ESTRO 38 Abstract book

S1098 ESTRO 38

images. The dose of the day is calculated with the isocenter of the original treatment plan shifted according to the clinically applied IGRT table shift. The rectum wall dose is accumulated in several steps, partly illustrated in fig. 1. 1) The whole rectum is contoured. Axial rectum slices are numbered starting at the caudal end at the tuber ischiadicum going cranially. 2) The rectal wall in every slice is represented in polar coordinates (around the center of mass “CoM”) and subdivided into equal angles (e.g. with a resolution of 1°). 3) The dose is extracted in every axial slice for every arc length element and every daily fraction. 4) Dose accumulation is performed for every small arc length element and transferred back to the corresponding pCT rectum wall element.

Fig.2: DVHs of the rectum wall. Doses are scaled to the treatment dose of 74 Gy. Conclusion The developed dose accumulation approach is a promising tool for dose estimations to the rectum wall, as it has a high geometrical precision and does not rely on potentially inaccurate DIR for dose accumulation. The method is applied to several patient cases which will be presented at the congress. A future goal is to validate the accumulated rectum wall doses by correlating them with patient reported side effects. EP-2006 Dosimetric benefit of the first clinical SBRT of lymph node oligometastases on the 1.5T MR-linac D. Winkel 1 , G.H. Bol 1 , A.M. Werensteijn-Honingh 1 , M.P.W. Intven 1 , W.S.C. Eppinga 1 , B. Van Asselen 1 , J. Hes 1 , B.W. Raaymakers 1 , I.M. Jürgenliemk-Schulz 1 , P.S. Kroon 1 1 UMC Utrecht, Department of Radiotherapy, Utrecht, The Netherlands Purpose or Objective The 1.5T MR-linac system (Unity, Elekta AB, Stockholm, Sweden) recently became available for clinical use in august 2018 and allows for MR-guided external beam radiotherapy. At our department, SBRT using the Unity has been initiated for patients with pelvic lymph node oligometastases. The purpose of this study was to compare the clinical dosimetric outcomes of online re-planning based on the new contours with an alternative online plan adaptation based on the online patient position with pre- treatment contours as well as with CBCT-linac VMAT back- up plans. Material and Methods In August 2018 clinical treatment with the Unity commenced in our clinic. At this time three patients received SBRT of 5x7Gy prescribed to 95% of the PTV with a 3mm PTV margin. For each patient a 7-beam pre- treatment IMRT plan was created using Monaco TPS (Elekta AB, Stockholm, Sweden), taking into account the 1.5T magnetic field. Also, CBCT-linac VMAT back-up plans were created with the clinical PTV margin (8, 3 and 3 mm for patient 1, 2 and 3, respectively, based on target visibility). With online MR imaging as provided in the Unity, the pre-treatment plan can be adapted by either 1) taking the new target position into account (adapt to position) and optimizing on the pre-treatment CT and contours or 2) using the new patient anatomy (adapt to shape) and optimizing on the daily image and adapted contours (Figure 1). Optimization was done by optimizing both segment shapes and weights for both methods. We compared these clinical plans with adapt to position plans and with VMAT CBCT back-up plans, using the daily anatomy of all online MRIs. All plans were evaluated using clinical dose criteria: PTV V 35Gy >95%, PTV D 0.1cc <47.25Gy, ureter D max <40Gy and bladder, bowel bag, rectum and sigmoid V 32Gy <0.5cc.

Fig.1: Axial slice through the CBCT cor of an exemplary patient. Isodoselines are shown together with the PTV (red) and rectum wall (pink). The second row depicts the corresponding rectum wall in polar coordinates around their center of mass (CoM) for two axial slices. Dose accumulation is performed in arc length elements with an angel resolution of 1° over all fractions and mapped back to rectum wall of the planning CT. Results In fig. 2 resulting dose-volume-histograms (DVH) are shown for an exemplary patient case irradiated with 37 fractions of 2 Gy. For this particular patient the dose to the rectum wall was higher in the planning CT than in the single fractions. Differences between the mean DVH over all fractions (“spatially not resolved accumulation”) and the accumulated DVH in fig. 2 indicate the effect of the improved geometrical accuracy of the novel method. The developed dose accumulation approach is robust against changes in the angle resolution (0.1°-5°).