ESTRO 37 Abstract book

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ESTRO 37

treatment plan no longer represented the implant geometry at treatment, and so, plan adjustment was performed for improved comparison.

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

Dose metrics were generated from dose grids calculated from measured dwell positions as inputs. Good agreement was seen for metrics less sensitive to small uncertainties. For the above example, the PTV D90% and V100% exhibited differences of 3% and <1%. When measurement uncertainties due to implant shift are accounted for, measured dose metrics (table 1) show good agreement with the plan, except for urethra D10%, where systematic uncertainties that contribute to individual dwell positions to be measured cranial of their true location, producing artefacts of higher urethra dose.

Conclusion Our source tracking system provides valuable information for routine clinical treatment verification. Pre-treatment imaging provides confidence the implant position is correct and tracked source dwells confirm treatment is delivered as expected. Post treatment dosimetric analysis provide valuable metrics that represent actual delivered patient dose. References: 1: Smith, R. L., et al (2017) (in press) An integrated system for clinical treatment verification of HDR prostate brachytherapy combining source tracking with pretreatment imaging. Brachytherapy . 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