ESTRO 38 Abstract book

S35 ESTRO 38

various clinical environments (delivery in the operating room (OR), CT scanner room (CT), and patient bed). To determine the accuracy of EMT-based dwell-position detection, the residual error, i.e. the distance between the EMT-measured and planned dwell positions, was averaged per catheter. In addition, we determined the minimum error that can be detected by EMT by calculating the detection limit, defined as the mean residual error in the x-, y- and z-direction + 2 standard deviations. Results For the prostate phantom the residual error was on average over all catheters 0.6mm, 0.5mm and 0.3mm in the OR, CT room and the patient bed, respectively, compared to 0.3mm in the interference free set-up. For the catheters in the cervix applicator, the mean residual error was <0.4mm in the interference free set-up (Fig. 1). The minimum error that could be detected with EMT was <0.9mm in all cases (Table 1).

Purpose or Objective HDR interstitial brachytherapy (iBT) is a common treatment option for breast cancer patients. Despite its clinical success, errors which could occur during treatment planning or treatment delivery can have a dosimetric impact on the planned dose delivery. Currently, there is no extended quality assurance (QA) system available to detect these errors. The hybrid treatment delivery system (HTDS) is one of the possible QA methods for iBT. The system consists of an afterloader prototype system (Flexitron, Elekta, Veenendaal, The Netherlands) with an integrated electromagnetic tracking sensor, combined with an EMT system (Aurora, Northern Digital Inc., Canada). The HTDS allows the automatic measurement of the sensor's position in the implanted catheters. To test the feasibility of the system for error detection for breast cancer patients, possible planning and delivery errors in iBT were simulated using two phantoms. Material and Methods A CT-based treatment plan for each phantom was used as the reference data. Planning errors such as: an incorrect offset value, an incorrect indexer length, tip/connector end swaps, partial swaps, and delivery error such as whole implant displacement were simulated by altering the treatment plan. Other delivery errors such as catheter shifts and catheter connection swaps were simulated using two different phantoms: a rigid phantom with 9 straight catheters and a breast equivalent phantom with 10 bent catheters. To simulate catheter shifts, 1.1 mm thick spacer discs were attached between the phantom’s surface and the button at the catheter’s tip end. Catheter connection swaps were simulated by connecting the catheters to the incorrect channels. Geometrical deviations between the dwell points of reference data and error-induced data were assessed using an in-house Matlab routine. The median deviation was used to confirm different errors, which were then differentiated visually. For catheter shifts, the median value corresponds to the detected magnitude of the shift. Results The following errors were detected: Incorrect offset values (-10 mm and -15 mm), tip/connector end swaps, partial swaps between two catheters, and whole implant displacement (5 mm in lateral direction) in both phantoms. A total of 37 catheter connection swap measurements were conducted. Catheter connection swaps were detected 100% of the time. Shift measurements of one catheter were conducted 25 times for 1.1 to 5.07 mm shifts in each phantom. ROC analysis for shifts in the rigid phantom showed that 1.1 mm shifts in one catheter can be detected 98.36% of the time, while shifts larger than 2.21 mm can be detected 100% of the time. The measurement to determine the minimum detectable catheter shift in the breast equivalent phantom is still ongoing. Conclusion Simulated treatment planning and delivery errors in iBT were detected. This study showed that it is feasible and beneficial to use the HTDS to detect planning and delivery errors prior irradiation in iBT for breast cancer patients. OC-0076 Real time treatment verification in HDR brachytherapy: an in-phantom proof of principle M. Hanlon 1 , R. Smith 2 , V. Panettieri 2 , J. Millar 2,3 , R. Franich 1,2 1 RMIT University, School of Science, Melbourne, Australia; 2 The Alfred, Alfred Health Radiation Oncology, Melbourne, Australia; 3 Monash University, Central Clinical School, Melbourne, Australia Purpose or Objective HDR brachytherapy treatment verification can be achieved by tracking the source during treatment to confirm correct dwell delivery. Our system utilises a flat

Conclusion Using the integrated EMT/BT system, we quantified errors due to EM field interference in the BT clinical setting for pelvic tumours. We found a mean residual error<0.6mm, which is smaller than the dose calculation grid size (1mm). With our current method we are able to detect dwell position shifts>0.9mm. We therefore conclude that the effect of EM field interference due to medical equipment is limited and that EMT is sufficiently accurate for dwell position detection in prostate and cervical cancer. OC-0075 Error detection using an electromagnetic tracking system in multicatheter interstitial brachytherapy S. Masitho 1 , K. Kallis 1 , V. Strnad 1 , R. Fietkau 1 , C. Bert 1 1 Universitätsklinikum Erlangen, Radiation oncology, Erlangen, Germany