ESTRO 36 Abstract Book

S861 ESTRO 36 2017 _______________________________________________________________________________________________

Physics, Aarhus C, Denmark 3 Heidelberg University Hospital, Department of Radiooncology, Heidelberg, Germany

patients.

Purpose or Objective Dose planning constraints, such as the volume receiving xGy (V x ), are often extracted from clinical outcome data. These data are tainted by uncertainties in dose- and output recording, large patient heterogeneity, small sample size and -variability. Our study is dedicated to the investigation of the fundamental uncertainty with which dose planning constraints can be extracted from clinical radiation pneumonitis data and how this relates to patient number and complication incidence rate. Material and Methods In order to measure the reliability of a V x logistic regression model, the dose-response mechanism generating the complication events needs to be known. For this reason, we generated cohorts of patients using real-life dose distributions of patients treated for advanced lung cancer, combined with a postulated V x logistic dose-response model. In each of the 1000 cohorts, the patients were randomly assigned complication/no- complication based on the individual risks given by the postulated model. Thus, “alternative reality” cohorts comprised of the same patients, but with different outcomes from the same dose distributions were created. Each cohort thus represented a possible result of a clinical study. They were analyzed with a number of logistic V x models, and the best fitting model was selected. This was matched to the postulated model to determine its recognition rate. The postulated model was varied to produce low, intermediate and high incidence rates. Results For a patient cohort of 100 individuals, a postulated model with an incidence rate of 15/100 was recognized in 31% of the cohorts. For a cohort size of 500, the correct- recognition rates increased to 75%. For a lower incidence model (7/100), these recognition frequencies dropped to 20% and 56%, respectively. To ensure a recognition rate >90%, large cohorts of between 500 and 2000 patients were required, see Figure 1(a). Figure 1(b) shows that the distribution width for the 15/100 incidence rate model decreased from a standard deviation of 10Gy for 100 patients to 1Gy for 2000

Conclusion For realistic dose distributions and cohort sizes, a state- of-the-art analysis failed to identify the postulated dose- response in about 2-in-3 cases for the low incidence of the large-volume effect complication radiation pneumonitis. Very large patient cohorts were required to ensure recognition rates above 90%. This fundamentally low success rate could explain the persistent difficulties to derive dose constraints from clinical data for complications in large-volume effect, “parallel” organs. EP-1615 Second cancer risk after radiation of localized prostate cancer with and without flattening filter M. Treutwein 1 , M. Hipp 2 , R. Loeschel 3 , O. Koelbl 1 , B. Dobler 1 1 Klinik und Poliklinik für Strahlentherapie- Unive, Regensburg University Medical Center- Department of Radiation Oncology, Regensburg, Germany 2 Klinkum St. Marien, Department of Radiotherapy, Amberg, Germany 3 Ostbayerische Technische Hochschule, Faculty of Computer Science and Mathematics, Regensburg, Germany Purpose or Objective Radiotherapy is a standard treatment modality with curative intent for localized prostate cancer. Prostate cancer is a disease of elderly men. Nevertheless these patients have a remaining life span of ten years or more. Radiotherapy compared to surgery may increase the risk for second cancer. Minimizing this risk can be one criterion in deciding for a specific technique. Therefore we compared the organ equivalent dose (OED) and excess absolute risk (EAR) for second cancer for different For ten patients four different plans were calculated, using a seven field intensity modulated radiotherapy (IMRT) and a single arc volumetric modulated arc therapy (VMAT) with and without flattening filter. The treatment techniques. Material and Methods