Geodesic Registration for Cervical Cancer Radiotherapy Sharmili Roy 1 , John J. Totman 1 , Joseph Ng 2 , Jeffrey Low 2 , and Bok A. Choo 2 1 A*Star-NUS Clinical Imaging Research Centre, Singapore sharmili@nuhs.edu.sg 2 National University Cancer Institute, Singapore Abstract. Uterus, bladder and rectum are the maximally exposed or- gans during cervical cancer radiotherapy and are at high risk of radiation exposure. Estimation of dose accumulation in these organs across mul- tiple fractions of external beam radiotherapy (EBRT) and brachyther- apy (BT) is extremely challenging due to structural mis-correspondences and complex anatomical deformations between the EBRT and BT im- ages. This paper proposes a unified registration framework that aligns multiple EBRT and BT images of a patient to a single coordinate frame for a cumulative dose assessment. The proposed method transforms the radiotherapy anatomical images to their distance maps from the critical organs (uterus, bladder, and rectum) and registers the distance maps. A Markov random field model is used to fuse the resulting dense defor- mations and transform the anatomical image. Registration accuracy is evaluated on 42 clinical image pairs and it is shown that the proposed system outperforms existing methods in the literature. Keywords: Brachytherapy, dose accumulation, cervical cancer, geodesic registration 1 Introduction Radiotherapy has proven to be very effective for cervical cancer treatment. Con- current weekly chemotherapy, external beam radiotherapy (EBRT) and high- dose-rate brachytherapy (BT) is the standard treatment strategy for locally advanced cervical cancer. Brachytherapy, many a times in multiple fractions, is used to augment EBRT. Treatment outcome highly depends on organ doses over multiple treatment fractions [1]. Treatment evaluation should, therefore, consider the combined EBRT and BT dose in each tissue over all sessions. In current clinical practices, however, each treatment fraction is optimized indepen- dently resulting in multiple dose distributions correlated with their correspond- ing anatomical images. The anatomical images are collected at different points in time and exhibit large anatomical variations. Accurate registration of the un- derlying anatomical images is essential for dose accumulation [2]. Many factors make this registration very challenging. The brachytherapy applicator and the bladder balloon used for brachytherapy delivery introduce missing structural