IMRT - Defining Precision in Treatment

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Intensity modulated radiation therapy (IMRT) offers innovative techniques for planning and delivering cancer treatment to a wide variety of solid tumors. The hallmark of this approach is its ability to conform radiation beams or beamlets of varying intensities that maximize treatment dose to malignancies while sparing critical structures and normal tissues close to the target.

For example, prostate cancer treated with highly targeted 3D conformal planning results in a much higher dose administered safely that increases cure rates without escalating toxicity to adjacent anatomic structures. In some diseases, this may be purely a quality of life issue, but in others where dose escalation is paramount, normal tissue can be spared while the tumor receives maximum dosages of radiation.

Since IMRT was first introduced in 1992, the adoption rate has skyrocketed and additional, more sophisticated techniques have been developed as a result of improved medical imaging and advances in the planning and treatment systems.

Arno Mundt, MD, associate professor of radiation and cellular oncology at the University of Chicago Hospitals, has conducted a series of surveys to study adoption rates of IMRT.

In 2002, with an objective of assessing the level of IMRT use in the United States, his group surveyed 333 randomly selected radiation oncologists. Thirty-two percent of these clinicians were using IMRT, with academic physicians more likely to use these techniques than their private practice counterparts. In a follow-up study completed last year, the rate of IMRT use rose to 73.2 percent, including 62 percent of identified non-users from the first survey.

In addition to those studies, Mundt's group asked chief residents about whether or not they had learned about using IMRT in their training program, to provide a barometer of future practice. Seventy-one percent of the respondents reported receiving formal IMRT didactic education and 87 percent of them had received "hands-on" training.

As adoption rates of the basics of IMRT have become more widespread, Mundt finds that many people are using advanced techniques such as dose painting.

"In our latest survey, 50 to 60 percent of respondents said they had dose-painted at least once in their practice…" says Mundt. "This tells you that they are using IMRT in more sophisticated ways than just conforming the dose."

Advances in medical imaging have propelled treatment progress. Mundt, who uses a Varian system, describes additional information provided by MRI and PET scans to guide dose-painting activities.

PET can reveal metabolically active areas within the tumor that may require additional dose levels, he explains. "Nuclear medicine scans also can look at things like oxygen levels. Low oxygen levels can impair the efficacy of radiation, so if you find hypoxic or low oxygen levels of the tumor, that might be an area where you would want to provide an extra boost to overcome that situation." This is experimental work being conducted at MD Anderson Cancer Center, according to Mundt.

Advances in technology make the difference

Daniel Galmarini, DABR, director of physics at 21st Century Oncology, a wholly-owned subsidiary of Radiation Therapy Services, of Fort. Myers, Fla., explains that they use a number of different systems in their more than 50 treatment centers.

The Planning Target Volume (PTV) includes the tumor in addition to areas where microscopic disease could be present plus the critical structures that surround the tumor.

"One of the problems we had was with the speed of delivery," says Galmarini as he describes some of their patients who required 45 minutes for each of their daily treatments. "With the new algorithms that optimize the workings of the MLCs [multi-leaf collimators], we could expedite delivery." Future enhancements should provide respiratory-gated treatment for lung and upper abdominal tumors.

With current treatment activities, such as for prostate cancer, great care must be taken to insure that the tumor is in the precise position reflected in the plan. Several linear accelerators include kilovoltage auxiliary beams that provide diagnostic quality images at the point of treatment.

"You have a head that delivers radiation with a megavoltage portal vision system, and placed 90 degrees to that, there is a kilovoltage tube with a detector panel on the opposite side," explains Galmarini. Different manufacturers offer a variety of configurations to produce images that verify placement