Assessment of the Implant Geometry in Fractionated Interstitial HDR Breast Brachytherapy

Abstract

To determine the inter-fraction stability of the implant geometry in fractionated HDR interstitial brachytherapy (HDR-iBT) partial breast irradiations (PBI) by means of electromagnetic tracking (EMT). In our contemporary phantom study, an EMT protocol to track 6F catheters with an accuracy of 1±0.3 mm (maximum: 2 mm) in our typical clinical environment was established [Kellermeier et al. Med. Phys. 42(6):3533]. Based on that protocol, the implant geometry of 21 patients (17 patients: 9-fraction PBI and four patients: 2-fraction boost after teletherapy) was quantified during their iBT treatment. For each patient, the implant geometry was measured on the CT table just after acquiring the treatment planning scan (EMCT) and on the HDR treatment table directly after each treatment fraction (EMFx). Three 6 degrees of freedom (DoF) sensors were placed on the breast surface to obtain fiducial positions used to derive relative positions of a 5 DoF sensor manually and consecutively inserted in the catheters. To compensate for patient movements, e.g. respiratory motion, the implant sensor data were corrected against the mean position of the fiducial sensors. Using the catheter traces, dwell positions (DPs) were determined for each measurement. In analogy to the already published phantom studies, the rigid coherent point drift (CPD) algorithm was used to register corresponding DPs. For each patient, the determined DPs from EMCT and EMFx were compared against the CT-derived DPs from treatment planning (CTRef). In addition, to assess quantitatively the changes in the implant geometry - without the need for registration - the distances from each DP to all DPs within a 3 cm spherical range were determined. For each measured implant geometry, the change in these intra-implant DP distances (IIDD) was compared with those from CTRef. EMT of the whole catheter implant per patient (median: 17 catheters) took per fraction on average 6.8 minutes plus a few minutes for setup of the EMT system. The registered implant geometry from the CT table showed over all DPs a mean deviation of 1.3 mm (median: 1.1 mm, inter-quartile range IQR: 0.9 mm). Registration of the EMT-based DP from the fractions (EMFx) against CTRef resulted in an overall mean deviation of 2.4 mm (median: 2.2 mm, IQR: 1.7 mm). Throughout the fractionated treatment, no significant trend could be determined (see the Figure for the various measurement sessions). Regarding the change in IIDD over CTRef, EMCT showed a mean deviation of 0.9 mm (median: 0.6 mm, IQR: 1.1 mm). In the EMFx data, IIDD changed by 1.0 mm (median: 0.8 mm, IQR: 1.3 mm). EMT measurements in 21 HDR-iBT breast patients were feasible within the clinical workflow and well tolerated by the patients. EMT determined DPs based on the patient measurements on the CT table differed by 1.3 mm (mean) from the CT-derived DPs used for treatment planning. Throughout the fractionated treatment, the EMT determined DP deviation increased to 2.4 mm (mean) without a significant trend over the treatment duration.