ESTRO 36 Abstract Book

S142 ESTRO 36 _______________________________________________________________________________________________

In HDR brachytherapy, image guidance is crucial for accurate and safe dose delivery. Accordingly, MR-guided HDR brachytherapy is in development at our institution. This study demonstrates the testing of a recently developed MR-compatible afterloader, while operating simultaneously with MR imaging, as well as an MR-based method for real-time source position verification. A prototype of an MR-compatible afterloader (Flexitron, Elekta) was developed. This afterloader was made MR- compatible by providing every part as well as the cover with RF shielding. The source cable was replaced by a plastic cable containing a piece of steel at its tip, serving as a dummy source. The afterloader was placed next to the MRI scanner and connected to a catheter positioned in an Agar phantom (doped with MnCl2), see Fig. 1. Afterloader management: The afterloader was programmed to send the source (I) to 10 dwell positions, with a 10 mm step size, remaining 10 s at each position, and (II) to 20 dwell positions, with a 5 mm step size, remaining 0.5 s at each position. MRI acquisition: While sending the source to its predefined dwell positions, MR imaging was carried out on a 1.5 T MR scanner (Ingenia, Philips) using a 2D gradient echo sequence (TR/TE 2.2/1.0 ms, slice thickness 10 mm, FOV 192x192 mm, acq. matrix 96x96, flip angle 30°, SENSE=2), scanning two orthogonal slices interleaved with a temporal resolution of 0.114 s per image. HDR source localization: The MR artifact induced by the magnetic susceptibility of the metallic source was exploited. The artifacts (complex data) were simulated based on the susceptibility induced B0 field disturbance [1]. The localization was executed offline in a post processing operation by phase-only cross correlation [1,2], to find the translation between the experimental image and the simulated artifact. Results The experiments demonstrated that the prototype MR- compatible afterloader and the MRI scanner fully functioned while operating simultaneously, without influencing each other. The afterloader was able to send the source to the predefined dwell positions when placed next to the MRI scanner, without being attracted to or being disturbed by the scanner. The HDR source positions could be determined by the described localization method (now accomplished offline), see Fig. 2. The average distances between the determined 3D source positions for cases (I) and (II) were 9.9±0.2 mm and 5.0±0.2 mm, respectively. The short dynamic scan time (~0.15 s) and the fast reconstruction/post processing (<0.15 s) guarantee that source localization will be possible in real time. Conclusion The MR-compatible afterloader developed in this study and a commercial 1.5 T MRI scanner were demonstrated to fully function while operating simultaneously, enabling real-time HDR source position verification for MR-guided HDR brachytherapy, using a phase-only cross correlation localization method. [1] Beld E. et al. 2015 Proc. Intl. Mag. Reson. Med. 24, #4151. [2] De Oliveira A. et al. 2008 MRM 59 1043-1050. Material and Methods Experimental set-up:

OC-0276 Toward adaptive MR-guided HDR prostate brachytherapy – Simulation study based on anatomy movements M. Borot de Battisti 1 , B. Denis de Senneville 2 , G. Hautvast 3 , D. Binnekamp 3 , M. Peters 1 , J. Van der Voort van Zyp 1 , J.J.W. Lagendijk 1 , M. Maenhout 1 , M.A. Moerland 1 1 University Medical Center Utrecht, Departement of Radiotherapy, Utrecht, The Netherlands 2 UMR 5251 CNRS/University of Bordeaux, Mathematics, Talence, France 3 Philips Group Innovation, Biomedical Systems, Eindhoven, The Netherlands Purpose or Objective Dose delivery during a single needle, robotic MR-guided HDR prostate brachytherapy may be impaired by: (1) needle insertion errors caused by e.g. needle bending, (2) unpredictable anatomy movements such as prostate rotations (induced by the insertion or retraction of the needle), prostate swelling or intra-procedural rectum or bladder filling. In this study, a new adaptive dose planning strategy is proposed to assess the second challenge. The