The physical, biologic, and clinical basis of radiosurgery.

Since Leksell's description of the concept of radiosurgery in 1951, probably more than 20,000 patients worldwide have been treated with this technique. Initially designed as a tool for functional neurostereotaxis, it has found widespread applicability for conditions as diverse as vascular malformations, benign tumors such as acoustic neuroma, meningioma, pituitary adenoma, and also malignant tumors such as brain metastases and malignant glioma. From rudimentary knowledge of the ability to produce focal necrotic lesions, the biologic understanding of the process of single-fraction, small-volume, high-dose brain radiation has evolved into a multicompartmental model, with reasonable appreciation of the dose, volume, and time factors involved. With the explosion of technology on several fronts in the 1980s and 1990s, a multitude of devices for radiosurgery, ranging from cyclotron-generated particle beams to multisource cobalt-60 units to an immense variety of modified linear accelerators has become available. A parallel explosion of technology in the fields of imaging and computing will ensure that this is just the beginning; already, technologies for automated image segmentation and target identification, long the physician's monopoly, are around the corner; image fusion now allows simultaneous visualization of target and normal tissue anatomy, physiology, and other exciting possibilities such as chemical composition and vascular characteristics. Advances in physics and robotics have led to development of prototypical machines that will blur the distinction between radiosurgery and conformal radiotherapy. Already, several "first generation" devices to free stereotaxis from its fixation to frames are available. Substantial enthusiasm among clinicians has ensured that, unlike many fleetingly and momentarily exciting technologies of the last 2 decades, radiosurgery has made and will continue to make a strong commitment for clinical efficacy, safety, and cost-effectiveness through the process of thorough multiinstitutional clinical trials, as opposed to seeking validation from anecdotal experiences. In this regard, the Radiation Therapy Oncology Group (RTOG) and the Gamma Knife User's Group (GKUG) are to be commended; if the plethora of radiosurgery-related publications is evidence of scientific interest, the field will likely continue to expand. In the future, issues pertaining to appropriate regulatory review, patient selection, quality assurance, and training will need to be addressed. Major clinical and biological studies still need to be undertaken.