history of prostate bed irradiation. The 2-year disease-free survival evaluation needs the corresponding follow-up to be achieved. Such salvage strategy has to be compared to the standard long-term ADT through a prospective phase 3 trial. PV-0624 Longitudinal analysis of the microbiota by GI toxicity during IMRT for high-risk prostate cancer. M. Reis Ferreira 1 , H.J.N. Andreyev 2 , S. Gulliford 3 , K. Mohammed 4 , J. Marchesi 5 , D.P. Dearnaley 1 1 Institute of Cancer Research and Royal Marsden NHS Foundation Trust, Academic Radiotherapy, London, United Kingdom 2 The Royal Marsden NHS Foundation Trust, Gastrointestinal Unit, London, United Kingdom 3 Institute of Cancer Research, Radiotherapy Physics Modelling Team, London, United Kingdom 4 The Royal Marsden NHS Foundation Trust, Statistics and Computing, London, United Kingdom 5 Imperial College, Department of Surgery and Cancer, London, United Kingdom Purpose or Objective In the UK, about 15,800 men/year receive radiotherapy for prostate cancer (PCa). High-risk patients receive extended-field pelvic radiotherapy, with about 40% and 20% experiencing significant acute and late gastrointestinal side-effects respectively. We assessed differences in the microbiota of patients with and without radiotherapy-induced gastrointestinal side- effects induced for high-risk PCa, with the aim of identifying bacteria asscociated with toxicity. Material and Methods A single-center exploratory prospective study included a cohort of PCa patients to be treated with pelvic radiotherapy. Clinical assessment (patient-reported IBDQ- bowel scale) and stool sampling was performed pre- radiotherapy and at 2/3 weeks, 4/5 weeks, 12 weeks, 6 months and 12 months post-radiotherapy initiation. Microbiota community information was obtained by sequencing of the V1-2 regions of the 16s-rRNA gene. Patients were divided in three groups: no symptoms, moderate symptoms (symptoms at 4/5 weeks or 6 months) and persistent symptoms (symptoms at 4/5 weeks and 6 months). Differences in bacterial α-diversity (Chao index) were assessed at each timepoint (Kruskal- Wallis test). Longitudinal changes in diversity and in the microbiota of interest were assessed with linear mixed- effect models. Results Recruitment ran from 04/2014 to 01/2015. 32 patients were recruited in the longitudinally-followed cohort.