THE PHYSICAL, BIOLOGIC, AND CLINICAL BASIS OF RADIOSURGERY

Since Leksell's description of the concept of radiosurgery in 1951, pr obably more than 20,000 patients worldwide have been treated with this technique. initially designed as a tool for functional neurostereotax is, it has found widespread applicability for conditions as diverse as vascular malformations, benign tumors such as acoustic neuroma, menin gioma, pituitary adenoma, and also malignant tumors such as brain meta stases 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 ha s evolved into a multicompartmental model, with reasonable appreciatio n of the dose, volume, and time factors involved. With the explosion o f technology on several fronts in the 1980s and 1990s, a multitude of devices for radiosurgery, ranging from cyclotron-generated particle be ams to multisource cobalt-60 units to an immense variety of modified l inear accelerators has become available. A parallel explosion of techn ology in the fields of imaging and computing will ensure that this is just the beginning; already, technologies for automated image segmenta tion and target identification, long the physician's monopoly, are aro und the corner; image fusion now allows simultaneous visualization of target and normal tissue anatomy, physiology, and other exciting possi bilities such as chemical composition and vascular characteristics, Ad vances in physics and robotics have led to development of prototypical machines that will blur the distinction between radiosurgery and conf ormal radiotherapy. Already, several ''first generation'' devices to f ree 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, radiosurg ery has made and will continue to make a strong commitment for clinica l efficacy, safety, and cost-effectiveness through the process of thor ough multiinstitutional clinical trials, as opposed to seeking validat ion from anecdotal experiences. In this regard, the Radiation Therapy Oncology Group (RTOG) and the Gamma Knife Users' Group (GKUG) are to b e commended; if the plethora of radiosurgery-related publications is e vidence of scientific interest, the field will likely continue to expa nd. In the future, issues pertaining to appropriate regulatory review, patient selection, quality assurance, and training will need to be ad dressed. Major clinical and biological studies still need to be undert aken.