MRI-Guided High-Dose-Rate Gynecologic Brachytherapy Using an MR-Linac as an MR Simulator: A Single Institutional Experience.

PURPOSE: The goal of this study was to commission the use of a magnetic resonance linear accelerator (MR-linac; Unity) for imaging of gynecologic high-dose-rate (HDR) brachytherapy. This included optimizing imaging protocols and workflow development. METHODS AND MATERIALS: T1-weighted and T2-weighted HDR imaging protocols were optimized on the Unity for HDR gynecologic imaging and treatment planning. Phantom measurements using these protocols were performed to determine geometric distortion and to assess reconstruction accuracy of the applicator compared with the ground truth computed tomography image. A treatment plan was created within the treatment planning system that was then delivered to a phantom. New workflows were developed which were tested with a full dry run with a healthy volunteer including patient transfer, anesthesia considerations, and data transfer. Validation of the workflow was completed on 1 patient who received imaging on both the Unity magnetic resonance imaging (MRI) and on a dedicated 3 Tesla MRI simulator. RESULTS: Imaging analysis results were favorable with MR-linac images with a maximum distortion of 0.96 mm and a 1.36-mm over a 350-mm diameter spherical volume on the T1- and T2-weighted images, respectively, and the maximum effect of the applicator was 0.36 ppm of the main magnetic field. Reconstruction uncertainties of the Venezia applicator's tandem and 2 lunar-ovoids on the MR-linac images were within the 2-mm tolerance of the International Commission on Radiation Units and Measurements Report 89. Treatment planning and delivery was performed on the MR-HDR quality assurance phantom without issue. Dry run and healthy volunteer imaging showed adequate performance of both vital monitoring and HDR equipment. For the patient for which both the Unity MRI and 3 Tesla images were acquired, 95.78% and 95.80% of the high risk clinical target volume received 100% of the dose, respectively. Both plans were considered clinically acceptable. CONCLUSIONS: Unity MR-linac images were successfully used in gynecologic HDR brachytherapy treatment planning, and a usable workflow was established.

A quality assurance tool for daily checks of the gamma knife high definition motion management system.

PURPOSE: This note describes the performance of a quality assurance (QA) tool built for daily checks of the Gamma Knife's high definition motion management (HDMM) system. METHODS: The tool is a three-dimensional (3D)-printed platform with a raised corner in the center. A reflector post is placed at the corner and the HDMM tool is zeroed to this position. Gage blocks produce very accurate gaps between the post and corner and the HDMM system's readout is compared to the gage block thickness. The HDMM system and tool were tested for noise, stability, reproducibility, linearity, accuracy and overall setup times plus ease of use. RESULTS: The QA tool performed with accuracies better than 0.1 mm. The setup and use of this tool take less than two minutes making it a suitable tool for daily use. CONCLUSION: This QA tool is a cost-effective solution that provides a fast and easy confirmation of the HDMM accuracy, making it suitable for daily QA checks of the HDMM system.