Evaluation of Pelvic MRI-to-CT Deformable Registration for Adaptive MR-Guided Particle Therapy.

Purpose: We aim to assess the magnetic resonance imaging (MRI)-to-CT deformable image registration (DIR) quality of our treatment planning system in the pelvic region as the first step of an online MRI-guided particle therapy clinical workflow. Materials and Methods: Using 2 different DIR algorithms, ANAtomically CONstrained Deformation Algorithm (ANACONDA), the DIR algorithm incorporated in RayStation, and Elastix, an open-source registration software, we retrospectively assessed the quality of the deformed CT (dCT) generation in the pelvic region for 5 patients. T1- and T2-weighted daily control MRI acquired prior to treatment delivery were used for the DIR. We compared the contours automatically mapped on the dCT against the manual contours on the MRI (ground truth) by calculating the Dice similarity coefficients and mean distances to the agreement for organs at risk, targets, and outer contour. We assessed the dosimetric impact of the DIR on the clinical treatment plans, comparing the dose-volume histograms and the value of the clinical goals achieved for each dCT. The water equivalent path lengths and dose range 80% (R80%) maps were compared by casting on the beams' eye view. Results: The T1 sequences performed better for the DIR with ANACONDA compared against the T2. ANACONDA's performance agreed with Elastix. The bladder and rectum led to the worst agreement. For the remaining structures analyzed, Dice similarity coefficients above 0.80 were obtained. Maximum median deviations of 7.1 and 2.1 mm were observed for water equivalent path lengths and R80%, respectively, on the PTV. Conclusion: This work shows a good agreement on the DIR quality achieved with ANACONDA for the structures in the beams' path. By comparing the R80% generated with ANACONDA and Elastix, we give a first quantification of the uncertainties to be considered in an online MRI-guided particle therapy workflow for pelvic treatment.