New advance in radiation therapy may improve cancer treatment

Until recently, radiation therapy has employed beams of nearly uniform intensity or with rudimentary devices for modifying the intensity. As a result, there were some instances in which the desired dose of radiation could not be delivered to the entire tumor without harming healthy tissue in the process.

IMRT solves this problem by allowing the intensity of each radiation beam to be varied or "modulated." In other words, each beam can send out a sophisticated radiation pattern that varies in time and space. Firing non-uniform beams from several angles could deliver the desired dose to the tumor while minimizing doses to surrounding organs.

IMRT is currently being used at several locations, such as the Memorial Sloan-Kettering Cancer Center in New York City and the Washington University School of Medicine in St. Louis. Researchers expect the technique to spread rapidly to many medical centers in the coming months and years. As a possible indicator of its potential importance, the technique will be featured at the upcoming World Congress on Medical Physics and Biomedical Engineering, co-sponsored by the American Association of Physicists in Medicine. To take place in Chicago from July 23-28, the meeting will have a two-day series of invited lectures on IMRT and over 100 additional contributed presentations on the new technique.

"In so many situations, you would like to send a larger dose of radiation to a tumor, but you can't because it would damage other organs," says Raj Varadahn, a medical physicist at Minneapolis Radiation Oncology, a cancer treatment center in Minnesota. "IMRT is a significant advance in maximizing the radiation dose to tumors and minimizing the dose to healthy tissues that surround it," he says.

The new technique could potentially improve radiation therapy in all forms of cancer. In prostate cancer, for example, doctors could send larger radiation doses to a prostate tumor, while minimizing doses to surrounding organs such as the rectum and bladder. Early results from clinical trials have indicated that increasing the radiation dose to prostate tumors improves the chances of surviving past five years with that cancer. In some cancers of the head and neck, IMRT can deliver more radiation to the tumor while minimizing doses to the sensitive organs such as the parotid glands, which regulate the flow of saliva. In preliminary data, researchers at the Washington University School of Medicine have found that patients receiving IMRT radiation therapy have better saliva function compared to those who receive traditional radiation therapy. "It's currently the low-dose sensitive structures in which IMRT is particularly useful," says Washington University medical physicist Daniel Low, an internationally known IMRT authority who is organizing the two-day lecture series on the new technique at the upcoming Chicago Meeting.

Acquiring precise, three-dimensional images of the body is the first step that made IMRT possible. Using techniques such as CT scans and magnetic resonance imaging, scientists can now obtain precise, 3-dimensional images of tumors inside the body. This helps physicians know the exact location of the tumor -- and where to direct tumor-destroying radiation beams.

In the last decade, researchers developed the ability to get a "beam's eye view" of a tumor. In other words, researchers can determine the exact dose that a radiation beam delivers to a particular tumor. With this information, physicians can send an exact dose of radiation to the three-dimensional region where a tumor resides.

Perhaps the greatest breakthrough enabling IMRT is the development, by medical physicists, of powerful computer programs, or "algorithms," to determine the right combination of beam angles and intensities that will do the best job of providing the desired dose. These algorithms work backwards: physicians can assign radiation dose limits to both the tumors and sensitive nearby organs, and then get a computer to generate the instructions for how to deliver that dose.

Medical physicists view this "inverse planning" approach as a major advance in cancer treatment. Previously, researchers could only use a trial-and-error-based "forward planning" approach, in which they would first select beam angles and intensities, observe the results, and make the appropriate modifications to approach the desired dose.

What's also vital to the IMRT approach is the hardware, and there are several ingenious systems for modifying the intensity of the beam. In one technique, researchers can split a single beam into hundreds of thin beams -- each with a different intensity. In another approach, the radiation beam passes through a special opening known as a multi-leaf collimator (MLC). Made of tungsten plates that can be arranged under computer control, the MLC can change shape from moment to moment to vary the shape and intensity of the beam which passes through. Finally, University of Wisconsin researchers have built a special radiation delivery device that can deliver radiation in a helix-shaped spiral to the body. Combined with MLCs, the helical delivery system may allow for more effective 3D coverage of the tumor.

Medical physicists see IMRT as an effective cancer-treatment tool for the dawn of the 21st century -- and possibly beyond.