While one of the Holy Grails in radiation oncology is to spare as much healthy tissue as possible during therapy, patients undergoing treatment for weeks at a time physically change. Patients can lose weight during a period of therapy. They might lose or gain fluid. Tumors may shrink or unfortunately, continue to grow. As a result, radiation target sites change, which can be problematic for treatment.
Thomas Jefferson University Hospital in Philadelphia and the Kimmel Cancer Center at Jefferson are among the first centers in the nation to study the effect of incorporating a new technology – cone beam CT – into a source of radiation, namely a linear accelerator, in an attempt to find an answer to this vexing problem.
The technology creates three-dimensional axial CT slices of a patient’s tumor, enabling therapists and doctors to compare these images with initial treatment planning images to determine how precisely focused the radiation set-up is. They can then make position adjustments if necessary to deliver a more targeted therapy to the patient. The hope is that this technology will lead to more highly customized radiation treatments, where higher doses are directed at the tumor while sparing the patient’s normal body structures.
“Right now, cone beam is used as one additional means of verifying the accuracy of the radiation treatments that we deliver,” says Mitchell Machtay, M.D., the Walter J. Curran Jr., M.D., associate professor of radiation oncology at Jefferson Medical College of Thomas Jefferson University and vice chair of the Department of Radiation Oncology. Traditionally, patients are positioned for their daily radiation treatment by making marks on their skin, based on earlier tests, Dr. Machtay explains. “Once that is done, unless dramatic changes are seen, it’s hoped that those marks will hold up for two or three or six weeks of radiation and that the treatment will be given accurately.
“Ultimately, we believe that cone beam will lead to more highly customized radiation treatments, such as higher doses to the gross tumor and lower doses to normal structures within the body,” explains Dr. Machtay, who specializes in treating head and neck cancers. He notes that this is particularly important for tumors of the head and neck, “where tumors and critical normal structures are tightly packed next to one another.”
Richard Valicenti, M.D., associate professor of radiation oncology at Jefferson Medical College, uses cone beam CT for treatment planning for prostate cancer patients. “We’ve never had a way to directly visualize a target for radiation therapy before,” he notes.
“Cone beam is a potential paradigm shift in checking the accuracy of a treatment,” adds Dr. Machtay.
Jefferson is the most experienced center in the Delaware Valley in using the technology, notes Walter J. Curran Jr., M.D., professor and chair of radiation oncology at Jefferson Medical College and clinical director of Jefferson’s Kimmel Cancer Center.
In cone beam, the CT scanner is attached to a radiation delivery machine. Prior to the actual treatment, a set of 3-dimensional CT scan images is obtained. This is compared to the conventional CT scan that was used for planning the patient’s radiation treatment. If there are any differences in the patient’s current position, this is corrected before treatment is actually given
“Positioning no longer depends on the road map marks on the skin,” says Dr. Machtay. “With cone beam, we are actually looking at a CT scan of the inside of the patient to see if he or she is lined up properly and if there’s been a change in the size or shape of the individual.
“We’re using it to collect quality assurance data, to see how much movement there is from the first day of treatment to every other day of treatment. We’re making the measurements and the adjustments necessary to hit the tumor.
“What we haven’t done yet is actually change the number of radiation treatments, the dosages of treatment or the size of the beams based on the CT scans,” he notes. In theory, cone beam is more accurate and the radiation beams can be more pinpointed, meaning less radiation exposure to the rest of the patient. That can mean fewer side effects, and perhaps a higher dose of radiation.
In treatment planning for head and neck cancer radiation therapy, Dr. Machtay explains that at least a 5 millimeter safety margin around the cancer is typical. “If this margin is reduced to 2.5 mm, and we think of it three-dimensionally, that’s a lot less area of radiation exposure.”
Cone beam can be applied to any cancer type. And while clinical trials using cone beam radiation are in the planning stages, says Dr. Machtay, “it’s something that we would consider state of the art but not yet standard of care.” The device is not yet approved by the FDA and currently is used on only about 25 percent of the patients receiving radiation therapy at Jefferson.
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