Error assessment during "image guided" and "imaging interactive" stereotactic surgery.

The mechanical accuracy of several available stereotactic instruments is extremely high as measured in tests with rigid phantoms. The author's experience with stereotactic-guided resection by craniotomy using both "frameless" and framed methods simultaneously is that neither the image accuracy nor the mechanical accuracy of the instrument is the limiting factor in the usefulness of the guidance. Rather, it appears that the errors encountered in actual use have to do with tissue position changes which occur during the procedure. The accuracy may be better for extrinsic lesions rigidly attached to the skull, but for intrinsic lesions, tissue position changes occur following the release of cerebrospinal fluid, air entry into the subdural spaces, tumor debulking, or cyst drainage. The potential error appears to be worse with hydrocephalus, intraoperative dehydration, collapse of larger cysts, and debulking of large tumors. Even with very small intrinsic tumors in young, not atrophic patients, the error may be 5 mm. The need for intraoperative update of the guidance image is obvious if greater accuracy is required. The advantages of such "imaging-interactive" stereotactic surgery have long been apparent from stereotactic biopsy procedures performed in the CT scanner where errors such as needle deflection or hemorrhage can be appreciated and corrected promptly. With intraoperative scanning it is also possible to monitor the progress of a cyst aspiration and confirm the site of a biopsy to avoid unnecessary sampling in cases where the pathology is inherently equivocal.