Department of Radiation Oncology, Universitätsklinikum Erlangen, Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU), Erlangen, Germany.
Comprehensive Cancer Center Erlangen-EMN (CCC ER-EMN), Erlangen, Germany.
Med Phys. 2023 Sep;50(9):5793-5805. doi: 10.1002/mp.16659. Epub 2023 Aug 4.
Electromagnetic 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.
The 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.
A 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.
ECT 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.
The 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.
电磁跟踪（EMT）系统已被证明可以为接受间质近距离放射治疗（iBT）的乳腺癌患者的导管植入物的几何形状提供有价值的信息。在扩展的患者特定的治疗前验证的背景下，EMT 在确定潜在需求方面发挥着关键作用，如果适用，则可以确定治疗适应的适当时间。为了检测剂量不足，必须知道导管之间的相对位置以及靶体积和关键结构。由于 EMT 不能提供解剖背景，并且标准成像技术（例如锥形束 CT）在大多数近距离放射治疗套件中尚不可用，因此无法每天或每次分割检测解剖变化，因此无法识别适应性计划的需求。
本可行性研究的目的是开发和评估一种能够基于初始治疗计划 CT（PCT）和 EMT 系统中有关植入物几何形状变化的替代信息来估算任何时间的随访 CT 的技术。
从 EMT 在治疗位置获取的两个不同的植入物重建中计算变形向量场，第一个重建是在 PCT 之后立即进行的，第二个是在治疗过程中的另一个时间点进行的。计算是基于控制和目标点对的离散位移向量进行的。这些向量通过不同的径向基函数进行外推，以覆盖整个 CT 体积。确定了计算变形场的适当参数。通过根据变形场对 PCT 进行变形，得到反映几何变化的估计 CT（ECT）。为了概念验证，在治疗流程中嵌入第四次分割后的临床随访 CT（FCT）时间点，计算了 ECT。
在计划靶区（PTV）和包含导管的凸壳（CH）方面，将 20 名患者的 ECT 和临床 FCT 进行了定量比较。CH 和 PTV 的中位数差异分别为 31.2HU 和 29.5HU。
所提出的 ECT 方法能够近似于“当天的解剖结构”，因此原则上允许在每次分割前对治疗计划质量进行剂量评估。通过这种方式，它可以为乳腺癌的 iBT 提供更详细的患者特定质量保证，并有助于确定潜在治疗适应的时机。
