PHSPP05 Presentation Time: 3:10 PM

Purpose

Image-guided interstitial and intracavitary gynecological brachytherapy (IGBT) is advancing towards MR-only workflows because of MR's superior soft-tissue contrast for tumor delineation and tissue segmentation. However, current treatment planning still necessitates the use of CT/MR fusion to aid in needle reconstruction and quality assurance (QA). In this work, we present strategies to visualize needles on MR and provide QA procedures for MR-only IGBT with comparisons to the conventional CT/MR approach.

Materials and Methods

Three 6F interstitial plastic needles and one CT/MR-compatible intrauterine tandem and ring (T&R) applicator set were examined in this study with the Flexitron afterloading system (Elekta Brachytherapy, Netherlands). To visualize the needles, they were immersed in demineralized water and scanned without markers in a 3T MRI system (Signa Premier; GE Healthcare, Milwaukee, WI). MR images were acquired with three protocols: 3D T1W (TR\TE 552\10 ms, and 1 mm voxel), 3D T2W (TR\TE 3051\104 ms, and 1 mm voxel) and silent Zero TE sequence (TR\TE 1060\20 ms, and 0.58 mm voxel). For QA, a 2D MR protocol (T1W, 7-10 mm thick slice) was utilized that allowed a projection-like image of an EBT3-film-based setup which was also imaged (with CT/x-ray markers) in CT (Philips, Netherlands) and x-ray imager (Varian Medical Systems, Palo Alto, CA). This setup consists of attaching catheters and four vitamin-D pills to EBT3 film pieces. The four corners of each film piece (5" x 4") were marked with skin markers that are visible in CT and MR images. These skin markers served as surrogates for a 4-point registration system between all images, and the vitamin-D pills were used to assess registration quality. An efficient film handling/scanning protocol was designed for this work and the source first dwell position (FDP) on the film was determined using a commissioned source localization algorithm. The FDP was compared between film, x-ray/CT markers, and applicator library models.

Results

For needle visualization, ZTE images generated a positive contrast for the interstitial needles as compared to conventional T1 and T2 images (See Figure). For QA, the 2D MR slice and x-ray image clearly displayed applicators, needles, vitamin-D pills, and skin markers allowing accurate registration to film images, visualization, and localization of the catheters, source positions, and markers in one fused image (See Figure). The tandem tip from MR was 1.1 mm longer than that of the x-ray and model which was not evident in the vitamin-D matching, implying a potential MR artifact at the tip (Sub-Figure B) which should be taken into account while planning. FDP reconstruction errors from the needle tip in MR with respect to film and x-ray markers were less than 1 mm (Sub-Figure D).

Conclusions

While further investigation is required, ZTE imaging offers a promising alternative to CT imaging for needle reconstruction supporting MR-only gynecological brachytherapy treatment planning. However, MR-only workflow requires special considerations in QA to account for possible systematic shifts that could exceed the level of accuracy achieved in x-ray-based IGBT.