ESTRO 36 Abstract Book

S144 ESTRO 36 _______________________________________________________________________________________________

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

HDR prostate plan. The fluctuation could be reduced to <0.5% by mixing the Y 2 O 3 :Eu and YVO 4 :Eu phosphors in a ratio 1-to-10. The stem signal of the ruby, Y 2 O 3 :Eu and YVO 4 :Eu ISDs was up to 3%, 1% and 2%, respectively, of the total signal, and the photoluminescence was <1%, when the BT source moved 8 cm away from the detector and 1 cm from the fiber-optic cable. In contrast, the stem signal of the PSD was up to 70%.

Conclusion Red-emitting ISDs based on ruby, Y 2 O 3 :Eu are suitable for HDR BT treatment verification in real time. Their large signal intensities and emission properties facilitate accurate detector systems that are straightforward to manufacture and use which can result in widespread dissemination and improved patient safety during BT. OC-0279 Removing the blindfold - a new take on real- time brachytherapy dosimetry J. Johansen 1 , S. Rylander 1 , S. Buus 1 , L. Bentzen 1 , S.B. Hokland 1 , C.S. Søndergaard 1 , A.K.M. With 2 , G. Kertzscher 3 , C.E. Andersen 4 , K. Tanderup 1 1 Aarhus University Hospital, Department of oncology, Aarhus C, Denmark 2 Örebro University Hospital, Department of Medical Physics, Örebro, Sweden 3 The University of Texas MD Anderson Cancer Center, Department of Radiation Physics, Houston- TX, USA 4 Technical University of Denmark, Center for Nuclear Technologies, Roskilde, Denmark Purpose or Objective Although in-vivo dosimetry has been available for decades it is still not a standardized verification tool in brachytherapy (BT). Major limitations are that in-vivo dosimeters only provide point dose information and that the steep dose gradient leads to strong positional dependency. The aim of this study is to examine whether it is possible to utilise in-vivo dosimetry for evaluation of the implant geometry during irradiation in addition to post :Eu and YVO 4

hoc dose verification. Material and Methods

This study includes in-vivo dosimetry measurements from 22 HDR BT procedures for prostate cancer. Needles were placed in the prostate guided by TRUS with a subsequent