• 10/15/2006
  • Escondido, CA
  • Bradley J. Fikes
  • NCTimes.com

Radiation therapy for cancer gets on the beam

Radiation therapy, a treatment for cancer more than a century old, has become much more powerful and precise with the help of the computer. Harmful side effects of radiation on normal tissue have been lessened, and tumors once untreatable by radiation can now be reached.

Two relatively new technologies, called the CyberKnife and intensity modulated radiation therapy (IMRT) , make plentiful use of computing power and precise medical imaging to determine the tumor location. The details of how they work are different, but both deliver X-ray radiation in a concentrated pattern from many directions that converge at the tumor.

The concept resembles that of another targeted radiation therapy, called the Gamma Knife, which sends about 200 beams of gamma radiation into the tumor. But while the Gamma Knife can be used only for lesions of the brain, the CyberKnife and IMRT can be used over the entire body.

IMRT and the CyberKnife are “complementary,” said Cihat Ozhasoglu, a medical physicist at the University of Pittsburgh Medical Center. The CyberKnife, the newer of the two therapies, is more precise in delivering radiation. But IMRT is superior for treating large and irregular tumors.

Because radiation damage to normal tissue is lessened, both treatments cause less fatigue in patients, a common side effect of radiation, than in conventional radiation.

“It’s much more pleasant than I thought it would be,” said Eugene Fundum, a Temecula resident who received IMRT for prostate cancer. Fundum, 76, was treated at the Vantage Oncology Redhawk Radiation Therapy Center, which opened earlier this year in Temecula.

Earle Grueskin took CyberKnife therapy at the San Diego CyberKnife center in Kearny Mesa. Grueskin is also 76 and also has prostate cancer.

“There was absolutely no pain before, during or after the treatments,” Grueskin said.

Both Fundum and Grueskin said their levels of PSA (prostate specific antigen, an indicator for prostate cancer), have declined dramatically since their treatment in July. They’ll need periodic checkups to check for tumor regrowth.

IMRT

The name gives a good indication of how intensity modulated radiation therapy works. It delivers patterns of radiation that vary in intensity. That’s important because certain parts of the body are more sensitive to radiation than others. The prostate, for example, is located between the bladder and rectum. Radiation to these organs must be controlled to avoid damaging bladder and bowel function.

First, the patient is given a CT scan to map the area, so the staff can plan the treatment. A CT scan is a series of X-rays, each focused to show a sharp image of adjacent thin slices of the area. These are digitally combined in a computer to form a 3-D image. This shows where the tumor is in the patient.

That’s not enough, however. The patient’s position must be tracked so if he or she moves, the radiation’s path also moves. An ultrasound guidance system called SonArray provides that information, said Mary Ann Rose, M.D., a radiation oncologist who is medical director of Redhawk Radiation Therapy Center and Wildomar Radiation Therapy Center. The centers, owned by Vantage Oncology, provide a variety of other radiation therapies.

Once the positioning is determined (a process that is repeated throughout the procedure), the radiation is delivered. A device called a linear accelerator produces a radiation beam that is modulated by “leaves” that interpose between the beam and the patient.

“There are 120 small tungsten leaves that flutter in and out as they rotate,” Rose said. The X-rays are lowered in power when they pass through important structures on their way to the tumor. When the path is clearer, the beams increase in intensity.

Everything is computer-controlled, so the overlapping paths of radiation meet at the tumor location. The computer calculates the paths and doses in accordance with what’s prescribed.

“It makes a huge difference,” Rose said. “Prior to the ability to localize the radiation, patients have significantly more bladder and rectal symptoms, more burning, more diarrhea.”

The radiation center also can treat a moving target, such as a lung tumor, through a technology called “respiratory gating,” which models a patient’s pattern of respiration. The model, which must be periodically updated, allows the radiation beams to be moved in sync with the patient’s breathing.

CyberKnife

Much of CyberKnife’s technology, such as the use of X-rays, CT images and respiratory gating, is the same or similar to that of IMRT. Both rely on many sources of overlapping radiation to deliver the therapeutic dose of radiation at the tumor site.

The CyberKnife differs mainly in providing more precise “target tracking” and delivery, said Donald B. Fuller, M.D., a radiation oncologist at the San Diego CyberKnife Center. That allows more radiation to be given at once, which reduces the number of treatments needed to deliver the cumulative dose.

Before the radiation is given, the tumor’s location is noted, using points of reference such as the patient’s bones or opaque objects implanted near the tumor to outline the treatment area. And during the treatment, the image is scanned again. And again.

“The location of the target is tracked with submillimeter accuracy,” Fuller said. The image is updated 50 to 100 times during each treatment.

The precise positioning means the CyberKnife is suited for tumors in especially delicate locations, such as tumors on the spinal cord or in the brain, Fuller said.

“Because the CyberKnife keeps on interrogating the target, you’re able to get treatment to your target with much less collateral damage,” Fuller said.

However, the way the CyberKnife is constructed also limits its area of treatment, said the University of Pittsburgh Medical Center’s Ozhasoglu.

The CyberKnife emits circular fields of radiation. If the cancer-killing dose is given in a small circle, the radiation diminishes relatively rapidly as it emanates from the source. This can be compared with the rapidly diminishing light from a penlight. It’s bright 2 inches away, but dim if shone across the room. A more powerful circle of radiation will deliver the tumor-fatal dose in a larger area that diminishes more slowly. This creates a larger margin of error for damaging healthy tissue.

When treating a moving tumor, as in the lung, the CyberKnife has a margin of error of 5 millimeters, Ozhasoglu said. The CyberKnife achieves this accuracy through a computer program called Synchrony, which Ozhasoglu and his colleagues developed. The device includes “self-checks,” he said: If it detects something is wrong, it shuts down.

To treat a lung tumor, the target is first marked with gold “seeds” that mark the site for CT scanning, Ozhasoglu said. And during the treatment, the movement of the chest is tracked with red LEDs placed on the chest. A camera observes the movement, and the information fed into a computer that corrects for the motion. This provides tracking in real time, he said.

As an added measure of safety, X-ray images of the tumor site are taken periodically in brief snapshots. This is used to update the computer model of the patient’s respiration.

The evidence

In theory, then, IMRT and the CyberKnife are substantial improvements over previous radiation therapies. But where’s the proof? It’s in a growing number of medical studies that show the effectiveness of these therapies.

IMRT saves more salivary glands than conventional radiation for head and neck cancers, according to a study by doctors from Washington University School of Medicine in St. Louis. This is important because patients with destroyed salivary glands suffer from dry mouth, and are at more risk for dental cavities and mouth infections. The study was presented Oct. 24, 2000, at the annual meeting of the American Society for Therapeutic Radiology and Oncology.

Another study from the same Washington University team found that IMRT with chemotherapy enables patients with advanced nasal cancer to live longer than with just chemotherapy alone. The study was presented on May 14 of that year at the annual meeting of the American Society of Clinical Oncology.

Good news for prostate cancer patients treated with IMRT: 89 percent of them are disease-free eight years later, according to a study in this October’s issue of The Journal of Urology. The study tracked 561 prostate cancer patients treated with IMRT at Memorial Sloan-Kettering Cancer Center.

As a newer therapy, CyberKnife has less of a track record. But here, too, the reports are favorable. CyberKnife treatment can safely treat benign tumors outside of the brain, according to a study by doctors at the University of Pittsburgh School of Medicine, a team that included Ozhasoglu. The study was presented Oct. 5, 2004, at the annual meeting of the American Society for Therapeutic Radiology and Oncology.

And this August, Ozhasoglu and his colleagues reported that CyberKnife treatment destroyed lung tumors in some patients within three to four months of treatment. The findings were delivered at the annual meeting of the American Society of Physicists in Medicine.