ESTRO 38 Abstract book

S608 ESTRO 38

Material and Methods The first patient treated with MRL had oligometastatic prostate cancer and was treated for a pararectal lymph node metastasis. The workflow consisted of: CT planning with MRL table top and acquisition of an MRI at the MRL. An 8 beam step-and- shoot IMRT plan was created. The staff consisted of RTTs, Radiation Oncologists and Medical Physicists. The daily MRL workflow included a pretreatment MRI. Based on this, plans were optimized with the “adapt to position” algorithm in order to adjust the plan of the daily anatomy. The patient received real time MRI (4D) during plan optimization and beam-on to confirm sufficient target volume coverage. Post-treatment MRI was performed for in-silico studies after treatment. Time required for individual steps (CT planning, patient positioning, MRI, plan optimization and daily treatment) of the treatment process were recorded. Results At Unity the total duration for planning was 395 minutes on average, for IMRT at a conventional linac it was 330 minutes for CT (no additional MRI). An average of 46 minutes was required for daily treatment on MRL, 16 minutes for the linac. The radiation delivery time is comparable on both devices with approximately 2-3 minutes. The total time required for planning, radiation and post- MRI and offline verification of the adapted and treated plan was 503 minutes at MRL and 357 minutes at linac (CBCT, no MRI). The daily pre-treatment scans took 3-7 minutes at MRL and 2-3 minutes for linac with CBCT. Conclusion Due to the new workflow and the involvement of all professional groups for daily radiation, each individual step took more time than with conventional linac treatment (IMRT). With each additional patient, we expect significantly reduced treatment times. With increasing experience, the planning and treatment times can be further reduced to enhance efficiency. PO-1096 Geometrically correct MR imaging with optimal Signal to Noise Ratio for Hippocampus delineation S. Takken 1 , M. Frantzen-Steneker 1 , T. Vijlbrief-Bosman 1 , L. Ter Beek 1 , M. Kwint 1 , J. Belderbos 1 , U.A. Van der Heide 1 1 Netherlands Cancer Institute, Radiotherapy, Amsterdam, The Netherlands Purpose or Objective Following the RTOG0933 contouring atlas [1], the hippocampus is visualized with a T1-weighted 3D gradient TFE sequence. Unfortunately, this can have a Water Fat Shift (WFS) in the order of 2mm. When matching the fatty skull bone tissue to the planning-CT, this may result in a 2mm mismatch of the hippocampus (fig.1a). For Radiotherapy (RT), scanning in an immobilization mask is debated as the benefit to obtain the scan in RT position and minimize motion artifacts comes at the cost of a reduced Signal to Noise Ratio (SNR). Patients can be scanned with flexible coils at either side of the mask combined with Anterior and Posterior (A&P) coil arrays to regain SNR. Purpose of this study is to obtain a geometrically correct MR sequence with sufficient image quality to use for delineation of the hippocampus, in head coil or in RT position. Poster: RTT track: Imaging acquisition and registration, OAR and target definition

Results All patients tolerated the marker implantation, the abdominal corset and the treatment well with no treatment interruptions. In 4 out of the 300 applied fractions (1,3%) the tolerance level of the fiducial markers was exceeded. For these fractions, the recalculated dose distribution nevertheless showed sufficient target coverage (Fig. 2). The entire treatment fraction (including patient positioning, setup verification and dose delivery) did not exceed the standard time slot of 30 minutes.

Conclusion This study demonstrates that PBS-based proton therapy with an abdominal corset to reduce breathing motion of GI-tumors is feasible, well-tolerated by patients and provides a reproducible setup without prolonging the daily treatment time. PO-1095 Time management and hands-on experience with ELEKTA Unity 1.5T MRI-Linac C. Marks 1 , A. Stolte 1 , D. Thorwarth 2 , L.H. Braun 1 , S. Boeke 1 , D. Wegener 1 , J. Boldt 1 , C. Ortinau 1 , M. Kammler 1 , B. Holl-Henkel 1 , C. Gani 1 , D. Zips 3 , M. Nachbar 4 , O. Dohm 4 , D. Mönnich 4 1 University Hospital Tübingen, Department of Radiation Oncology, Tübingen, Germany ; 2 Section for Biomedical Physics Tübingen- Eberhard Karls University Tübingen, Department of Radiation Oncology, Tübingen, Germany ; 3 University Hospital Tübingen- Eberhard Karls University Tübingen, Department of Radiation Oncology, Tübingen, Germany ; 4 Section for Biomedical Physics Tübingen, Department of Radiation Oncology, Tübingen, Germany Purpose or Objective MR-Linac (MRL) hybrid devices are a new development in radiation oncology. Workflow and time management represent a challenge for all involved professions. Herein we present our initial experience with the ELEKTA Unity. Despite longer treatment times at MRL, patients may benefit from MRL and could be treated more accurately with hypofractionated radiotherapy. MRI allows better soft tissue contrast of the tumor and adjacent OARs. The mobility of the tumor and the patient movement can be detected online and adaptive response may be considered. In addition, the MRI, unlike a CBCT, does not generate additional radiation exposure to the patient.