ESTRO 36 Abstract Book

S142 ESTRO 36 2017 _______________________________________________________________________________________________

was also used to assess the registration error of CT T OTS . To prove the overall accuracy of this approach, another in- house developed phantom was used, equipped with eleven catheters and eight IR-markers. The feasibility of a daily patient data acquisition was examined in an institutional review board-approved study by using the afterloader prototype and the OTS in addition to regular iBT treatments. Results Based on different poses of the calibration tool, the root mean square errors for OTS T EMT and CT T OTS were 0.56 mm and 0.49 mm, respectively. The overall accuracy of CT T EMT resulted in 0.74 mm. The determined transformations were applied to the phantom measurements and showed a mean deviation to the CT data of 0.92 mm. Currently, 65 catheters from four patients were tracked by the EMT system in combination with the OTS. The deviations of the implant geometry, determined by this hybrid tracking approach, are comparable to the previous results, obtained using only the EMT procedure. Conclusion The novel hybrid tracking system allows direct mapping of EMT and CT data. So far, the system was successfully used to measure the implant of four patients. The clinical study is ongoing. OC-0278 Red-emitting inorganic scintillation detectors to verify HDR brachytherapy treatments in real time G. Kertzscher 1 , S. Beddar 1 1 The University of Texas MD Anderson Cancer Center, Department of Radiation Physics, Houston, USA Purpose or Objective Treatment verification during brachytherapy (BT) is presently limited because only few detectors can measure accurate and precise dose rates in the steep gradients that are characteristic for HDR BT. Red-emitting inorganic scintillation detectors (ISDs) are promising for BT because they can generate large signal intensities. Furthermore, they facilitate efficient background suppression because of the small overlap with the Cerenkov and fluorescence light contamination induced in the fiber-optic cable (the stem signal) primarily emitted in blue/green regions. The purpose of this study was to assess the suitability of red- emitting ISDs for real-time verification during BT. Material and Methods We investigated ISDs based on the 5 inorganic scintillators ruby, Y 2 O 3 :Eu, YVO 4 :Eu, Y 2 O 2 S:Eu and Gd 2 O 2 S:Eu, of which the first was rigid and the others in powder form. The ISDs were compared with plastic scintillation detectors (PSDs) based on the organic scintillator BCF-12. Each detector consisted of a 1 mm-diameter scintillator that was coupled to a 1 mm-diameter and 15 m-long fiber-optic cable. Optical filters were placed between the ISD volume and the fiber-optic cable to prevent the stem signal from striking the scintillator and inducing photoluminescence. The fiber-optic cable was coupled to a charge-coupled device camera or a spectrometer to measure signal intensities and emission spectra, respectively. The detectors were exposed to an 192 Ir HDR BT source in experiments dedicated to determine their scintillation intensities, the influence of the stem signal and photoluminescence, and time-dependent luminescence properties. Results Figure 1 shows the emission spectra of all detectors (left) and that the scintillation intensities were up to 19, 44, 16, 54 and 130 times larger than that of the PSD (right). Figure 2 shows time dependent luminescence properties of the ISDs. The Y 2 O 2 S:Eu and Gd 2 O 2 S:Eu ISDs are not recommended because their accuracy was compromised by their time dependence. The scintillation of the ruby, Y 2 O 3 :Eu and YVO 4 :Eu ISDs changed by +1.6%, -2.8% and +1.1%, respectively, during 20 Gy, which is the dose that the ISD inserted in urethra could absorb during a typical

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