Purpose: Interstitial and intracavitary gynecological brachytherapy (IGBT) is advancing towards MR-only treatment planning (MRTP) workflows. 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 a novel QA system to evaluate the positioning accuracy of MR-only and conventional IGBT approaches.
Methods: Three 6F interstitial plastic needles, one CT/MR-compatible tandem and ring applicator set were examined in this study with the Flexitron afterloader system. The proposed QA system consists of attaching these to EBT3 film pieces, designing scanning protocols for MR, x-ray, film, and an image registration system between them. The source first dwell position (FDP) on the film was determined using a commissioned source localization algorithm. This is combined with a 4-point registration routine using skin markers on the film as surrogates between all images and four vitamin-D pills to assess registration quality. The MR scanning protocol was a 7-10 mm thick T1W slice that allowed a projection-like image of the setup (immersed in demineralized water). The FDP was compared between film, x-ray/CT markers, and applicator library models.
Results: MR images clearly displayed all applicators, needles, and skin markers. 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. Only for the ring, Flexisource-192-Ir-c FDP source-path-model matched with the film-measured FDP, whereas the 192-Ir-mHDR-v2 model and x-ray markers lead to 3-4 mm errors, and their use is discouraged. FDP reconstruction errors from the needle tip in MR with respect to film and x-ray markers were less than 1 mm.
Conclusion: MRTP is possible given that a sensitive and robust QA system is implemented to account for possible systematic shifts that could exceed 2 mm.
Brachytherapy, MR, Radiochromic Film
TH- Brachytherapy: Imaging for brachytherapy: development and applications