ESTRO 36 Abstract Book

S143 ESTRO 36 _______________________________________________________________________________________________

performance of this approach is evaluated by simulating brachytherapy procedures using data of 10 patients diagnosed with prostate cancer. Material and Methods Throughout HDR prostate brachytherapy, unpredictable anatomy movements may cause errors in dose delivery and potentially, this may result in failure to reach clinical constraints (e.g. for single fraction monotherapy: D95% PTV>19 Gy, D10% urethra<21 Gy, D1cc bladder<12 Gy and D1cc rectum<12 Gy). In this study, a novel adaptive dose planning pipeline for MR-guided HDR prostate brachytherapy using a single needle robotic implant device is proposed to address this issue (Figure 1a). The dose plan (needle track positions, source positions and dwell times) and needle insertion sequence are updated after each needle insertion and retraction with MR–based feedback on anatomy movements (cf. Figure 1b). The pipeline was assessed on moving anatomy by simulating MR-guided HDR prostate brachytherapy with varying number of needle insertions (from 2 to 14) for 10 patients. The initial anatomy of the patients was obtained using the delineations of the prostate tumor and the OAR considered (urethra, bladder and rectum) on MR images. Each needle insertion and retraction induced anatomy movements which were simulated in 2 steps: (1) a typical 3D rotation of the prostate was imposed (2) a regularization of the movement in space was then applied. The initial and final dose parameters were compared in the situations with and without update of dose plan and needle insertion sequence. Results The computation time for re-planning was less than 90 seconds with a desktop PC. The actual delivered dose improved with vs. without update of dose plan and needle insertion sequence: On average, the dose coverage of the PTV was higher in the situation with vs. without update (Figure 1c). Moreover, the difference increased with the number of needle insertions. The dose received by the PTV in the situation with re-planning was not significantly different compared to the initial dose plan. Finally, the dose to the OAR’s was not significantly different between the initial dose plan and the dose delivered in the situation with and without update. Conclusion This study proposes a new adaptive workflow with feedback on the anatomy movements for MR-guided HDR prostate brachytherapy with a single needle robotic implant device. The assessment of the pipeline showed that the errors in the dose delivered due to movement of anatomy can be compensated by updating the dose plan and the needle insertion sequence based on MRI.

OC-0277 Assessment of the implant geometry in interstitial brachytherapy by a hybrid tracking system N. Pallast 1 , M. Kellermeier 1 , K. Kallis 1 , B. Steinmetz 1 , V. Strnad 1 , C. Bert 1 1 Universitätsklinikum Erlangen- Friedrich-Alexander- Universität Erlangen-Nürnberg, Department of Radiation Oncology, Erlangen, Germany Purpose or Objective Electromagnetic tracking (EMT) is a promising g approach to measure variations of the implant geometry in interstitial brachytherapy. The coordinate system for EMT data measurements is usually decoupled from the one of computed tomography (CT) used for treatment planning. Therefore, an optical tracking system (OTS) is introduced to associate EMT and CT coordinate systems. The accuracy of this hybrid tracking system was investigated in phantom studies and the system is currently used in a clinical feasibility study. Material and Methods EMT data providing the implant geometry were measured by an implant sensor integrated in the cable of an afterloader prototype (Flexitron, Elekta, The Netherlands). Breathing motion was compensated by three additional fiducial sensors on the chest. Simultaneously, an OTS (Polaris, NDI, Canada) collected data of eight infrared (IR) markers of which three were attached to the EMT fiducial sensors and five to the skin. A reproducible marker position was ensured by adhesive pads glued on the patient prior the first measurement. To align both coordinate tracking systems, a transformation between the OTS and EMT ( OTS T EMT ) was estimated by a so-called hand eye calibration. An additional registration from the OTS to the CT ( CT T OTS ) was determined to associate their coordinate systems. Finally, both transformations were combined to get a direct relation of EMT- and CT-derived data. The resulting calibration error of the OTS T EMT transformation was evaluated by measuring different poses of an in-house developed calibration tool. This tool