ESTRO 37 Abstract book

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ESTRO 37

standard linac for both lung and prostate treatments. QA for these treatments is challenging and requires dose measurements with high spatial and temporal resolution. A dosimetry system for 2D dose verification is tested for SBRT treatments combined with MLC tracking of real patients breathing patterns. Material and Methods MP512 is a 52x52 mm 2 array of 512 individual ion implanted diodes with 2 mm centre-to-centre spacing. Each diode has an area of 0.5x0.5 mm 2 and all 512 channels can be read out in parallel by a custom designed multichannel electrometer. MP512 was placed in a solid water phantom and a CT scan was acquired. A two arc VMAT treatment plan was created in Eclipse on the CT geometry, the target was defined as a hypothetical 2cm diameter quasi-spherical volume aligned to the central pixel of the detector. A 3mm PTV expansion was created and 5Gy prescribed dose was optimised to this volume to be delivered in one fraction. Three patient lung motions were applied to the phantom using a moving platform and MLC tracking was implemented to compensate for the motion. Two types of tracking algorithms were used; predictive and non- predictive. 2D dose maps were reconstructed from MP512 and compared with EBT3 film and Treatment Planning System (TPS) dose maps through the use of a 2D gamma analysis. Dose profiles were extracted from MP512 for each motion modality and compared to the static reference case. This gave a measure of the efficacy of the tracking system and allowed identification of point- to-point dose errors. Results Fig.1 shows the comparison of the 2D dose maps extracted from EBT3 film, MP512 and TPS. Also shown are the dose profiles extracted from MP512 for each motion modality during one of the patient treatments. Gamma pass rates were above 95% for both 3%-3mm and 2%-2mm criteria when comparing the MP512 dose map to EBT3 film or TPS. When the treatment experienced motion the pass rate was as low as 55% (2%-2mm). Both tracking algorithms restored the pass rate to above 95%, indicating that the MLC tracking is effective in compensating for the motion. Motion distorted the dose profile and resulted in dose of ±150cGy compared to the reference. MLC tracking is able to reduce this error by more than half.

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 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