Stanford Report, October 25, 2000
|Shooting a moving target:
planning for breathing movements in radiation therapy
by KRISTIN WEIDENBACH
The perennial challenge for radiation oncologists treating cancer patients is how to kill the cancer cells yet spare the surrounding, normal tissue. Intensity modulated radiation therapy (IMRT) is a promising new technique that holds the potential to tip the odds in favor of destroying the cancer while avoiding damage to healthy cells. The technique is used at Stanford Medical Center and elsewhere to treat cancers of the prostate and of the head and neck, among others. But difficulties unique to organs within the chest mean IMRT is not widely available for treatment of breast and lung cancers. Stanford researchers now have designed a computer plan that they hope will overcome this problem.
According to Arthur Boyer, PhD, professor of radiation oncology and director of the division of radiation physics, radiation oncologists at Stanford began using IMRT about three years ago. The ability to modulate (increase or decrease) the radiation beam as it passes through neighboring regions of tissue means that oncologists can be more selective about which areas are irradiated and can focus the beams on a specific, irregular-shaped target. For example, a prostate tumor and involved lymph nodes can be targeted without radiation damage to nearby areas of the rectum, bladder, small intestine and colon.
Conventional radiation therapy also can treat irregular target shapes pretty well, says Boyer, but intensity-modulated radiation adds the ability to spare critical structures that are very close to the target. The IMRT radiation beams can be mapped in such a way that they "wrap" around the tumor and conform to its shape. "Spillover" of radiation into surrounding normal tissues is reduced, even in cases in which the tumor surrounds a critical structure that must be preserved.
However, the ability to train the IMRT beams on a specific area, which is so advantageous for physicians attempting to destroy cancers in the lower trunk and head, creates a potential problem for physicians trying to treat tissues in the chest cavity that move rhythmically as a person breathes. This breathing-induced motion of the organs has been the main obstacle confronting physicians wishing to use IMRT to treat breast and lung cancers. Radiation oncologists and physicists are concerned about breathing motion, says Boyer, but have not yet reached consensus on a solution to the problem.
Now, Lei Xing, PhD, assistant professor of radiation oncology, has found a way to use computer optimization to compensate for this breathing movement. "Others using IMRT [on the chest] know the problem but don't know how to deal with it. We are the first to come up with a solution," said Xing.
A standard IMRT radiation treatment map assumes no movement of the tumor target. Xing and his colleagues produced a computer program that incorporates respiratory motion into the treatment design. Using image data and an optical tracking system that follows fixed points on the patient's skin, they determined how the breast and critical organs such as the heart and lungs move with each inspiration and exhalation. When they compared the "static-design" map with the "movement-design" map they found that a treatment plan based on the map that incorporates breathing motion produces a superior result.
When the researchers applied the static-design map to a computerized breast that moves in a way that simulates a person breathing, they found that the heart and lungs received an excessive dose of radiation. In addition, the amount of radiation in the targeted tumor varied unfavorably. Rather than achieving a uniform dose inside the target tissue, the researchers predicted that as the tumor moves with the rise and fall of the chest, some areas accumulate an excessive radiation dose that could prohibit future healing of the breast tissue, while others potentially receive too little radiation to kill tumor cells.
"If you can incorporate the breathing cycle into your treatment planning, you can then use IMRT to deliver an optimal treatment to the breast over the whole breathing cycle," said Boyer. "If you don't take into account the patient's breathing you might not deliver the dose you want to deliver."
Xing says that some medical centers are treating breast cancer with IMRT without compensating for breathing-induced movement. "I think that's wrong," said Xing. He and Boyer favor a more cautious approach. "We want to send the message that the breathing movement should be taken into account when planning the treatment. The conventional treatment is optimized only when the patient is static and breathing motion is ignored, and that's not real."
Xing plans to continue testing the new technique on the computer, but believes it will not be long before breast cancer and lung cancer patients at Stanford can begin being treated with IMRT.