Performance of an enhanced afterloader with electromagnetic tracking capabilities for channel reconstruction and error detection.

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To assess catheter reconstruction and error detection performance of an afterloader (Elekta Brachytherapy, Veenendaal, The Netherlands) equipped with electromagnetic (EM) tracking capabilities. The Flexitron research unit used was equipped with a special check cable integrating an EM sensor (NDI Aurora V3) that enables tracking and reconstruction capability. The reconstructions of a 24-cm long catheter were performed using two methods: continuous fixed-speed check cable backward stepping (at 1, 2.5, 5, 10, 25 and 50cm/s) and stepping through each dwell position every 1mm. The ability of the system to differentiate between two closely located (parallel) catheters was investigated by connecting catheters to the afterloader and moving it from its axis with an increment of 1mm. A robotic arm (Meca500, Mecademic, Montreal) with an accuracy of 0.01mm was used to move the catheter between each reconstruction. Reconstructions were obtained with a locally weighted scatterplot smoothing algorithm. To quantify the reconstruction accuracy, distances between two catheters were computed along the reconstruction track with a 5mm step. The reconstructions of curve catheter paths were assessed through parallel and perpendicular phantom configuration to the EM field generator. Indexer length and lateral errors were simulated and a ROC analysis was made. Using a 50cm/s check cable speed does not allow for accurate reconstructions. A slower check cable speed results in better reconstruction performance and smaller standard deviations. At 1cm/s, a catheter can be shifted laterally down to 1mm and all paths can be uniquely identified. The optimum operating distance from the field generator (50 to 300mm) resulted in a lower absolute mean deviation from the expected value (0.2±0.1mm) versus being positioned on the edge of the electromagnetic sensitive detection volume (0.6±0.3 mm). The reconstructions of curved catheters with a check cable speed under 5cm/s gave a 0.8mm±0.3mm error, or better. All indexer and lateral shifts of 1mm were detected with a check cable speed of 2.5cm/s or lower. The EM-equipped Flexitron afterloader is able to track and reconstruct catheters with high accuracy. A speed under 5cm/s is recommended for straight and curved catheter reconstructions. It allows catheter identification down to 1mm inter-catheter distance shift. The check cable can also be used to detect common shift errors.