Deformable registration when combining brachytherapy and external beam radiotherapy

Sammanfattning: Patients with cervical cancer are treated both with external beam radiotherapy(EBRT) and brachytherapy (BT) which involves an applicator. During the treatments, CT-images are taken and to perform dose calculations, deformable image registration (DIR) is performed. The image registration involves many challenges, for example, the organs may have different shapes and volumes in the images and the images have different intensities due to the applicator. Many DIR-methods are available but they fail in aligning the multiple organs correctly and simultaneously. This project aimed for developing a method that created triangle meshes of the rectum and bladder in EBRT- and BT-images.The generation of triangle meshes was driven by projection points on the organ and found pairs of points between the same organ in both images. The performance of using these triangle meshes in DIR was compared with that of using boundary conditions (also triangle meshes) and binary masks. Results show that the triangle meshes and boundary conditions performed similarly concerning the DSC, mean DTA and HD validated on rectum, bladder and bones, while the binary masks performed the worst. For the TRE results of the projection points, the triangle meshes outperformed the boundary conditions which shows potential for this method. One drawback of this method is its sensitivity to the initialization for generating triangle meshes, which can be improved. In its developing state, the method has proven to perform well but it remains to see how well it performs on other organs, such as the vagina, cervix and uterus combined and sigmoid.