Hutch News

Technology in three-D

Aug. 16, 2001
doctor demonstrating linear accelerator machine

Laser beams to align radiation mark the face of Dr. Jason Rockhill, posing as a patient to demonstrate the Alliance's linear accelerator, as Dr. Kenneth Russell and dosimetrist Tammy Korssjoen look on.

PHOTO BY MICHELLE HRUBY

Protected behind a five-ton lead-and-steel door as thick as that of a bank vault lies one of the Seattle Cancer Care Alliance's most potent weapons against cancer: a radiation beam that destroys diseased cells.

More remarkable than the power of radiation itself, though, is how radiation oncologists - the medical specialists trained in its use - manipulate this atomic energy with the pinpoint accuracy required to eliminate malignant cells deep within the body.

Their aim must be so precise that healthy tissue is spared from destruction while nearby cancer cells are penetrated by the DNA-damaging rays.

The secret, said Dr. Kenneth Russell, director of the Alliance's radiation oncology program, is a technique known as conformal radiation therapy.

"Conformal radiation therapy is a way of aiming the radiation beam so that we maximize the dose to the tumor while minimizing the dose to the normal organs," he said. "The method involves three-dimensional imaging technology that lets us focus the beam on a precise target."

This sophisticated technique is one of the services of the radiation-oncology program at the Alliance, which treats up to 30 patients a day. The program is part of a larger department that treats patients at University of Washington Medical Center, Overlake Hospital Medical Center in Bellevue, Veterans' Administration Hospital of Seattle and Harborview Medical Center.

Integrated care

"A patient will use radiation-oncology services at whichever institution his or her doctor is located," said Russell, also a UW professor of radiation oncology. "This lets us do integrated care in one building and streamline service for the patient."

For example, patients requiring bone-marrow transplants receive high-dose, full-body radiation at the UW, since they will be treated as inpatients on the transplant wards at that institution.

The less intensive radiation doses for mini-transplants, largely performed on an outpatient basis, are given in the radiation-oncology clinic on the first floor of the Alliance outpatient facility.

Although all tumor types are treated in the combined Alliance and UW Medical Center radiation facilities, most of the patients treated at the Alliance have breast, prostate, lymphoma/leukemia, gastrointestinal or other genitourinary cancer. Conformal therapy is routinely used to treat many of these problems.

During the process of treatment planning, doctors, dosimetrists and radiation therapists map out an entire therapy plan before a patient has the first treatment on the radiation source, an instrument called a linear accelerator.

"Images of the body are obtained using magnetic resonance (MR) or computerized tomography (CT)," Russell said. "Then we reconstruct the anatomy in three dimensions, using treatment-planning computers."

For complicated treatment plans, often involving breast or prostate cancers, it may hours to develop a strategy for targeting radiation, said Tammy Korssjoen, whose job as dosimetrist involves analyzing the anatomical reconstructions.

"Many patients will already have had CT or MR scans taken for diagnostic purposes, but these are insufficient for developing a radiation treatment plan," she said. "For example, a treatment-planning CT would include the skin surface - which wouldn't have been important when the cancer diagnosis was made but is important for dose calculations - so we work with the radiology department upstairs to have new scans taken."

With patients who do not undergo surgical removal of a tumor, the dosimetrist can visualize the tumor's location with the aid of the CT scan. But many patients receive radiation therapy as a follow-up to surgery to remove residual cancer cells, which are invisible to the naked eye.

"Often with a breast tumor, the surgeon will leave surgical clips in place to mark the position of the excised tumor," Korssjoen said. "We can see these in the CT scan."

The dosimetrist works with physicians to define essentially a topographic map of the radiation dose, which may target the affected area from as many as six different directions.

"We have to deliver treatment with great precision," Russell said. "Sometimes it's like making a quilt. The seams between adjacent radiation fields must be precise and not overlap."

Specialized molds

To direct the radiation to the exact spot, specialized molds, or blocks, may be designed that attach to the linear accelerator and shape beams along a specific path. Customized cradles, made of hardened styrofoam and designed during the simulation process, immobilize the patient and prevent movement during the therapy.

But the radiation boundaries typically must have some flexibility built in, for two reasons, Russell said. "We want to make sure the boundaries extend far enough so that we eliminate any malignant cells that remain, even if we can't see those cells," he said. "We also must accommodate any slight movement - caused by breathing, for example - of the patient during their radiation treatment."

The team checks its accuracy before treatment ever starts in a simulation room that houses a mock linear accelerator.

Once the team is sure the dose will be delivered accurately, members find the position where the center of the linear-accelerator will pass, and they mark it with a minute, indelible tattoo.

Unlike cobalt, a naturally occurring element, radiation from the linear accelerator is electronically generated.

"The linear accelerator generates X-rays from an accelerated electron," Russell said, "but it's much more powerful than a typical X-ray machine, which allows us to treat even deep-seated tumors. It's also a much more even dose. With diagnostic X-ray, different tissues absorb the radiation differently. That's good for taking pictures, but not good for treating tumors."

Despite the hours it takes to design a treatment plan, treatment time takes only minutes, usually administered five days a week for four to eight weeks. Patients undergoing radiation regimens as part of a mini-transplant receive a single dose on the linear accelerator of total body radiation. Sometimes unnerving for patients, the treatment nevertheless is painless.

In addition to Russell and Korssjoen, the Alliance radiation-oncology clinic is staffed by Dr. Skyler Lindsley, UW associate professor of radiation oncology, and a team of nurses, radiation therapists and physicists.

Because the Alliance is set up in multidisciplinary teams for oncology subspecialties, the department works closely with Alliance medical oncologists who treat the same spectrum of tumors using chemotherapy.

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