Bone Extraction
In the first step, the binary masks of the key organs in the pelvis region, including the rectum, bladder, and cervix, were obtained. It was also necessary to consider a binary mask of the bony structures for the simulation of image deformation. The binary masks of the bony structures were obtained from CT images by an intensity-based segmentation on the CT number (HU > 150). The segmented bony structures were visually verified and manually corrected to avoid any mis-segmentation errors.