Stereotactic radiosurgery for malignant extracerebral intracranial tumors: patient selection, efficacy, and technical nuances.

Intracranial tumors extrinsic to the brain include a variety of histological types, including meningiomas and pituitary tumors, both of which are most commonly benign but can present with malignant biology and clinical behavior. In the same compartment arise a number of frankly malignant tumors, which include chordomas, metastases (to bone or dura), and sarcomas (e.g., chondrosarcoma). These malignant tumors derive from bone, dura, or vascular elements and pose significant therapeutic challenges. Because of the anatomical constraints imposed by the cranial base and by venous sinuses, and because of the relentless tendency to recur shown by malignant tumors of meningeal origin, surgery often achieves incomplete removal. Some tumors are not resectable without the use of complex approaches that endanger adjacent neurovascular structures. For these reasons, stereotactic radiosurgery (SRS) has an important role in primary treatment of malignant intracranial extracerebral tumors and, most commonly, in treating residual or recurrent disease after resection has established the diagnosis and decompressed the tumor's environs. Here we review the role and technique of SRS in a variety of these unusual lesions, including malignant meningioma, glomus tumor, pituitary carcinoma, skull base metastasis, chordoma, and chondrosarcoma. Understanding the specific nuances of each is helpful in allowing optimal planning of patient selection, dose level, and dose contours for best treatment results. Currently, SRS can be useful in achieving effective palliation of these malignant tumors but does not usually provide a cure. In the future, better results are anticipated because of new methods of metabolic imaging for delineating tumor extent and new radiosensitizers that enhance tumor kill within a safe range of doses at the tumor margin.