Radiosurgery for the treatment of intracranial lesions.

Stereotactic radiosurgery is a technique of external irradiation that utilizes multiple convergent beams to deliver a high single dose of radiation to a small volume. In any stereotactic procedure, a coordinate system associated with a stereotactic frame is used both to reference the 10cation of intracranial structures with radiological studies and to direct intervention, such as biopsy, craniotomy, or the placement of radioactive sources (brachytherapy). In radiosurgery, multiple, highly collimated beams of radiation are stereotactically directed toward a radiographically discrete treatment site. 1 The hallmark of radiosurgery is the rapid dose fall-off at the target boundary. The radiobiologic effects of this focal radiation dose distribution are associated with small blood vessel thrombosis and/or reproductive cell death. 2 The development of frank radionecrosis is not a required pathological outcome for radiosurgery to be successful. The procedural goals of radiosurgery are: to define a small three-dimensional intracranial target volume, to deliver a clinically significant dose of radiation within the target volume, and to avoid delivering a clinically significant radiation dose beyond the target volume. These goals were technically difficult to achieve until the introduction of modern diagnostic imaging tools and high energy treatment machinery. Radiosurgery is currently performed with one of three types of high energy radiation technologies: high energy x-rays produced by linear accelerators or the Gamma Knife producing gamma rays (AB Elekta, Stockholm, Sweden) and less frequently charged particles such as protons or other ions produced by cyclotrons or synchrotrons. Currently, there are over 200 facilities in the United States that are treating patients with radiosurgery. The most common radiosurgery technology being used worldwide is linear accelerator-based. As the number of radiosurgery facilities increases, so does the annual number of patients being treated. It is estimated that over 20,000 patients have now been treated with some form of radiosurgery since the 1960s with the majority of patients being treated in the last five years. The proliferation and expense of radiosurgery systems, particularly the gamma