Radiation destroys sensitive dividing cells by mitosis-linked death. High-energy external beam radiation affects tissues surrounding the target area with wide margins and even with modern planning techniques can therefore not be applied for truly focal tissue ablation in radiosensitive organs. Brachytherapy permits sharply defined ablation, but requires placing of the radioactive material within the target, and therefore does not constitute an extracorporeal technique. These problems are in part overcome by using stereotactic techniques to apply highly focused radiation. Pioneered in neurosurgery for the treatment of intracranial tumors, they exceed the scope of this treatise. A frame-less image-guided radiosurgical device (Cyberknife) has recently been evaluated for focal renal ablation in a porcine model (1). The system combines a lightweight 6 mV linear accelerator mounted on a robotic arm with an image-to-image algorithm for target localization. An adequate conformal radiation dose is delivered by focusing a multitude of radiation beams at the target zone, yet directing the individual beams along different pathways so that the surrounding tissues are not exposed to a harmful dose. With a targeted dose of up to 40 Gy, complete fibrosis of the target zone was achieved without any apparent damage to surrounding structures (1).

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