PL02 Presentation Time: 1:45 PM

Abstract

Purpose

To design, construct, and evaluate a system for assisted placement of brachytherapy applicators using electromagnetic tracking (EMT) technology that has been registered to CT or MRI images. The system provides real-time localization of needles during the insertion process, a three-dimensional display of planned needle sites, visibility of the anatomy and needle position during placement, and reference tracking to account for generator or target anatomy shifts. Such a system might be used to reduce brachytherapy procedure times, improve correspondence between intended and actual needle positions, or decrease the trainee learning curve. The system is notable for the following features: 1) Real-time visual and quantitative feedback of needle placement with respect to the underlying anatomy, as visualized by MRI or CT image, without continuous or repeated imaging 2) Pre-planning capability with a graphical overlay of target needle trajectories 3) Reference tracking to account for electromagnetic field generator or target anatomy shifts 4) DICOM-coordinate digital reconstruction of applicator locations for treatment planning and/or pre-treatment quality assurance 5) Compatibility with standard brachytherapy workflows including fixed table CT and MRI systems, procedures within or outside of a brachy suite, and insertion of needles in dorsal lithotomy position 6) Consists of commercially available EMT technology components

Methods

The system was constructed using an Aurora (Northern Digital Instruments, Waterloo, ON) planar 20 × 20 cm² field generator (EFG), System Control Unit, Sensor Interface Unit, and 6DOF and 5DOF Flextube sensor tools. The graphical user interface was written as a Matlab application with native functions and toolboxes. EMT-to-DICOM registration was based upon intracavitary applicators (e.g., tandem and ovoids), placed prior to imaging and digitized automatically using thresholding methods. The EFG was positioned above the pelvis (Figure 1A), EMT sensors were translated through the tandem and ovoid channels, and the EMT system was registered to the DICOM image set using an iterative closest-point algorithm. Next, a 5DOF EMT sensor was loaded into the distal inner lumen of a brachytherapy needle for placement. The system display provides axial, coronal, sagittal, and 3D-volumetric CT/MRI views. Proof-of concept and system accuracy were evaluated in phantom and human cadaver by comparing EM-tracked needle positions with ground-truth, post-implant CTs.

Results

Proof of concept was demonstrated for EMT-assisted placement of brachytherapy needles in a realistic clinical environment and on a brachy suite CT table. Figure 1B provides an example of how a pre-planned needle location (blue) can be visualized alongside real-time needle placement (red) to provide feedback to the user. The left-hand panel in Figure 1B shows an initial attempt where a guided needle is not in the intended location. The right-hand panel shows placement after readjustment. Accuracy in phantom (mean ± standard deviation) was 0.76 ± 0.13mm for needle tips placed up to 75mm from the tandem/ovoids and 0.52 ± 0.27mm for needle shafts at distances up to 100mm from the tandem/ovoids. Figure 1C illustrates the human cadaver EM-tracked needle positions and manually digitized positions from a post-implant CT scan. Tip and shaft accuracies were 0.77 ± 0.14mm and 0.40 ± 0.21mm, respectively.

Conclusion

An EMT-based guidance system provided sub-millimeter accuracy for the placement of brachytherapy needles without repeated or continuous imaging. The technology can be used to reduce brachytherapy procedure times, improve the correspondence between intended and actual needle positions, or decrease the trainee learning curve.