A neurologically intact patient with a deep tumor that is poorly defined by MRI or CT is the ideal candidate for stereotactic biopsy. Such procedures are also useful for patients with recurrent tumors in whom a change in histopathology is anticipated and when the use of interstitial irradiation or hyperthermia is planned. Some patients with cystic recurrences obtain symptomatic relief from stereotactic aspiration of the cyst. The advent of computerized imaging and CT - and MRI -compatible stereotactic frames (e.g., the Leksell instrument) has greatly simplified the performance of stereotactic procedures for both large and small target lesions. Nevertheless, simple biopsy carries less than a 2 percent mortality rate and a 3 percent serious complication rate. Although the smear preparations from such procedures yield a correct diagnosis in less than 95 percent of glial tumors when adequate tissue has been obtained, in 11.8 percent of the cases either the diagnosis is incorrect or the material is inadequate. After stereotactic biopsy, all patients with a confirmed diagnosis of a glial neoplasm should undergo some form of external irradiation. Combined stereotactic biopsy and postoperative irradiation is an especially appropriate method of handling young and neurologically intact patients with large, non enhancing, low-density lesions on CT or ill-defined, nonenhancing lesions on MRI. Even patients with high-grade tumors can do well with such management. In cases of diffuse spread of a low-grade astrocytoma through large volumes of critical tissue, it is usually impossible at open craniotomy to do more than a biopsy, because the margins of the lesion are totally undefined. Because open biopsy without resection carries a higher complication and mortality rate than either closed biopsy or radical removal, it seems only prudent to subject such patients to stereotactic biopsy instead.

A recent development has been the evolution of hybrid or combined techniques in which the precision and accuracy of image­based stereotaxy is combined with the therapeutic advantages of open procedures. Patients are placed into a stereotactic frame and targets are calculated from an enhanced MRI or CT scan in the usual fashion. Upon return to the operating room, a stereotactic probe is used to guide the placement of the scalp incision and bone flap. After these have been turned, the probe is again lowered to the surface of the operative field and the dural incision selected. Finally, the probe can be used to guide the placement of the cortical incision and in some instances can be followed all the way down the transcortical tunnel until the tumor is reached. This method obviously avoids virtually all possible errors of localization in the management of deeply placed and ill-defined small lesions, Frameless stereotaxy is also in use as an intraoperative aid for craniotomy. In this method, a robotic arm is touched to the patient's head and acquires localization points for a computer in which the image of the lesion has been previously stored, Stacked slice representations of the tumor within the brain have also been used to perform computer-guided stereotactic resections of deep lesions by mounting a laser and computer­controlled motors on a stereotactic frame.