ESTRO 37 Abstract book

S1099

ESTRO 37

correlations between toxicity and different dose-volume parameters were analyzed. Results The incidence of late grade 1 and grade 2 GI toxicity in this patient group was 31 % and 12.5 %, while that of late GU toxicity of grade 1 and grade 2 was 32 % and 11 %, respectively. No grade 3 toxicity was recorded. For all endpoints, a statistically significant correlation between total dose to the GTV and toxicity could be established with a multivariate analysis. On the other hand, correlations between side effects and selected dose- volume parameters were not significant. In all risk groups, the mean NTCP for GI toxicity resulted to be lower than the recorded incidence while for GU toxicity the predictions agreed with the observed values within the standard deviation. Conclusion A retrospective analysis of the NTCP model predictions for the prostate cancer patients considered in this work revealed an underestimation of the late GI effects and a good agreement, although within a large spread of the values, with the observed GU toxicity. A preliminary analysis of intra-fractional internal organ movements revealed a negative effect on the NTCP prediction in the case of systematic reduction of the bladder volume or increase of the rectum volume that need further investigations. EP-2012 Sensitivity study of the Microdosimetric Kinetic Model input parameters for carbon ion radiotherapy T.J. Dahle 1 , G. Magro 2 , C.H. Stokkevåg 3 , K.S. Ytre- Hauge 1 , A. Mairani 2,4 1 University of Bergen, Dept. of Physics and Technology, Bergen, Norway 2 Centro Nazionale di Adroterapia Oncologica- CNAO, Medical Physics, Pavia, Italy 3 Haukeland University Hospital, Dept. of Oncology and Medical Physics, Bergen, Norway 4 Heidelberg Ion Beam Therapy Centre- HIT, Physics, Heidelberg, Germany Purpose or Objective An advantage of carbon ion therapy is the possibility of an increased relative biological effectiveness (RBE) in the tumor volume, compared to the RBE in the surrounding healthy tissue. The RBE is accounted for clinically by optimizing the RBE-weighted dose using biophysical models. In Japan, treatment planning for carbon ions is based on the Microdosimetric Kinetic Model (MKM) with parameters optimized for HSG cells. In this study we investigated the sensitivity of the MKM to variations in the model parameters, as the accuracy of the applied RBE model and its input parameters may influence the delivery of (homogeneous) RBE-weighted dose to the target. Material and Methods The MKM is based on estimations of specific energy (z) in small volumes called domains. To enable Monte Carlo (MC) estimations of z, a table connecting z to the kinetic energy of the particles was generated, using the Kiefer- Chatterjee track structure model with saturation corrections. The input parameters were the linear- quadratic model parameter β, the nucleus radius (R n ) and the domain radius (r d ). Based on this, several spread out Bragg peak (SOBP) scenarios were simulated in the FLUKA MC code. The MKM parameters (α, β, R n and r d ) for HSG cells were then varied one at a time by ±{5, 25, 50}%, and in addition changed to related parameter values of other cell-lines (V79, T1 and CHO). The resulting z-tables and SOBPs were compared to the nominal HSG z-table and SOBP. Results Variations in r d had the largest impact on the z-values (Figure 1A). While variations in z due to R n and β were

Conclusion Rectal toxicity prediction can be improved by incorporating parameters of delivered DSMs into NTCP modelling. Ultimately, we anticipate that accumulating delivered dose throughout the course of treatment will facilitate online risk prediction for decision making in adaptive radiotherapy. [1] Shelley, L.E.A., et al., Radiother Oncol. 2017 Jun; 123(3): 466–471. [2] Dearnaley, D., et al., Lancet Oncol. 2016 Aug ; 17(8) : 1047-1060. EP-2011 Accuracy of NTCP models for rectum and bladder toxicity in prostate cancer patients E. Gargioni 1 , S. Domanski 1 , R. Schwarz 1 1 University Medical Center Hamburg - Eppendorf UKE, Department of Radiology and Radiotherapy, Hamburg, Germany Purpose or Objective The use of radiobiological models for predicting toxicity after radiotherapy, for optimizing IMRT treatment plans, and for allowing plan adaptation during the course of treatment is being implemented in modern treatment planning systems. However, the reliability of such models and the accuracy of their predictions need to be thoroughly investigated before they can be used in the clinic. In this work, the treatment plans of prostate cancer patients that underwent helical tomotherapy were retrospectively analyzed in order to compare the observed incidence of gastro-intestinal and genito-urinary toxicity and the NTCP model predictions. Material and Methods This work is based on a group of 139 prostate cancer patients that were treated with helical tomotherapy between January 2007 and December 2010. The dose prescription to the PTV varied between 72 Gy and 76 Gy, with dose escalation to the GTV up to 80 Gy for high-risk patients according to the D’Amico classification. The dose volume-histograms (DVH) of bladder and rectum were retrospectively analyzed and the NTCP values were calculated for late gastro-intestinal (GI) and late genito-urinary (GU) toxicities of grade 2 and grade 1, respectively. The Lyman-Kutcher-Burman (LKB) model was used with parameters taken from the literature. The observed incidence of side effects was then compared to the model predictions for each risk group. Finally, possible