Dosimetric Impact of Deformable Image Registration versus Simple Summation in Multi-Fraction HDR Brachytherapy for Cervical Cancer under Standard Protocol

Abstract

Objective: In high-dose-rate brachytherapy for cervical cancer, cumulative dose is commonly estimated using simple summation (SS) of dose–volume histogram parameters, assuming high-dose regions remain spatially consistent across fractions. Deformable image registration (DIR) offers an alternative approach that accounts for interfractional anatomical variations. This study compared the dosimetric outcomes of SS and DIR under standardized clinical protocols.Material and Methods: This retrospective study included 54 patients with cervical cancer treated with four-fraction intracavitary brachytherapy using standardized applicators and a bladder-filling protocol with CT-based planning. Cumulative doses to the high-risk clinical target volume (HR-CTV), bladder, and rectum were calculated using both SS and DIR, aligned using applicator geometry. DIR accuracy was evaluated using the Dice Similarity Coefficient (DSC), mean distance to agreement (MDA), Hausdorff distance (HD), and Jaccard Index.Results: DIR yielded significantly lower accumulated HR-CTV doses across all the evaluated parameters (percentage differences were 8.2%, 6.0%, and 3.7% for D98, D90, and D50, respectively; all p-value<0.001) compared with SS, with moderate geometric agreement (DSC=0.72). For organs at risk, dose differences for the bladder and rectum were not statistically significant. Rectal geometric agreement was limited (DSC=0.60), whereas bladder agreement was good (DSC=0.89), likely influenced by the controlled bladder-filling protocol.Conclusion: DIR-based dose accumulation resulted in significantly lower HR-CTV dose estimates compared with SS, while dose differences for the bladder and rectum remained minimal. Geometric reliability varied across anatomical structures. Future research should correlate DIR-based dosimetry with clinical outcomes to support anatomically adaptive brachytherapy.