“CyberKnife and other noninvasive radiosurgery systems are producing ever more accurate energy beams, raising the possibility of extending the use of potentially lethal radiation to fight Parkinson’s, epilepsy and other afflictions”
By Larry Greenemeier Scientific American
Targeted beams of high-intensity radiation can shrink early-stage tumors with limited collateral damage to surrounding healthy tissue. The addition of robotics and image guidance systems in recent years has made these stereotactic, or directed beam, radiosurgery systems an even more versatile weapon against cancer, attacking not only brain tumors (for which they were originally designed) but also other diseases virtually anywhere in the body.
Researchers have begun pushing the technology to the next step by increasing beam accuracy so that physicians can safely administer higher doses of radiation to cancerous cells, making radiosurgery a viable alternative to conventional surgery in more cases. Such accuracy would prohibit radiation overdose, which has serious consequences—an errant high-power beam passing through healthy tissue and organs could cause severe damage, and even cause a fatal shut down, as a January article in The New York Times illustrated.
Surgery alternative
A prime target is lung cancer, where the standard treatment is surgery to remove diseased tissue, says Eric Lindquist, a senior vice president and chief marketing officer for Accuray, Inc., a Sunnyvale, Calif., company that makes the CyberKnife Robotic Radiosurgery platform. “But it’s highly invasive, meaning the patient will have scars, a long healing time and blood loss,” he adds.
Lindquist believes that early-stage non–small cell lung cancer patients treated with targeted, high-dose radiation delivered in three or four treatments can have the same, if not better, chance of survival as patients undergoing surgery. To prove this, Accuray has partnered with The University of Texas M. D. Anderson Cancer Center in Houston on a clinical study to compare survival rates between traditional surgery and CyberKnife three years after treatment. Accuray and M. D. Anderson hope to have their study, which began in 2007 and may include more than 1,000 patients, wrapped up by the end of 2013.
New targets
Accuray is also exploring other applications for CyberKnife, including the treatment of atrial fibrillation (irregular heartbeat), Parkinson’s disease, epilepsy and psychiatric disorders (pdf). Doctors have been using CyberKnife for the past decade to treat trigeminal neuralgia, a chronic pain condition arising from the trigeminal nerve, which relays facial sensation. In February doctors at the Memorial Cancer Institute in Pembroke Pines, Fla., used CyberKnife to successfully treat a woman suffering from this malady by focusing radiation on the culprit—a nerve connected to her brain stem that doctors believe was unprotected and highly irritable.
How it works
Radiosurgery (and radiation therapy, which relies on lower doses of radiation spread out over a longer period of time) uses a beam of energetic particles to ionize the atoms that make up the DNA chain. A treated cell becomes unable to reproduce and loses its structural integrity. Because the technology does not discriminate between healthy and cancerous cells, Accuray developed a tracking system to help CyberKnife maintain accurate targeting of soft-tissue tumors that shift position during respiration, says Brian Collins, an attending radiation oncologist and assistant professor in Georgetown University Hospital’s Department of Radiation Medicine.
As a result, CyberKnife’s linear particle accelerator can produce a radiation beam that moves in rhythm with a patient’s breathing, targeting the correct spot at all times. Anesthesia is not typically needed for a CyberKnife procedures, and the treatment itself is painless, Collins says, adding, “The number one thing I hear from patients afterward is, ‘Are you sure I got treated?’” Collins and his colleagues tend to administer CyberKnife radiotherapy treatments for 30 to 40 minutes at a time over one to five treatment sessions, typically during a single week.
3 types of radiosurgery
CyberKnife employs one of the three basic types of stereotactic radiosurgery technologies. To generate a radiation beam, CyberKnife (and Varian Medical Systems’s Novalis Tx) use a linear accelerator that can be moved to treat tumors in both the head and the body. Indeed, Georgetown physicians found that CyberKnife could not only treat brain tumors but also prostate, neck and other cancers, Collins says.
A second type of stereotactic radiosurgery is Elekta, AB’s Leksell Gamma Knife, a pioneering technology in the radiosurgery field. It uses a fixed beam generated by a cobalt 60 synthetic radioactive isotope. When Gamma Knife was developed in the late 1960s, it offered an alternative to conventional radiation therapy, which bathes portions of the body in lower-dose radiation for longer periods of time in an effort to kill cancer cells while limiting damage to surrounding healthy tissue. Unlike CyberKnife, Gamma Knife’s beam cannot move during treatment, making the technology suitable for treating tumors in the brain but not other areas of the body that move due to respiration.
The third form of radiosurgery relies on a beam of protons to irradiate tumors. This proton (particle) beam technique has not found wide deployment in the U.S., in part because the equipment can cost in excess of $100 million, but also because little research exists on the technology’s efficacy and safety.
Despite costing 10 times as much, proton therapy has emerged as a challenge to CyberKnife, because it uses a different type of radiation at a lower dosage, Collins says. Protons, he adds, also do not work well with a moving target. “If you could make a proton system that’s as accurate as a CyberKnife, it would be fantastic,” he says.
