With each breath, organs in the chest and upper abdomen move. A tumor in a person's lung, liver or breast may move within a 2-inch range between inhalation and exhalation. When doctors attack the tumor with external radiation, they must treat healthy tissue as well in order to adequately include the tumor if the radiation beam is on during the entire respiratory cycle.

"There is a correlation between the increased dose of radiation to normal tissue and an increased risk of complications," says Mayo Clinic radiation oncologist Dr. Laura Vallow. "We are trying to minimize the dose of radiation delivered to normal tissue while continuing to treat all of the disease."

Respiratory gating is the latest advance in radiation oncology to permit a reduction in the volume of normal tissue treated. Using a multislice, four-dimensional CT scanner (the fourth dimension is time) and computers capable of storing and manipulating the necessary 1,500 CT images, physicians measure the patient's range of motion during respiration, decide whether respiratory gating is appropriate and customize the treatment field to the patient. During actual treatment, the radiation beam is continuously turned on and off to synchronize delivery of radiation during the optimal point of the patient's respiration cycle.

"This is not perfect, because the way you breathe today and the way you breathe tomorrow are not necessarily the same," says Mayo Clinic medical physicist Christopher Serago, Ph.D. "If you try to breath like a machine, you can't do it, but this is a better approximation than we've ever had before."

It's usually necessary to treat some margin of healthy tissue with radiation for two reasons. The first is to include microscopic cancer cells not visible on imaging studies. The second is to allow for variables such as patient movement, including breathing, during treatment. "Respiration is hard to predict from patient to patient," Serago says. "In the past if you wanted to be very conservative, you might put up to a 3-centimeter margin around the tumor to account for respiratory motion. With gating, the treatment margin may be reduced and customized to the individual patient."

Drs. Serago and Vallow say that patients with breast, lung, liver, pancreas and possibly kidney cancer will be evaluated to see if respiratory gating can be used in their radiation therapy. In addition, they say respiratory gating technology will enable radiosurgery beyond intracranial application. This type of surgery uses a single powerful dose of tumor-killing radiation, which is not possible without extreme accuracy.

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Mayo Clinic is a multispecialty medical clinic in Jacksonville, Fla. The staff includes 328 physicians working in more than 40 specialties to provide diagnosis, treatment and surgery. Patients who need hospitalization are admitted to nearby St. Luke's Hospital, a 289-bed Mayo facility. Mayo Clinics also are located in Rochester, Minn., and Scottsdale, Ariz. Visit http://www.mayoclinic.org/news for all the news from Mayo Clinic.

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