ESTRO 2020 Abstract Book

S260 ESTRO 2020

to fit MR2 and anatomical changes between the images could be quantified. The inverse deformation was used to warp the dose distribution from MR2 to the geometry of MR1 in order to compare the two different dose distributions in the same geometry. Results A mean bladder volume increase of 40.9% (12.4% - 133.0%) was observed. Rectum volume changes varied between - 10.9% and 38.8%. The fraction of patients passing the local clinical CTV criteria of D min ≥41.5 Gy was 100% for MR1 and 91.4% for MR2. The fraction of patients passing the criteria for PTV of D 98% ≥40.6 Gy decreased from 100% to 28.6%, implying that 25 patients out of 35 would have been under- dosed in the PTV, assuming the same motion pattern for each fraction (Figure 1). For the patient with the maximum difference in D 98% , one fraction with that motion pattern would lead to a -7.5% difference in the total PTV D 98% . Mean gamma pass rate was 90.6 ± 7.5 % for 3% / 1 mm local dose.

Conclusion The relatively high similarity of breast MRL bulk ED and CT LUT doses suggests an acceptable level of accuracy for MR- based offline review of the delivered plan. However, due to significant error incurred in the chest wall and ipsilateral lung, the tested bulk ED approach may be unsuitable for clinical treatment planning. Further work is required to assess the significance of this error against other radiotherapy uncertainties. OC-0466 Hypofractionated radiotherapy of the prostate using a MRgRT system – are there any disadvantages? A. Mannerberg 1 , E. Persson 2,3 , J. Jonsson 4 , C. Jamtheim Gustafsson 2,3 , A. Gunnlaugsson 2 , L.E. Olsson 2,3 , S. Ceberg 1 1 Lund University, Department of Medical Radiation Physics, Lund, Sweden ; 2 Skåne University Hospital, Department of Hematology- Oncology and Radiation Physics, Lund, Sweden ; 3 Lund University, Department of Medical Physics, Malmö, Sweden ; 4 Umeå University, Department of Radiation Sciences, Umeå, Sweden Purpose or Objective Radiotherapy of the prostate is associated with inter- and intrafractional motion (Langen et al. 2008, McPartlin et al. 2016). With the high fractional dose that constitutes hypofractionation, it is crucial to manage the prostate motion properly. One possibility to achieve this is by using MR-guided radiotherapy (MRgRT) systems, which offer e.g. daily adaptive replanning and real-time imaging. However, the online workflow for MRgRT with imaging, recontouring, replanning, reoptimisation and quality assurance of the new plan can take 20-40 minutes (Acharya et al. 2015, Raaymakers et al. 2017, Tetar et al. 2019), with the patient lying in the treatment position. This implies that the patient is treated with a plan adapted to a situation that was valid up to 40 minutes earlier. The purpose of this study was to: - Quantify anatomical changes in prostate cancer patients that occur during the time frame corresponding to the procedure of adaptive replanning in MRgRT. - Evaluate how these anatomical changes affect the dose distribution. Material and Methods For each of the 35 patients enrolled in this study, two large field of view T2-weighted MR image volumes (MR1 and MR2), were acquired. Images were acquired 30 minutes apart, during which the patient was lying in treatment position. For dose calculation purposes, a synthetic CT (sCT, MriPlanner TM , Spectronic Medical) was created for MR1 and MR2 (sCT1 and sCT2). The CTV and organs at risk were delineated based on MR1 and the margin for the PTV was 7 mm. A 6MV flattening filter free VMAT treatment plan, 42.7 Gy / 7 fractions, was created for sCT1. This dose distribution was then recalculated onto sCT2. Using MICE Toolkit (NONPI Medical), MR2 was deformable registered to MR1. The structures of MR1 were deformed

Conclusion The time spent on daily plan adaption in MRgRT systems may result in patient motion and consequently under- dosage to the PTV. The results indicate that prostate cancer patients would benefit from a reduced MRgRT process time, and thorough position verification before beam-on. OC-0467 Focal salvage SBRT treatment for radiorecurrent prostate cancer on the MR-Linac: a planning study T. Willigenburg 1 , E. Beld 1 , J. Hes 1 , J.AN.J.W. Lagendijk 1 , H.C.J. De Boer 1 , M.A. Moerland 1 , J.R.N. Van der Voort van Zyp 1 1 UMC Utrecht, Radiation Oncology, Utrecht, The Netherlands Purpose or Objective One of the treatment options for radiorecurrent localized prostate cancer is MRI-guided focal salvage High-Dose-Rate brachytherapy (HDR-BT), delivering 19 Gy in a single fraction to the CTV. Due to the invasive nature and need for spinal anesthesia patients are not always eligible. The non-invasive nature and potential for high-precision dose delivery of magnetic resonance guided radiotherapy, such as with the Magnetic Resonance Linear Accelerator (MRL), could potentially open up treatment possibilities for patients who are not eligible for HDR-BT or even replace HDR-BT treatment for recurrent prostate cancer. We investigated the feasibility of delivering a 19 Gy dose to the recurrent lesion on the MRL by conducting a planning