Purpose: Eliminating the patient-Computed Tomography (CT) scan for patients undergoing tandem and ring (T&R) brachytherapy procedures reduces time with the applicator in place and eliminates imaging dose. However, visualizing titanium applicators in a patient-Magnetic Resonance Imaging (MR) is difficult. We evaluated the reproducibility of an MR-only based digitization, and assessed the feasibility of a new workflow where a reference-CT of the T&R applicator (without a patient) is used in lieu of a patient-CT.
Methods: For the MR-only workflow, 8 physicists replanned a case to assess reproducibility. We used PD weighted TSE-sequence with a 1.5T-Siemens and BrachyVisionâ„¢ solid-applicator library for contouring and digitization, respectively. For the reference-CT workflow, 8 physicists registered the reference-CT to the T2-weighted patient-MR. The patient-MR was used for planning, with the registered reference-CT displayed during digitization to assist in applicator reconstruction. We evaluated dwell positions and plan quality differences among the plans in terms of the high-risk-CTV (HRCTV) V100, D100 and D90, and D2cc of the rectum, sigmoid, bladder, and small bowel.
Results: MR-Only workflow: Comparison of MR-Only digitization showed a mean variation between plans of 1.3mm in the tandem (range:0.34-3.74mm), and 2mm in the ring dwell positions (range:0.27-3.68mm). Coverage of the HRCTV changed as much as 14%, and dose to OARs up to 19%. Reference CT workflow: The uncertainty in dwell positions was reduced by 30% in the tandem and 10% in the ring. Mean dwell-position variation between plans was 0.9mm (range:0.2-1.6mm) in the tandem and 1.8mm (range:0.5-3.4mm) in the ring. The standard deviation of resulting dose distributions decreased for the HRCTV V100 (6.6%to3.3%), and Rectum-D2cc (8%to3.3%).
Conclusion: Registering the reference-CT for applicator reconstruction to the patient-MR prior to digitization decreased the uncertainty in dwell position placement in MR-based planning, resulting in improved reproducibility and reduction of plan deviations.