Localization of reference points in electromagnetic tracking data and their application for treatment error detection in interstitial breast brachytherapy

Abstract

BackgroundElectromagnetic tracking (EMT) is a promising technology that holds great potential to advance patient‐specific pre‐treatment verification in interstitial brachytherapy (iBT). It allows easy determination of the implant geometry without line‐of‐sight restrictions and without dose exposure to the patient. What it cannot provide, however, is a link to anatomical landmarks, such as the exit points of catheters or needles on the skin surface. These landmarks are required for the registration of EMT data with other imaging modalities and for the detection of treatment errors such as incorrect indexer lengths, and catheter or needle shifts.PurposeTo develop an easily applicable method to detect reference points in the positional data of the trajectory of an EMT sensor, specifically the exit points of catheters in breast iBT, and to apply the approach to pre‐treatment error detection.MethodsSmall metal objects were attached to catheter fixation buttons that rest against the breast surface to intentionally induce a local, spatially limited perturbation of the magnetic field on which the working principle of EMT relies. This perturbation can be sensed by the EMT sensor as it passes by, allowing it to localize the metal object and thus the catheter exit point. For the proof‐of‐concept, different small metal objects (magnets, washers, and bushes) and EMT sensor drive speeds were used to find the optimal parameters. The approach was then applied to treatment error detection and validated in‐vitro on a phantom. Lastly, the in‐vivo feasibility of the approach was tested on a patient cohort of four patients to assess the impact on the clinical workflow.ResultsAll investigated metal objects were able to measurably perturb the magnetic field, which resulted in missing sensor readings, that is two data gaps, one for the sensor moving towards the tip end and one when retracting from there. The size of the resulting data gaps varied depending on the choice of gap points used for calculation of the gap size; it was found that the start points of the gaps in both directions showed the smallest variability. The median size of data gaps was ⩽8 mm for all tested materials and sensor drive speeds. The variability of the determined object position was ⩽0.5 mm at a speed of 1.0 cm/s and ⩽0.7 mm at 2.5 cm/s, with an increase up to 2.3 mm at 5.0 cm/s. The in‐vitro validation of the error detection yielded a 100% detection rate for catheter shifts of ≥2.2 mm. All simulated wrong indexer lengths were correctly identified. The in‐vivo feasibility assessment showed that the metal objects did not interfere with the routine clinical workflow.ConclusionsThe developed approach was able to successfully detect reference points in EMT data, which can be used for registration to other imaging modalities, but also for treatment error detection. It can thus advance the automation of patient‐specific, pre‐treatment quality assurance in iBT.

A comprehensive quality assurance protocol for electromagnetic tracking in brachytherapy

“…EMT is an elaborated tracking technology with the characteristic feature of accurate 3D position determination in the absence of a direct line-of -sight, which makes it particularly advantageous for clinical applications such as for the navigation of diagnostic and therapeutic procedures inside patients. Recently, the possibilities offered by EMT have attracted interest in the field of BT and various studies have shown promising results for tasks ranging from the detection of treatment errors 22,25,26 and uncertainties 20,23,24 to implant reconstruction. 28 In anticipation of a wider clinical application of EMT, a profound knowledge of the uncertainties associated with EMT in the context of BT are needed.…”

“…17 By integrating EMT functions into an afterloader and automatically driving the sensor through the BT applicator or implant, this so-called hybrid treatment delivery system allows assessment of the applicator/implant geometry inside the patient directly on the treatment table and without the need for other imaging techniques like CT, MRI, or US. [18][19][20][21] Various phantom and patient studies from different institutions have demonstrated that BT treatments benefit from the geometric information acquired by EMT, especially with respect to treatment error and uncertainty detection 20,[22][23][24][25][26][27] and automated applicator/implant reconstruction. [28][29][30][31] However, to establish EMT as an integral part of the clinical procedure, it must be ensured that the EMT system itself functions properly and achieves the necessary accuracy and precision.…”

Accuracy of an electromagnetic tracking enabled afterloader based on the automated registration with CT phantom images

“…for the same system 17 . A higher check cable speed would result in more outliers, due to a slight asynchronism of the timestamps of the EMT system compared to the afterloader device, according to Dürrbeck et al 22 …”

Estimating follow‐up CTs from geometric deformations of catheter implants in interstitial breast brachytherapy: A feasibility study using electromagnetic tracking

“…With an EMT‐equipped check cable, it is possible to determine the implant geometry in almost no extra time and without in‐room imaging facilities. The geometric information thus obtained is the basis for all applications of EMT in iBT, which range from catheter reconstruction 14,15 to error detection 16–18 and treatment verification 19–21 . A major drawback is that, unlike (cone‐beam) CT imaging, EMT cannot provide information about the anatomic context and hence about the target volume and the critical structures, without which the treatment plan quality cannot be appraised according to GEC‐ESTRO recommendations 11 .…”