Clinical implementation of MR‐guided vaginal cylinder brachytherapy

Abstract

We present an institutional experience on the clinical implementation of magnetic resonance (MR)-guided vaginal brachytherapy using commercially available solid applicator models. To test the fidelity of solid applicator models to digitize vaginal cylinder applicators, three datasets were evaluated. The first included 15 patients who were simulated with CT alone. Next, a water phantom was used to evaluate vaginal cylinders ranging from 20 to 35 mm in diameter with CT and MR. Finally, three patients undergoing HDR brachytherapy with vaginal cylinders that were simulated with both CT and MR were evaluated. In these assessments, the solid applicator models were aligned based on the outline of the applicators on the corresponding volumetric image, and deviations between the central source positions defined based on X-ray markers (on CT) and solid applicator models (on CT and MR), and the percent dose difference between select reference points were calculated. The mean central source deviation defined based on X-ray markers (on CT) and solid applicator models (on CT and MR) for the 15-patient cohort, the phantom, and the 3-patient cohort is 0.6 mm, 0.6 mm, and 1.2 mm, respectively. The average absolute percent dose difference for the bladder, rectum, prescription, and inferior reference points were 2.2%, 2.3%, 2.2%, and 2.4%, respectively, for the 15 patient cohort. For the phantom study, the average, absolute percent dose difference for the prescription and inferior reference points are 2.0% and 2.1% for the CT, 2.3% and 2.2% for the T1W, and 2.8% and 3.0% for the T2W images. For the three patient cohort, the average absolute percent dose difference for the bladder, rectum, prescription, and inferior reference points are 2.9%, 2.6%, 3.0%, and 4.2% for the CT, 6.5%, 1.6%, 2.5%, and 4.7% for the T1W, and 6.0%, 7.4%, 2.6, and 2.0% for the T2W images. Based on the current study, aligning the applicator model to MR images provides a practical, efficient approach to perform MR-based brachytherapy planning.

WITHDRAWN: Image-guided navigation systems for interstitial brachytherapy

“…Current solutions are to make MR compatible paramagnetic or some nonmetallic (plastic) devices and titanium applicators and seeds [91]. Further efforts to exploit alternative MR compatible materials, MR compatible markers, new sequences and reconstruction methods of applicator and seed visualization are required for MRI-based guidance and postimplant dosimetry assessment [92,[99][100][101]. So far, real-time MRI guidance is not generally used except at some major medical centers that have MR scanners available for brachytherapy procedures.…”

Magnetic resonance imaging-guided brachytherapy for cervical cancer: initiating a program

“…The use of intracavitary titanium applicators for gynecologic brachytherapy is feasible at field strengths of 3T (shown in Figure 2 ) and lower [ 38 ], and some MRI pulse sequences such as T1w [ 34 ], proton-weighted MRI [ 39 ] and 3D pulse sequences [ 40 ] may improve applicator visualization. Although, source channel digitization has been aided with the use of X-ray markers for CT - based imaging, reliably and reproducibly fabricating MR compatible markers has been challenging [ 41 , 42 ]. Additionally, the presence of applicators introduces artifacts in MRI images, and while these artifacts are very minimal with plastic applicators, they are pronounced when titanium applicators are in place.…”

“…There have been several proposed methods for applicator reconstruction using MRI including the use of an MR - compatible marker [ 34 , 43 ], applicator models, and/or plan libraries [ 10 , 44 ]. Each of these methods have known limitations, such as the limited availability of commercial MR-compatible markers, difficulties fabricating leak-tight MR markers, and availability of models for commercial applicators [ 41 ].…”

“…Reports on applicator reconstruction using MRI images with in - house fabricated MR-visible markers have been published [ 41 , 42 ]. However, the authors reported difficulties maintaining a water tight seal of these markers.…”

Simple phantom fabrication for MRI‐based HDR brachytherapy applicator commissioning

“…Previous work has described commissioning performed in preparation for MRI‐based and MRI‐guided HDR programs, including phantom measurements. This has been done using water phantoms: for example, Owrangi et al 6 and Zhang et al 7 performed MRI measurements in water phantoms and described their implementation processes for incorporating MRI into their HDR brachytherapy programs. Zhang et al noted that a gel phantom would have been preferable to water, but that the institution did not have a gel phantom available.…”