MRI-Only Intracavitary Brachytherapy: Commissioning and Clinical Implementation
Abstract
Purpose
This study reports on a comprehensive MRI-only intracavitary brachytherapy (MR-ICB) commissioning framework with quantitative isodose-based validation against CT.
Methods
3D T2, CISS (Constructive-Interference-in-Steady-State), and TRUFI (True-Fast-Imaging-with-Steady-State-Precession) MRI sequences were optimized on a 1.5T scanner (Siemens MAGNETOM) for visualization of Elekta’s MRI line markers that can be securely inserted into Elekta's MR-conditional tandem and ovoid applicators. An in-house ultrasound gel phantom with external fiducials was constructed to validate applicator geometry and marker tip localization. Vendor-supplied applicator 3D models containing source path definitions were utilized for applicator reconstruction. Five retrospective patient cases with 15 treatment fractions were reconstructed on CT and MRI and planned using a Point-A-prescription. The CT-derived pear-shaped dose distribution served as the reference. Following manual rigid CT–MRI registration, agreement of 100%–250% isodose lines (IDLs) was quantified using 95th-percentile-Hausdorff-distance (H95), mean-distance-agreement (MDA), and Dice-coefficient (DC), enabling evaluation in dose-gradient-regions. First-dwell-position accuracy was also evaluated by loading a single dwell and comparing 100%–500% IDLs at applicator tips.
Results
For applicator-tip verification, MDA, DC, and H95 were 1.12 ± 0.07 mm, 0.85 ± 0.012, and 1.21 ± 0.02 mm for the 500% IDL; 1.08 ± 0.03 mm, 0.91 ± 0.009, and 1.21 ± 0.02 mm for the 200% IDL; and 1.06 ± 0.03 mm, 0.94 ± 0.006, and 1.10 ± 0.21 mm for the 100% IDL. For Point-A–based-plans, H95 values ranged from 1.29–1.47 mm, with DC ≥ 0.97 and MDA ≤ 1.33mm across the 100%–250% IDLs. The maximum observed H95 (1.73mm) was within the ICRU Report 89 recommended uncertainty for applicator reconstruction of 2 mm.
Conclusion
This work introduces an MR-ICB commissioning procedure with quantitative isodose-based validation that demonstrated millimeter-level agreement with CT-based planning thereby providing evidence for clinical adoption.