The Cyberknife is a new high-technology system that delivers fractionated LINAC radiosurgery (FSR) using a robotic arm controlled by an image-guided computer technology similar to that used by the military in cruise missiles. The Cyberknife was approved by the FDA in July 1999. 

Unlike conventional radiotherapy systems where the treatment machine is not able to verify the position of the tumor, the Cyberknife can define and confirm critical information concerning the position of the tumor during the treatment. Through the use of this "smart beam" technology, the accuracy of targeting is improved. According to latest research results (see Chang et al, Neurosurgery, page 140, January 2003), the accuracy of Cyberknife targeting is comparable to that of fixed (attached) frames such as in Gamma Knife treatments, and more accurate than for relocatable frames based on either bite blockes or masks, such as used for other FSR protocols.

Dr. John R. Adler (Professor of Neurosurgery, Stanford University) has provided more details about the increase in accuracy:

the RMS error measured was 1.2 mm in this paper [by Chang et al], which is in fact considerably less than the 1.8-2.0 mm errors reported for skeletally attached stereotactic frames. 

This improvement comes at the price of exposure to additional radiation, in the form of X-rays.  There are two ceiling mounted X-ray cameras that target the regional anatomy around the tumor, and a high speed computer that analyzes the images. The data is converted into information about the location of the patients' features, such as bone structure. The computer can account for any small changes in the position of the patient during treatment, and can deliver the radiation dose to the target with high precision.

According to Dr. John R. Adler, this additional X-ray exposure is not a concern:

the exposure to the head during a standard single treatment or even the more typical 3 stage procedure used at Stanford for acoustic neuroma, adds up to no more than the x-ray dose administered in 1 or 2 CT scans.  Such a dose is much less than scatter given off by a treating linear accelerator for example, and is widely considered by most radiosurgical practioners, to be inconsequential. 

It is important to note that Cyberknife differs from other types of FSR in more than just the method of positioning.  A crucial difference is that the robotic arm delivering radiation during Cyberknife treatment does not perform a continuous motion, as for standard FSR treatment with LINAC. Rather, it makes many short stops; the X-ray based targeting as well as the radiation shots are only applied during these stops rather than throughout the motion.  Conceptually, this is more akin to Gamma Knife, where the radiation comes in individual beams that intersect at the tumor.

While the Cyberknife is a great new technology for those parts of the body which cannot be accurately repositioned by other means, it is not yet clear that there is a benefit to using it on the head, where the skull allows for other repositioning technologies.  While the Cyberknife's targeting may be more accurate than FSR, the combination of additional X-rays and the change in radiation pattern from a continuous sweeping arc to discrete shots may offset the benefits. This issue can only be resolved after we have treatment outcome data for large samples of Cyberknife and FSR patients.

Those who wish to know more about the Cyberknife technology are referred to the website of the CyberKnife Society, located at This is a non-profit organization is dedicated to patient education and scholarly exchange regarding the Cyberknife.  

Last Edited: Monday, November 17, 2003