Uncertainties in MR-to-CT Image Registration for HDR Cervical Brachytherapy

MR-based planning provides many benefits to planning of the HDR brachytherapy component of treatments for cervical cancer patients but requires the acquisition of different imaging sets that need to be registered for planning. However, registration between images introduces unknown uncertainties which impact the dose metrics that are essential to assessing brachytherapy plans. The goal of this project is to determine the overall effects of uncertainties in image registration between 1) T1- and T2-weighted MR images, and 2) planning CT and T1-weighted MR. The study looks at a total of 60 treatment fractions from thirteen patients for tandem and ovoid treatments. Workflows were created to compare clinical registrations with an automated intensity-based registration. In evaluating the overall uncertainty budget due to registration, the two different contour sets from the two different registrations were compared by calculation of the percent differences in important dosimetric values for the treatment target and also organs at risk. The data collected show that for both T1-MR to T2-MR and T1-MR to CT registration the average deviation for the target was very close to 0 with a reasonable tight standard deviation within 3%. However, the average deviation for organs at risk is greater which may mean patients are receiving more dose to those organs at risk than indicated due to uncertainties of registration caused by shifts and deformation between CT and T1- and T2-weighted MR.