“The use of 3D images in some ways is one of the biggest factors benefiting our patients,” says Per Halvorsen, chief clinical physicist in the Wake Section at the University of North Carolina (Chapel Hill, N.C.). “3D conformal therapy has made significant advances, but that is not possible without these images.”
Image taken from Varian’s Helios inverse planning system.
Halvorsen says CT images have “very good spatial accuracy and have fairly accurate density information contained in the image,” which is important in dose calculations. “Unfortunately,” he adds, “CT doesn’t always localize soft tissue tumors very well, especially in the case of the brain. MRI often does a better job of showing us different composition of soft tissue.”
In addition to CT and MRI studies, many centers employ functional nuclear medicine scans from SPECT (single photon emission computed tomography) or PET (positron emission tomography) to produce images of tumor activity for planning purposes. Because tumors “light up” during radiolabeled glucose uptake, these studies help to pinpoint tumor activity once they are fused with a CT image.
There are other uses for SPECT studies in the treatment of cancer. For example, the Wake Section uses SPECT studies to quantify lung function, primarily perfusion, to determine which part of the lung functions well, and where function is impaired before a patient’s tumor is radiated.
“On a lung patient,” notes Halvorsen, “if we’re planning to take him to a fairly high dose, and he presents with poor pulmonary function test values, then we certainly don’t want to wipe out whatever remaining lung function he has.”
Radiation therapy planning essentially begins with computer simulation, says Timothy D. Solberg, Ph.D., associate professor at the University of California-Los Angeles’ (UCLA) Jonsson Cancer Center (Los Angeles). “We mimic the entire procedure on a computer, including looking at what the dose distribution is going to look like before we deliver the treatment,” he adds. “The planning is done with image guidance.”
When the center is planning any kind of tumor treatment, it always uses a combination of CT and MR image sets, because each has its strengths, says Solberg. “CT is very accurate in a geometric sense. Brain tumors are often better visualized on MRI. So, we use image fusion to map the MR onto the CT.”
IMRT treatment plan for radiation therapy of the prostate. This image is taken with ADAC Laboratories’ Pinnacle system.
Harry Tschopik, director of global marketing for radiation therapy products at ADAC Laboratories Inc. (Milpitas, Calif.), agrees. “We view the planning process as central to radiotherapy,” he says. “The planning process involves everything from gathering the images that are needed to find tumor tissue and normal tissue, localizing both of those, planning how the radiation is to be delivered, evaluating alternative plans, and then the plan is exported to the treatment device.”
Setting the goals
There is a crucial balancing act involved in radiation therapy. Healthcare providers must destroy the tumor, while sparing critical structures that surround it.
Vivek Mishra, Ph.D., director of medical physics at Baptist Hospital of Miami (Fla.), uses ADAC’s Pinnacle radiation therapy planning system. The center treats between 600 and 700 cancer patients per year.
Pinnacle includes a software program and imaging workstation with an intuitive clinically based interface that is used in conformal treatment planning. Pinnacle offers automatic, semi-automatic and manual tools to accomplish the treatment planning process, because different clinical circumstances call for different tools.
Given the sophistication of some treatment options, aiming at the correct target and providing the appropriate dose are imperative to success.
“Cancer can always be killed by radiation,” Mishra says. “The fight is about whether or not the patient comes out alive at the end of the treatment. The more accurately you hone in on this target [the tumor], the better off you are.”
Gerald E. Hanks, M.D., chairman of the department of radiation oncology at Fox Chase Cancer Center (Philadelphia), concurs.
“The principle of 3D conformal therapy is directed at accurately determining where the target is, hitting the target every day you treat it, while maximally protecting the adjacent normal tissues,” he adds.
In treatment for prostate tumors, Hanks says that if the beam is directed at the prostate from the front, “where the prostate may look like a ball, it will be a circular field with a centimeter of margin around the prostate. If it comes from the side, where the prostate looks like a sausage, you have a sausage-shaped field with a centimeter of margin around it.”
Regardless of the approach, the beam is directed to conform to the target from that angle.
Hanks explains that using these techniques, Fox Chase has reduced complication rates significantly, while improving the cure rate.
“My published cure rates are the same or better than any of the surgical series, but our complication rates are clearly better,” Hanks says. “Our potency preservation is better and we don’t have any of the incontinence [problems] they [surgical interventions] have.”
He also says there are fewer bowel and bladder problems in patients during treatment.
Pinpoint accuracy
There are several techniques to insure that the targeted tumor will be found in the same field for each treatment. Prostate cancer patients lie in a soft half-cast to reduce day-to-day variability of positioning. Using this technique, healthcare providers can reduce the margin around the target by as much as a centimeter.
Back at UCLA’s Jonssen Cancer Center, Solberg uses the Novalis planning/treatment system developed in a collaborative effort with BrainLab USA Inc. (Redwood City, Calif) and others to treat brain tumors using radiosurgery. The planning process is guided by 3D image sets acquired through CT and MRI. A metal frame is affixed to a patient’s skull during the imaging and treatment phases to provide stereotactic coordinates for defining treatment parameters.
“Obviously, the tumor is the structure of interest,” Solberg says. Structures that treatment specialists try to avoid that are particularly radiation-sensitive include the brain stem and cranial nerves, which must be carefully delineated. Precision is paramount.
One of the principles of radiosurgery is that the target is bombarded from many directions, using from six to 24 beams.
“Another nice feature is the integration of our treatment planning system with the delivery machine, the Novalis accelerator,” Solberg says. “When the plan is finished, and the physicians are happy, then we can export all of those parameters to the machine. This feature minimizes opportunities for data input errors.”
Solberg’s group also uses this system to treat acoustic neuromas, benign tumors that can cause hearing problems. Historically, these tumors were removed surgically with almost a 100 percent chance of hearing loss in that ear.
“What we’ve been able to do with Novalis, with shaped beam and fractionation combined, is that instead of giving [the dose] all at once, we give it over a course of about six weeks, a little bit each day,” Solberg says. “That controls the tumor and we’re seeing that it allows the patient’s hearing to be preserved.”
In the treatment of soft-tissue tumors with radiosurgery, a stereotactic frame is not an option. Trying to hit a moving target, the soft tissue that moves when a patient’s heart beats or he breathes, is a major challenge. The University of Rochester (N.Y.) Medical School uses gating techniques combined with their ability to turn the linear accelerator on and off in 30 to 50 milliseconds to treat soft tissue tumors with radiosurgery.
“I have a couple of imaging scientists on my faculty and their job is to hit a coronary artery on a beating heart in a breathing patient,” says Paul Okunieff, M.D., chairman of radiation oncology at the University of Rochester. “They can do that. The technology Novalis offers can achieve that.”
Using Novalis in treating liver tumors, Okunieff utilizes a technique he calls virtual immobilization. The patient is imaged with a CT in treatment position with surface markers placed on the skin. The motion of the diaphragm during respiration is analyzed using the surface markers as reference points. Once providers determine a pattern of motions that correspond with tumor placement in a certain spot, they can determine where the target will be at a certain point in the respiratory cycle. When the patient is moved into position, a ghost image on a computer suggests where the patient’s body should be, and the tumor position is confirmed with an ultrasound study. A gating mechanism triggers the accelerator to destroy the tumor at the appropriate time.
One use of these techniques involves cases when both surgery and radiosurgery are required to improve patient outcome.
“We know that there is a risk on certain kinds of procedures of spreading the tumor during the operation,” explains Okunieff. In those cases, the surgeon requests that the radiation therapist destroy the tumor prior to surgery. This eliminates the opportunities for tumor proliferation.
Nomos Corp. (Sewickley, Pa.) offers an ultrasound-based targeting system — BAT (B-mode Acquisition and Targeting) — that localizes mobile targets. Once initial treatment planning is accomplished using CT, views of the target appear on the BAT screen. BAT provides a real-time ultrasound image as an overlay that helps to align those images with the treatment coordinates.
An exact science
Perhaps the hottest area today in radiation therapy planning is intensity modulated radiation therapy (IMRT). One key element to IMRT is inverse planning.
Corvus is an inverse planning system developed by Nomos.
“You start with the clinical results you would like to achieve with the plan and then Corvus tries tens of thousands of different alternatives of how to achieve those, evaluates which one is best, provides you with the results, and summarizes how well it was able to do in satisfying the clinical goals,” explains Robert Riker, Nomos’ IMRT product manager.
“It’s a different process for the physician to sit down in front of a planning system and describe at the outset his clinical goals in the clinical terms he’s used to using to evaluate a treatment plan,” Riker says. “To put it in at the front of the process makes it more physician-oriented.”
One of the primary goals of inverse treatment planning is to reduce significantly margins around tumors. Combining inverse planning with IMRT means that treatment margins around the tumor can be reduced, which spares normal tissue.
“If you can have a centimeter less of “rind” around a circular prostate, volumetrically that’s a lot of normal tissue,” notes Hanks.
Varian Medical Systems Inc. (Palo Alto, Calif.) offers Helios as its inverse planning system.
“With inverse planning, the physicist specifies the dose he or she wants to deliver to the tumor and the maximum amount of dose to be delivered to critical structures,” says Varian’s Corey Zankowski, product manager for treatment planning systems. “You tell the computer your objectives, and it will figure out how to modulate the beam and add shielding for the best dose distribution to meet your objectives. The algorithm produces intensity patterns that would be impossible for a human to come up with on his or her own. It can do a million calculations per second.”
On the treatment side, Zankowski says Varian is developing a newly designed multileaf collimator with 120 separate tungsten leaves, each seven centimeters thick, 20 centimeters long and three millimeters wide. This system is designed to provide a resolution of five millimeters near the center of a patient.
Bradley A. Kramer, M.D., medical director at Midwestern Regional Medical Center (Zion, Ill.), uses the Helios system with the Varian Clinac 2100 CD linear accelerator. Once they have described the desired clinical outcomes and the Helios system develops a plan, providers verify those treatments before the patient is treated.
“When we plan for IMRT, we tell the computer what we want to go where, and it comes back with the best plan by modulating the beam using a dynamic multileaf collimator,” says Kramer. “We then take the same plan and radiate a phantom. We make sure that the computer is giving us what it tells us it is going to give.”
Economic issues
One of the negative aspects to inverse planning is the cost, most of which is not reimbursed by third-party payors. While there can be cost savings through reduced complication rates, the benefits may be difficult to prove on paper.
“We’re in a position now that we can say in our hearts that we know this works,” says Rochester’s Okunieff. “We’ve had patients that are now over two years out after failing chemotherapy with liver metastases, who are now disease free. And we’ve had people with tumors that are not expected to respond to any therapy who are now disease free.”
As healthcare providers negotiate the complex balancing act of radiation therapy — destroying tumors without damaging nearby healthy structures — advances in medical imaging have provided essential information to permit these treatments to the benefit of patients.
