ESTRO 36 Abstract Book

S859 ESTRO 36 2017 _______________________________________________________________________________________________

symptoms, factor analyses was used. Dose-response relationships were estimated fitting the data to the Probit model. ROC curve analyses were used to identify which organ at risk is correlated the most with the clinical outcome. Results The maximum likelihood estimates of the dose-response parameters for Probit model, the three organs at risk and ‘radiation induced urgency syndrome’, the Log Likelihood (LL) value and the AUC are: D 50 = 51.3 (48.3-54.6), γ 50 = 1.19 (0.98-1.42), α/β=0.59 (0.034-1.56), LL = -50.2 and AUC=0.63 for rectum, D 50 = 51.6 (48.7-54.9) Gy, γ 50 = 1.20 (0.98- 1.44), α/β =2.02 (0.85-4.73), LL = -51.0 and AUC=0.66 for the sigmoid and D 50 = 61.4 (56.4-67.5) Gy, γ 50 = 0.90 (0.73- 1.08), α/β = 10.3 (2.1- 1e+06 Gy), LL = - 51.4 and AUC=0.60 for small intestines.

Conclusion The rectum dose maps and the connected NTCP models used in this study identified clinically relevant changes in rectum dose distributions caused by organ motion during the course of therapy. This model validation study showed that these maps and models are useful tools to evaluate the risk of normal tissue reactions in the rectum. EP-1611 Dose-response relationships for radiation- induced urgency syndrome after gynecological radiotherapy E. Alevronta 1 , V. Skokic 1 , U. Wilderäng 1 , G. Dunberger 2 , F. Sjöberg 1 , C. Bull 1 , K. Bergmark 1,3,4 , G. Steineck 1,4 1 Institute of Clinical Sciences- Sahlgrenska Academy at the University of Gothenburg, Department of Oncology- Division of Clinical Cancer Epidemiology, Gothenburg, Sweden 2 Ersta Sköndal College University, Department of Health Care Sciences, Stockholm, Sweden 3 Sahlgrenska University Hospital, Department of Oncology, Gothenburg, Sweden 4 Karolinska Institutet, Department of Oncology and Pathology- Division of Clinical Cancer Epidemiology, Stockholm, Sweden Purpose or Objective To find out what organ and doses are most relevant for ‘radiation-induced urgency syndrome’ in order to derive the corresponding dose-response relationships as an aid for avoiding the syndrome in the future. Material and Methods Of the 99 survivors treated with radiation therapy for gynecological cancer, 24 developed ‘radiation-induced urgency syndrome’. The survivors included in the study had not received brachytherapy, but other treatment combinations of external radiation therapy, surgery, and chemotherapy at the Karolinska University Hospital, Stockholm or the Sahlgrenska University Hospital, Gothenburg during the period 1991 to 2003. The rectum, the sigmoid and small intestines were delineated and the dose-volume histograms were exported for each patient. ‘Radiation-induced urgency syndrome’ consists of different self-reported bowel symptoms. To combine the

Conclusion For the studied organs at risk, the dose to the sigmoid is the best predictor of ‘radiation-induced urgency syndrome’ among gyneocological cancer survivors. Dose planners having the ambition to eliminate the syndrome may consider to delineate the sigmoid as well as rectum in order to incorporate the dose-response results. EP-1612 Estimates of the α/β ratio for prostate using data from recent hypofractionated RT trials. S. Gulliford 1 , C. Griffin 2 , A. Tree 3 , J. Murray 4 , U. Oelfke 1 , I. Syndikus 5 , E. Hall 2 , D. Dearnaley 3 1 The Institute of Cancer Research and The Royal Marsden NHS Foundation Trust, Joint Department of Physics, London, United Kingdom 2 The Institute of Cancer Research, Clinical Trials and Statistics Unit, London, United Kingdom 3 Royal Marsden NHS Foundation Trust, Academic Urology Unit, London, United Kingdom 4 Guy’s & St Thomas' NHS Foundation Trust, Department of Clinical Oncology, London, United Kingdom 5 Clatterbridge Cancer Centre, Department of Clinical Oncology, Wirral, United Kingdom Purpose or Objective The α/β ratio for prostate cancer has been widely studied with growing evidence for a value significantly lower than the standard value for tumours of 10 Gy. Previous studies have also indicated that there may be a time factor whereby tumour repopulation should be considered during the course of radiotherapy[1]. Recent reporting from 4 separate phase III clinical trials comparing hypofractionated schedules with standard schedules for prostate radiotherapy allow for further exploration of the α/β value. Material and Methods The α/β ratio for prostate was derived independently for each of the CHHiP[2], HYPRO[3], PROFIT [4]and RTOG 0415 studies[5] by comparing the outcomes in the standard and hypofractionated trial arms. This approach