PSOR11  Presentation Time: 5:20 PM: On the Use of a GYN Applicator for the Registration of an EMT-Enabled Afterloader and CT Scan in Interstitial GYN Brachytherapy

• Published in: Brachytherapy Vol. 22; no. 5; p. S32
• Main Authors: Sarmiento, Isaac Neri Gomez, Tho, Daline, Dürrbeck, Christopher, de Jager, Wim, Laurendeau, Denis, Beaulieu, Luc
• Format: Journal Article
• Language: English
• Published: Elsevier Inc 01.09.2023
• ISSN: 1538-4721; 1873-1449
• DOI: 10.1016/j.brachy.2023.06.041

Electromagnetic tracking (EMT) can be used to reconstruct the 3D geometry of implants used in interstitial gynecological (GYN) brachytherapy, but it does not share the same reference frame (RF) as the patient's medical image. This work validates the use of a tandem and ring (T&R) GYN applicator and its 3D model counterpart from a treatment planning system (TPS), to register the RF of an EMT-enabled brachytherapy afterloader with the RF of a CT scan image-set of a rigid phantom. Eleven 6F, 20 cm long interstitial catheters and a T&R applicator (Elekta, CT/MR 60°, 30 mm diameter ring and 40 mm long tandem) were placed in a rigid custom-made phantom (allowing different parallel and curved catheter paths) to reconstruct their geometry using a 5 Degree of Freedom EMT sensor attached to the check-cable of an EMT-enabled afterloader prototype. A magnetic field generator (NDI Aurora V3) was placed parallel to the catheters and were reconstructed using two different acquisition step-and-record (SR) and continuous motion (CM). In SR, the sensor moved forward every 2 mm and stopped for 3 s inside all five straight catheters and the tandem channel, and every 1 mm inside the six bent catheters and ring channel. In CM, after the sensor reached the channel tip, the sensor was retracted at a constant speed of 1 cm/s and the resulting point cloud, representing the reconstruction of the channel, was smoothed using a Savitzky-Golay filter. In all cases, the acquisition was performed at a rate of 40 Hz. A CT scan of the phantom inside a water cube was captured (0.5 mm slice thickness, 120 kV & 420 mAs). A rigid registration of CT and EMT RFs was performed using coherent point drift (CPD) based on three different subgroups of corresponding point clouds: the T&R applicator, four s-shape calibration templates, and four 5 mm diameter ceramic marbles mounted on each of the four templates. The CT-based reconstruction of the applicator was obtained by manually aligning the TPS applicator 3D model to the CT scan. The centroid of the marbles in the EMT RF is estimated by aligning the CAD model of the four calibration templates to their EMT counterpart using CPD. The absolute mean deviation from the expected value for EMT measurements is 0.33 ± 0.27 mm, while for the CT scan is 0.71 ± 0.30 mm, when compared to the 3D model of the phantom. The total mean registration errors between EMT and CT scan RF using the applicator-based registration are 0.85 ± 0.54 mm and 1.35 ± 0.88 mm, for SR and CM EMT acquisition methods respectively. For the S-shape template-based registration, the registration errors are 0.63 ± 0.32 mm (SR) and 0.62 ± 0.36 mm (CM), while for the marble & CAD-based registration the errors are 0.64 ± 0.30 mm (SR) and 0.65 ± 0.36 mm (CM). The calibration template of the proposed rigid phantom constitutes an efficient approach for accurate, submillimeter registration between EMT and imaging data, allowing for the design of efficient quality control tests in the future. While further evaluation is still needed, this work shows that the use of the TPS applicator library model placement within the image-set might be accurate enough (recommended accuracy ≤2 mm) for registration of implants obtained from EMT data to the CT image RF.