Abstract Book

S1099

ESTRO 37

(D 2% ) values of RBE-weighted doses were estimated for crucial organs at risk (OARs), including the heart, brainstem, lungs and thyroid. Finally, (dose-weighted) organ-mean RBE values were calculated from the RBE- weighted- and physical organ-mean doses. Results Organ-mean RBE (Figure A) and RBE-weighted dose (mean and D 2% ) (Table and Figure B) for OARs varied both with choice of model and applied (α/β) x values as well as between patients. The largest inter-patient variations in RBE were observed for the thyroid (WIL:1.3-2.1), heart (WIL:1.4-2.0) and lungs (WIL:1.1-1.5), while only minor variations were seen for the brainstem. However, as the brainstem received high doses in all patients (RBE 1.1 dose range: 52.5-54.2 Gy(RBE)) also small variations in RBE resulted in non-negligible variations in RBE-weighted doses. For the thyroid, the variations in RBE could be strongly influenced by the large inter-patient variation in mean physical dose (RBE 1.1 dose range:1.3-14.3 Gy(RBE)). Overall, the highest RBE values were estimated with the WIL and the MCN (α/β)x=2Gy models. The LWD and MCN (α/β)x=10Gy models gave moderate RBE values in the range 1.1-1.3 for all OARs.

Conclusion Considerable variation in mean doses to HPA substructures was found in four of eleven patients, indicating that specific sparing of these structures may be clinically feasible. The predicted risk of GHD from the HPA substructures increased with time reaching close to 100% at 5 years post treatment. EP-2014 Organ-specific RBE estimates for cranio-spinal irradiation with protons K. Ytre-Hauge 1 , L. Fjæra 1 , E. Rørvik 1 , J. Dale 2 , S. Thörnqvist 2 , C. Stokkevåg 2 1 University of Bergen, Department of Physics and Technology, Bergen, Norway 2 Haukeland University Hospital, Department of Oncology and Medical Physics, Bergen, Norway Purpose or Objective Proton therapy offers improved dose conformity compared to conventional radiotherapy with photons for patients receiving cranio-spinal irradiation (CSI). In current clinical practice, proton treatment planning is based on a constant relative biological effectiveness (RBE) of 1.1. However, proton RBE is known to vary with linear energy transfer (LET), tissue type and dose, properties included in variable RBE models. The RBE for a treatment plan can vary with parameters including beam arrangement and patient geometry. The objective of this study was to investigate inter-patient variations in organ- specific RBE values for patients receiving CSI with protons. Material and Methods Intensity Modulated Proton Therapy (IMPT) plans for 10 children (5-11 years) were generated using Eclipse (Varian Medical Systems). The PTV included the cranio- spinal axis with vertebra bodies prescribed 23.4 Gy(RBE) and posterior fossa boost up to 54 Gy(RBE). The plans were recalculated with the FLUKA Monte Carlo code, obtaining the physical dose and dose-averaged LET (LET d ) voxel by voxel. Due to the uncertainty in the proton RBE, we applied several models: RBE=1.1 (RBE 1.1 ), the McNamara (MCN) and Wilkens (WIL) models as well as LET-weighted dose (LWD). For the MCN model, (α/β) x values of 2 and 10 Gy were applied. Mean and maximum

. Conclusion Modest inter-patient variations in organ-mean RBE values were observed for the heart, lungs and thyroid, while only small variations were seen for the brainstem. Greater variations in RBE were seen between estimates from different RBE models, thus RBE models should be used with awareness of this. The estimated median RBE values could be used in assessment of proton CSI treatment plans optimized with RBE 1.1 . EP-2015 Radiomics can detect changes in lung after low dose irradiation: a preclinical study T.A.G. Refaee 1 , A. Ibrahim 1 , H. Woodruff 1,2 , R.T.H. Leijenaar 1 , A. Jochems 1 , R.T.H. Larue 1 , E.E.C. De Jong 1 , F. Verhaegen 2 , L. Dubois 3 , P. Lambin 1 1 Maastricht university, The D-Lab: Decision Support for Precision Medicine- GROW - School for Oncology and