ESTRO 38 Abstract book

S441 ESTRO 38

representative until next CBCT available fraction (as the figure shown). Recalculated dose was deformed into planning CT image by using deformable image registration from Velocity AI software version 3.2 to accumulate estimated actual dose in the treatment course. Dosimetric parameters were then studied, including V50, V60, V65, V70 and V75 for rectum and V65, V70, V75 and V80 for bladder based on QUANTEC study. Clinical toxicities of each case were reviewed from the patient records according to CTCAE version 5.0. Differences of mean volume between patients with grade 0-1 and grade 2-5 toxicities were compared using student T-test. Relationships between toxicities and volumes receiving doses were analyzed using logit analysis.

Department of Radiation Oncology- University of Toronto, Toronto, Canada ; 4 Department of Radiation Oncology- Odette Cancer Centre- Sunnybrook Health Sciences Centre, Department of Radiation Oncology- University of Toronto- Institute of Health Policy- Measurement & Evaluation- University of Toronto, Toronto, Canada Purpose or Objective Ultra-hypofractionation is appealing for prostate cancer (PCa) due to the low α/β ratio, and increasing the dose per fraction could improve the therapeutic index. Here we report the outcomes of a phase II trial in prostate SABR using a two-fraction protocol. Material and Methods Patients had low or intermediate risk prostate cancer. Three gold fiducials were implanted for image guidance. The clinical target volume (CTV) included the prostate only, and the planning target volume (PTV) was a 3mm expansion on the CTV, enabled through the use of a rectal immobilization device. The dose prescribed to D99 CTV was 26 Gy in 2 weekly fractions (EQD2 110 Gy 1.4 ). The primary endpoint was quality of life using the EPIC questionnaire, and the minimal clinically important change (MCIC) was defined as an EPIC QOL decrease > 0.5 SD. Biochemical failure (BF) was defined as nadir +2. Four- year PSA response rate (4yPSARR) was defined as < 0.4 30 patients were accrued in 2014 with a median follow-up of 49.3 months. 10% had low-risk, 33% had favourable intermediate-risk and 56% had unfavourable intermediate- risk PCa. Five patients received a short course of ADT. Median nPSA was 0.2 ng/ml. One patient had BF and is being observed. 56.6% of patients had a 4yPSARR. The mean EPIC QOL change from baseline was -1.1 for the urinary domain, -1.04 for the bowel domain, and -3.8 for the sexual domain. Six patients had a MCIC in the urinary or bowel domain, and 3 had a MCIC in the sexual domain. The cumulative rate of grade 3-4 late GI/GU toxicity was 3%, one patient had a grade 3 hemorrhoid and one had a grade 3 urinary retention. Conclusion Two-fraction SABR in prostate cancer is safe and feasible, with a minimal change in QOL experienced by patients and a low rate of late grade 3-4 toxicity. The PSA kinetics and biochemical control rates are encouraging given that the majority had unfavourable intermediate-risk disease, although longer follow-up is required. PO-0841 Salvage SBRT for local prostate cancer recurrence after radiotherapy: a GETUG retrospective study G. Martinage 1 , G. Janoray 2 , D.P. Rojas 3 , D. Zerini 3 , F. Goupy 4 , R. De Crevoisier 4 , E. Bogart 5 , G. Calais 2 , A. Toledano 6 , L. Chauveinc 6 , N. Scher 6 , P.Y. Bondiau 7 , J.M. Hannoun-Levi 7 , M. Silva 8 , E. Meyer 8 , P. Nickers 1 , T. Lacornerie 9 , E. Lartigau 1,10 , B.A. Jereczek-Fossa 3 , D. Pasquier 1,10 1 Centre Oscar Lambret, Academic Department of Radiation Oncology, Lille, France ; 2 CHRU de Tours, Oncology-Radiotherapy Department, Tours, France ; 3 European Institute of Oncology IRCCS- Istituto Europeo di Oncologia IRCCS Milano, Department of Radiotherapy- Department of Oncology and Hemato-oncology, Milan, Italy ; 4 Centre Eugène Marquis, Department of Radiotherapy, Rennes, France ; 5 Centre Oscar Lambret, Department of Biostatistics, Lille, France ; 6 Hartmann Radiotherapy Center, Department of Radiotherapy, Levallois-Perret, France ; 7 Antoine Lacassagne Cancer Center, Department of Radiation Oncology, Nice, France ; 8 Centre François Baclesse, Department of Radiation Oncology, Caen, France ; 9 Centre Oscar Lambret, Department of Medical Physics, Lille, France ; 10 Lille University, CRIStAL UMR CNRS 9189, Lille, France ng/ml. Results

Results All patients with acute rectal toxicity grade 2 or more showed significant increase in volume receiving 50, 60, 65, 70, and 75 Gy radiation compared to those with acute rectal toxicity grade 0 and 1 (p=0.002, 0.007, 0.008, 0.021, and 0.018 respectively), while in late rectal toxicity grade 2 or more showed significant increase in volume receiving 50, 60, 70 and 75Gy radiation compared to group with late rectal toxicity grade 0 and 1 (p=0.011, 0.011, 0.011, 0.003 respectively). For the correlation between rectal volume receiving 75 Gy and acute and late toxicities, the significant dose-response relationships were exhibited (p<0.001). The probability for developing toxicities of 10%, 15% and 20% were related with volume receiving 75 Gy dose of 10.9, 15.7 and 19.3 cc in acute rectal toxicity and 10.9, 14.7 and 17.5 cc in late rectal toxicity. Acute and late bladder toxicity revealed no significant relationship with volume receiving radiation in all doses (65, 70, 75 and 80 Gy).

Conclusion Estimated actual volume of rectum receiving high dose (V75) from CBCT-based recalculation was significantly related with grade 2-5 acute and late rectal toxicities in dose-response relationship. Adaptive planning should be considered for a novel approach in order to reduce toxicity. PO-0840 Two StereoTactic Ablative Radiotherapy Treatments for Localized Prostate Cancer (2STAR) Y. Alayed 1 , P. Cheung 1 , W. Chu 1 , H. Chung 1 , M. Davidson 2 , A. Ravi 2 , J. Helou 3 , L. Zhang 2 , A. Mamedov 2 , A. Commisso 2 , K. Commisso 2 , A. Loblaw 4 1 Department of Radiation Oncology- Odette Cancer Centre- Sunnybrook Health Sciences Centre, Department of Radiation Oncology- University of Toronto, Toronto, Canada ; 2 Odette Cancer Centre- Sunnybrook health sciences centre, Department of Radiation Oncology, Toronto, Canada ; 3 Princess Margaret Cancer Centre,