ESTRO 38 Abstract book

S276 ESTRO 38

OC-0525 4D Monte Carlo dose calculations on different CT image sets for SBRT using patient breathing data P. Freislederer 1 , A. Von Münchow 1 , F. Kamp 1 , C. Heinz 1 , S. Gerum 1 , F. Roeder 1,2 , S. Corradini 1 , R. Floca 3,4 , M. Alber 3,5 , M. Söhn 1 , M. Reiner 1 , C. Belka 1,6,7 , K. Parodi 8 1 University Hospital LMU Munich, Department of Radiation Oncology, Munich, Germany ; 2 German Cancer Research Center DKFZ, CCC Molecular Radiation Oncology, Heidelberg, Germany ; 3 National Center for Radiation Research in Oncology NCRO, Heidelberg Institute of Radiation Oncology HIRO, Heidelberg, Germany; 4 German Cancer Research Center DKFZ, Division of Medical Image Computing, Heidelberg, Germany; 5 Heidelberg University Hospital, Department of Radiation Oncology, Heidelberg, Germany; 6 Comprehensive Pneumology Center Munich CPC-M, Member of the German Center for Lung Research DZL, Munich, Germany ; 7 DKTK, German Cancer Consortium, Munich, Germany; 8 LMU Munich, Department of Experimental Physics - Medical Physics, Munich, Germany Purpose or Objective The need for four-dimensional treatment planning becomes indispensable for radiation therapy of tumors with breathing-induced motion. In this study, we combined the actual patient breathing trace with the Linac's log file and a Monte Carlo (MC) dose engine to recalculate a 4D dose distribution for 3D-conformal radiation therapy (3D-CRT) and volumetric modulated arc therapy (VMAT) for lung SBRT and compared the 4D dose to MC dose calculations based on different CT image datasets. Material and Methods For 5 lung patients, 3D-CRT and VMAT treatment plans were calculated on four different 3DCT image datasets: a three-dimensional CT (3DCT), an average intensity projection (AIP) and a maximum intensity projection (MIP) CT both generated from a 4DCT, and a 3DCT with density overrides (DO) in the internal target volume (ITV) and planning target volume (PTV) based on the 3DCT. The MC 4D dose has been calculated on each 4DCT phase using the Linac's log file and the patient's breathing trace (acquired during clinical treatment). Dose calculation on 10 4DCT breathing phases was performed using MCverify, a scriptable sub-module XVMC dose engine of Hyperion V2.4.5 (University Tübingen, Germany; research version of Elekta MONACO 3.2). Dose was accumulated to the gross tumor volume (GTV) at the 50% breathing phase (end of exhale) using deformable image registration with AVID (Analysis of Variations in Interfractional Radiotherapy, DKFZ, Germany). Dose-volume-histograms (DVHs) were created and ΔD98% and ΔD2% were calculated as the difference between 4D recalculated and planned dose. Results For 3DCT-based planning, ΔD98% is higher in mean over all patients for all treatment plans while D2% is lower for 3D- CRT. AIP-based planning increases the low dose areas in the GTV in 3D-CRT and slightly less using VMAT, while D2% stays within the same range for both treatment planning methods. Absorbed dose in high dose areas is in good agreement for MIP-based planning over the patient cohort, although less dose was calculated and both treatment techniques. DO-based planning leads to a lower D2% in the GTV due to the higher density in ITV and PTV but also to fewer cold spots for low dose areas (D98%) in the GTV.

Fig. 1: GTV 50% DVHs of 4D MC recalculated (solid) and optimized dose (dashed) for 2 patients: 3DCT (blue), AIP (magenta), MIP (green), DO (brown) Conclusion AIP and MIP served as the most stable base for treatment planning and both tend to be superior to DO, but the results indicate a dependency on breathing variability, tumor motion, and size. Both methods can be recommended for SBRT treatment planning but DO could perform better in individual cases due to the highly patient-specific character of breathing motion. An interplay effect could not be observed in the small patient cohort. Our workflow, after adaptation for clinical use, could help to provide a 4D dose calculation tool in modern treatment planning systems, where evaluation tools for the effects of tumor motion have not been incorporated adequately. OC-0526 Dependency of the interplay effect on the fractionation for proton therapy of pancreatic cancer K. Dolde 1,2,3 , Y. Zhang 4 , N. Chaudhri 3,5 , C. Dávid 2,6 , M. Kachelriess 6 , A.J. Lomax 4,7 , P. Naumann 8 , N. Saito 8 , D.C. Weber 4 , A. Pfaffenberger 1,3 1 German Cancer Research Center DKFZ, Medical Physics in Radiation Oncology, Heidelberg, Germany ; 2 University of Heidelberg, Department of Physics and Astronomy, Heidelberg, Germany ; 3 National Center for Radiation Research in Oncology, Heidelberg Institute for Radiooncology, Heidelberg, Germany ; 4 Paul Scherrer Institute, Center of Proton Therapy, Villigen, Switzerland ; 5 Heidelberg Ion-Beam Therapy Center HIT, Heidelberg Ion-Beam Therapy Center HIT, Heidelberg, Germany ; 6 German Cancer Research Center DKFZ, Division of X-Ray Imaging and Computed Tomography, Heidelberg, Germany ; 7 ETH Zurich, Department of Physics, Villigen, Switzerland ; 8 University Clinic Heidelberg, Department of Radiation Oncology, Heidelberg, Germany Purpose or Objective In particle therapy treatments of pancreatic cancer, interplay effects between the scanning pencil beam and intrafractional abdominal organ motion may lead to pronounced hot and cold spots in the resulting dose distributions. These effects can be assessed and quantified by means of 4D dose calculations. To furthermore account for interfractional motion variability, repeated 4D image acquisitions are required. In this study, we performed 4D dose calculations based on repeated time-resolved volumetric MR images (4D-MRI) to investigate both the magnitude and the mitigation effectiveness of the interplay effect by means of fractionation for pancreatic cancer patients. Material and Methods For a cohort of 9 pancreatic cancer patients (P1-P9), up to six repeated 4D-MRI were acquired, which were