Abstract Book

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ESTRO 37

OC-0171 Quality assurance for interstitial brachytherapy using an EMT system integrated into an afterloader K. Kallis 1 , V. Strnad 1 , R. Fietkau 1 , C. Bert 1 1 University Hospital Erlangen, Radiation Oncology, Erlangen, Germany Purpose or Objective Irradiation of the tumor bed after breast conserving surgery using interstitial brachytherapy (iBT) is a common treatment option for breast cancer. However, so far no extensive quality assurance (QA) protocol has been introduced to detect possible errors prior to irradiation and hence ensure the planned treatment delivery. Therefore, this study explores the feasibility of electromagnetic tracking (EMT) integrated into an afterloader for QA in high-dose-rate multi-catheter iBT. Material and Methods The phantom and patient measurements were conducted with a hybrid Flexitron afterloader (Elekta, Veenendaal, The Netherlands) equipped with an EMT sensor. The system consists of the Flexitron prototype in combination with an Aurora EMT system (NDI, Waterloo, Canada). After connecting the prototype to the applicator and placing the field generator lateral to the patient, all catheters were sequentially and automatically tracked. Based on phantom measurements, the accuracy and precision of the system were determined and a reliable measurement routine, including the sensor step size and stopping time, was identified. Further, different fitting and interpolation techniques for dwell position (DP) reconstruction, were evaluated. After rigid registration of the catheter traces and reconstruction of the DP, all estimated points were compared to the DP defined in treatment planning. Three different phantoms were used for the evaluation. Until now, the geometry changes of 18 patients treated with iBT were acquired and explored. Treatment of those patients was delivered using a microSelectron afterloader (Elekta, Veenendaal, The Netherlands) before conducting the EMT measurement. Results A measurement step size of 10 mm showed the best trade-off between measurement time (4 min) and reconstruction precision after registration. For reconstruction of the DP, interpolation of the EMT raw data yielded overall the best results (RMSE=1.27 mm), however no difference between linear, cubic and B-spline interpolation was detected. Although, fitting a third degree polynomial to the phantom measurements seemed to be more stable considering motion and distortions, overall the reconstruction by fitting was inferior (RMSE=1.32 mm) to interpolation. First patient measurements indicate that reconstructing DP by fitting a cubic function (RMSE=2.46 mm) or interpolating additional points (RMSE=2.04 mm) were both reliable techniques for data analysis. The measurement lasted between 6-9 minutes depending on the amount of catheters. Moreover, first results showed that even small shifts of individual catheters could be detected. Conclusion Using an EMT system integrated into an afterloader for QA in iBT of breast cancer has proven to be feasible and beneficial. The developed algorithm is able to detect within seconds deviations from the treatment plan prior irradiation. From a workflow and technical perspective the prototype could be well integrated into the clinical routine.

OC-0172 Development of an inorganic scintillation detector system for in vivo dosimetry for brachytherapy G. Kertzscher 1 , S. Beddar 1 1 The University of Texas MD Anderson Cancer Center, Department of Radiation Physics, Houston, USA Purpose or Objective Brachytherapy (BT) clinics presently do not verify treatments in real time because there is a lack of affordable technology that precisely can monitor the treatment progression. Inorganic scintillation detectors (ISDs) are promising for in vivo dosimetry during BT because their large signal intensities can generate a wide dynamic range and result in a negligible Cerenkov and fluorescence light contamination induced in the fiber- optic cable (the stem signal). The purpose of this study was to develop an in vivo dosimetry system based on ISDs that can be widely disseminated to BT clinics for real- time treatment verification. Material and Methods We have manufactured miniature ISDs based on ruby (Al 2 O 3 :Cr), Y 2 O 3 :Eu, YVO 4 :Eu, ZnSe:O or CsI:Tl. The ISDs consisted of a 1 mm-size scintillator that was optically coupled to a 1 mm-diameter and 15 m-long fiber-optic cable. The ISDs were connected to a charge-coupled device camera or a spectrometer spectrograph, and placed in a water phantom during experiments with a 192 Ir BT source, to measure their emission spectra, scintillation intensities and influences of the stem signal, photoluminescence and time-dependent luminescence properties. The ISDs were compared with detectors based on the commonly used organic scintillators BCF-12 and BCF-60. A new in vivo dosimetry system based on ISDs was developed using photodetector and data acquisition components that matched the luminescence characteristics of the ISDs. The new system was tested in water phantom experiments to determine its dynamic range and signal-to-noise ratio. Results The scintillation intensities of the ISDs were between 20 and 900 times greater than the intensity of the BCF-12 based detector (Figure 1). The large scintillation intensity of the ZnSe:O and CsI:Tl based ISDs made it possible to develop an in vivo dosimetry system based on low-cost photodetector and data acquisition components. The final detector system (Figures 2A and B) measured dose rates with <0.2% precision for source-to-detector distances up to 10 cm (Figure 2C and D). The stem signal was <0.5% of the total signal for the ZnSe:O and CsI:Tl based ISDs, and up to 5% and 3% for the ruby and Y 2 O 3 :Eu and YVO 4 :Eu based ISDs, respectively. The photoluminescence background was up to 1% for the ruby based ISD and <0.5% for the other ISDs. All ISDs exhibited stable scintillation during constant irradiation and negligible afterglow.