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

Abstract

BackgroundElectromagnetic tracking (EMT) systems have been shown to provide valuable information on the geometry of catheter implants in breast cancer patients undergoing interstitial brachytherapy (iBT). In the context of an extended patient‐specific, pre‐treatment verification, EMT can play a key role in determining the potential need and, if applicable, the appropriate time for treatment adaptation. To detect dosimetric shortcomings the relative position between catheters, and target volume and critical structures must be known. Since EMT cannot provide the anatomical context and standard imaging techniques such as cone‐beam CT are not yet available in most brachytherapy suites, it is not possible to detect anatomic changes on a daily or fraction basis, so the need for adaptive planning cannot be identified.PurposeThe aim of this feasibility study is to develop and evaluate a technique capable of estimating follow‐up CTs at any time based on the initial treatment planning CT (PCT) and surrogate information about changes of the implant geometry from an EMT system.MethodsA deformation vector field is calculated from two different implant reconstructions acquired in treatment position through EMT, the first immediately after the PCT and the second at another time point during the course of treatment. The calculation is based on discrete displacement vectors of pairs of control and target points. These are extrapolated by means of different radial basis functions in order to cover the entire CT volume. The adequate parameters for the calculation of the deformation field were identified. By warping the PCT according to the deformation field, one obtains an estimated CT (ECT) that reflects the geometric changes. For the proof of concept, ECTs were computed for the time point of the clinical follow‐up CT (FCT) that is embedded in the treatment workflow after the fourth fraction.ResultsECT and clinical FCTs of 20 patients were compared to each other quantitatively in terms of absolute Hounsfield unit differences in the planning target volume (PTV) and in a convex hull (CH) enclosing the catheters. The median differences were 31.2 and 29.5 HU for the CH and the PTV, respectively.ConclusionThe proposed ECT approach was able to approximate the “anatomy of the day” and therefore, in principle, allows a dosimetric appraisal of the treatment plan quality before each fraction. In this way, it can contribute to a more detailed patient‐specific quality assurance in iBT of the breast and help to identify the timing for a potential treatment adaptation.

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.…”