Objective Preoperative planning of complete mastoidectomy based on postprocessing, interactive processing, and visualization of CT datasets was implemented and tested. The synthesized images are used to define an access path for the operative treatment of diseases at the lateral skull base. Alternatively, the mastoid may be resected to allocate space for implants such as hearing aids. Methods For the preoperative planning of such surgery, accurate segmentation of the mastoid is required. A semiautomatic pipeline for segmentation of the mastoid was developed. The approach requires minimal user interaction, using a method based on generating a fast 3D-oriented primary segmentation, which (if necessary) can be refined interactively, both in 3D-and slice-based manner. Results To verify the usability of the segmentation and visualization tools for surgery planning, an evaluation study was conducted with datasets of 16 operated patients. For each patient, at least on preoperative CT scan of the skull, with focus on the region of the mastoid to be operated, was available. Qualitative and quantitative evaluation of the images was performed, and the results were tabulated. Conclusion Semiautomated mastoid segmentation provides a feasible means for preoperative planning, based on postprocessing analysis and visualization of CT datasets.