Abstract Book

S209

ESTRO 37

is set to the intermediate density between surrounding tissue and the GTV (DO2). IMRT plans were created for five lung cancer patients taking into account magnetic field to beam orientations from MR-Linac setups described in literature. Optimized plans were forward calculated to the corresponding 4DCTs (no DO) and dose was accumulated to the FBCT by means of deformable image registration and energy/mass transfer mapping. Results Figure 1 shows the average dose metrics of , and in the GTV with corresponding standard deviations. All plans perform with similar plan quality for FBCT, DO1 and DO2 approaches in the 0 T setup. In perpendicular magnetic fields, and are distinctly deteriorated for the forward calculated dose compared to the planned dose; decreases from 96.7% to 94.5% and increases from 103.3% to 107.4% (DO1) for the 1.5 T setup. In inline magnetic fields, more dose is delivered to the GTV when DO are applied; increases up to 101.8% (DO1) for a 1 T inline setup. The mean dose in lung is reduced on average by 5.3% and 5.1% when using the DO1 and DO2 approaches (results not shown).

Conclusion Patient lung motion during EBRT significantly distorts the planned dose profile. This can result in under-dosing of the target and normal tissue complications. MLC tracking is an effective motion management strategy for reducing dose errors, as demonstrated by the gamma pass rates and dose profiles of the tracking modalities. MP512 is able to reconstruct the dose delivered to the target accurately and is an effective tool for evaluation of the dosimetric impact of patient motion. OC-0408 The effect of density overrides on treatment planning for MR-Linacs O. Schrenk 1,2,3 , C.K. Spindeldreier 1,3,4 , L.N. Burigo 1,3 , M. Bangert 1,3 , A. Pfaffenberger 1,3 1 German Cancer Research Center, Medical Physics in Radiation Oncology, Heidelberg, Germany 2 Heidelberg University, Medical Faculty, Heidelberg, Germany 3 Heidelberg Institute for Radiation Oncology HIRO, National Center for Radiation Research in Oncology NCRO, Heidelberg, Germany 4 Heidelberg University, Faculty of Physics and Astronomy, Heidelberg, Germany Purpose or Objective To compensate for tumor motion, enlarged planning margins are introduced into treatment planning. Consequently, when treating lung cancer patients, lung tissue will be included into the PTV. This introduces density heterogeneity inside the PTV and presents a challenge for Monte-Carlo based treatment planning. To provide homogeneous dose to the PTV, a higher fluence has to be delivered to low density tissues than to high density areas of the GTV. This results in an increased inhomogeneity of the fluence map and can lead to increased dose to the GTV and healthy tissue, when the GTV moves to positions where low density tissue was expected. Overriding the density of the PTV can improve the dose delivery to the GTV and reduce the dose to surrounding tissue. In this work, we investigate the effect of magnetic fields on the dosimetric outcome when density overrides (DO) are used during treatment planning. Material and Methods Monte-Carlo based plans were generated with the in- house developed matRad/EGSnrc treatment planning framework accounting for magnetic fields during optimization, directly based on free-breathing planning CTs (FBCT) and using two methods of DO, where 1) the density in the PTV is replaced with the mean GTV density (DO1) and 2) the density of the ITV is replaced with the mean GTV density and the density of the remaining PTV

Conclusion Performing DO to improve treatment quality appeared to be unsuitable for MR-Linacs with perpendicular magnetic fields. The dose conformity in the GTV deteriorates due to electron return effects which were underestimated in overridden densities during treatment planning. Since MR-Linacs are expected to enable reduced planning margins, the ratio of lung tissue inside the PTV is likely to decrease. The results of this study, however, show that density overrides need to be considered with great care for MR-Linacs. OC-0409 A film-based end-to-end test for MR-guided online adaptive radiotherapy D. Hoffmans 1 , O. Bohoudi 1 , N. Niebuhr 2,3,4 , A. Pfaffenberger 2,3 , L. Battum 1 , B. Slotman 1 , M. Palacios 1 , A. Bruynzeel 1 , F. Lagerwaard 1 1 VU University Medical Center, Radiation Oncology, Amsterdam, The Netherlands 2 German Cancer Research Center DKFZ, Medical Physics in Radiation Oncology, Heidelberg, Germany 3 National Center for Radiation Research in Oncology, Heidelberg Institute for Radiooncology, Heidelberg, Germany 4 Heidelberg University, Department of Physics and Astronomy, Heidelberg, Germany Purpose or Objective The implementation of a MRIdian system (Viewray Inc., Mountain View) allows for Stereotactic MR-guided Adaptive Radiation Therapy (SMART), including online re- planning based on the daily anatomy. As a consequence, existing methodologies for end-to-end testing need to be revised in order to perform QA for daily adaptation. The purpose of this study is to present the first results of a film-based end-to-end test for MR-guided adaptive radiotherapy.