Factors associated with deformation accuracy and modes of failure for MRI-optimized cervical brachytherapy using deformable image registration

Purpose: To identify factors associated with MRI-to-CT image deformation accuracy and modes of failure for MRI-optimized intracavitary high-dose-rate treatment of locally advanced cervical cancer. Methods and Materials: Twenty-six patients with locally advanced cervical cancer had preimplantation MRI registered and deformed to postimplantation CT images using anatomically constrained and biomechanical model–based deformable image registration (DIR) algorithms. Cervix (primary) and cervix plus 10-mm margin (secondary) were used as controlling regions of interest for deformation. High-risk clinical target volume defined on pre-MRI was propagated to CT and evaluated for clinical utility in optimizing target volumes using scores 0 (low performing) to 4 (high performing). Quantitative evaluation of deformation performance included Dice index, distance to agreement, center of mass (COM) differences, cervical/uterus volume, and geometric change in organ position for MR-projected structures. Statistical analysis was performed to identify predictors of clinical utility and modes of failure. Results: Anatomically constrained and biomechanical model–based deformable image registration algorithms achieved clinical utility >3 in 65% and 81% of patients, respectively. This improved to 81% and 85%, respectively, if cervix plus margin was used to drive deformations. Total COM displacement (cervix plus uterus) had the highest sensitivity in predicting low from high clinical utility in optimizing target volumes. Deformation failure (low clinical utility) resulted from high COM displacement, high cervical volume change, and retroverted uterine anatomy. Conclusions: MRI-to-CT deformable image registration using a cervix-controlling region of interest can aid clinical target delineation in cervical brachytherapy and potentially improve brachytherapy implant quality and clinical workflow. Deformation failures warrant further study and prospective deformation validation.