Evaluation of deformable image registration in HDR gynecological brachytherapy

Introduction: In brachytherapy, as in external radiotherapy, image-guidance plays an important role. For GYN treatments it is standard to acquire at least CT images and preferably MR images prior to each treatment and to calculate the dose of the day on each set of images. Then, the dose to the target and to the organs at risk (OAR) is calculated with worst case scenario from ICRU recommendation that propose add maximum doses to the volume of 2cc of every organ (D2cc) from every session. Especially for the OAR, it makes the safe conservative assumption that the same part of the OAR always receives the highest dose. The dose calculated this way often limits the prescription dose or the target coverage. In this work, we propose to investigate an alternative method to assess the cumulative dose for an HDR GYN treatment course; the use of image deformation software to track the deformations of the OAR from one fraction to the next. Materials and Methods: Thirty patients with locally advanced cervical cancer who had been treated by external beam radiotherapy (EBRT) and three to four fractions of HDR brachytherapy (HDR-BT) were introduced to this study. Intensity-based free form deformation algorithm has been used as the primary DIR techniques using MIM software. In the dose accumulationn stage, total doses of rectum and bladder from all brachytherapy fractions have been integrated by deformable image registration techniques. Results: The DICE, jaccard, MDA and HD indexes of deformable registration for bladder and rectum were (0.94±0.03), (0.89±0.06), (0.83±0.3), (9.85±3.8) and (0.86±0.05), (0.78±0.07), (1.46±0.7), (14.4±0.1) respectively compared to rigid registration for bladder and rectum that were (0.74±0.07), (0.61±0.07), (4.2±2.3), (18.73±8.6) and (0.59±0.13), (0.43±0.11), (5.01±1.7), (23.4±1.1) respectively. Dose accumulation of D2cc values were (5.1±2,3)% and (2.7±1.1)% lower for the bladder and rectum respectively compared to adding D2cc of each fractions. Conclusion: Using deformable image registration, based on image intensity, allows to accumulate the dose from different brachytherapy fractions. The results from our study indicate that it is feasible to utilize deformable image registration for dose accumulation.