The XirusCAD plugin has evolved into Rhino3DMedical plugin. Mirrakoi’s website says: “Directly read and process all common medical image formats (DICOM, NIFTI, mhd).”
Free programs called 3DSlicer, MIPAV, MITK, and Osirix can convert Dicom images to STL format. 3DSlicer is very popular. Another free program called AMI Medical Imaging toolkit is posted on Github. It can read Dicom, NRRD, and Nifti files.
Mayo Clinics Analyze and MIMICS are expensive commercial programs which convert DICOM data to STL.
Dornheim Segmenter is mature commercial program. It makes 3D models from medical scan data. videos: https://www.youtube.com/channel/UCb-YtwJlzR3yw2QYgTva-2w/playlists
Rhino3DMedical is a small step in the right direction. In my opinion, it will become useful tool if it reads and interprets data coming from microcameras (e.g. Awaiba) in real time, during surgery. Every surgery is a bloody mess, so having clear, 3D image of what is going on is desirable.