WHY ON-BOARD IMAGING?
Standard radiation therapy is limited in many cases by normal shifts within human anatomy. Tissues and organs can settle around the bones differently each time a patient climbs onto the treatment table. Patients may gain or lose weight over the course of 25 to 35 daily treatments, causing repositioning of organs. Tumors can move several centimeters as patients breathe during treatments.

Oncologists have had to compensate for tumor movements by making the radiation beam larger, exposing a significant volume of healthy tissue around the tumor. Unfortunately, to avoid causing complications in the surrounding healthy tissue, the radiation doses have had to be limited—sometimes to a point below the optimal amount needed to kill the tumor.

Using current procedures, clinicians typically verify tumor locations on a weekly basis, often using the high-energy treatment beam to generate an image that can be used to make any needed adjustments in patient positioning and treatment plans. Many physicians believe that daily imaging and adjustments would make treatments more precise, especially if these can be done using low-energy diagnostic X rays rather than the high-energy treatment beam. With the treatment beam, says Emory’s Fox, “you don’t get a high-quality diagnostic image.”

Varian’s On-Board Imager device solves these problems by using low-energy X rays (about one-sixtieth the energy of the treatment beam) to yield much higher quality images, rapidly and automatically. Clinicians at Emory have now used the device to refine patients’ positions for more than 400 radiotherapy treatment sessions for prostate, brain, and central nervous system cancers.

A VERSATILE DEVICE
The On-Board Imager device produces radiographic, fluoroscopic, and cone-beam CT images, providing still shots, X-ray movies of moving tumors, or 3D images that can provide excellent contrast between tumors and the surrounding normal soft tissue. Physicians can choose the optimal imaging technique for each patient’s disease characteristics.

So far, the On-Board Imager device has been used primarily to track shifts in tumor locations immediately before treatment sessions. In the near future, however, doctors expect to be able to track and adjust for tumor movement during treatment sessions, using fluoroscopic imaging in combination with Varian’s Real-Time Position Management (RPM™) respiratory gating system. The gating system tracks tumor movement caused by respiration and enables physicians to activate beams at optimal moments in their patients’ normal breathing patterns. Because breathing can move tumors as much as two to four centimeters, tracking this motion using fluoroscopic imaging and gating could significantly reduce the margin of healthy tissue exposed to the beam. This could be particularly important in lung or breast cancer treatments.

The On-Board Imager device is designed to be integrated and synchronized with other Varian hardware and software, including treatment planning and information management systems, all working through a single database to communicate in real time with one another and with treatment devices. Such linking helps automate and expedite imaging and treatment, making IGRT processes fast and practical.

Ingemar Naslund, MD, head of the radiotherapy unit at Karolinska, says his institution will be increasing its use of On-Board Imager devices. “Images are high quality and can be integrated automatically and easily into the treatment process to make the On-Board Imager practical for use in busy radiotherapy units,” he says. Naslund is particularly enthusiastic about using fluoroscopic imaging to visualize gold markers implanted in tumors that are subject to respiratory motion.

Doctors and clinicians using the On-Board Imager device say that the amount of additional time required to take daily X-ray images and make positioning changes is not significant: three to five minutes or less in a typical treatment session.

The increased precision afforded by an On-Board Imager device raises the possibility of treating tumors with higher daily doses over shorter periods of time using Varian’s new Trilogy™ linear accelerator. “IGRT will be especially important for ensuring a precision treatment, particularly when we are escalating the dose,” says Fang-Fang Yin, PhD, director of medical physics at Duke University Medical Center in Durham, North Carolina. During 2004, Yin was instrumental in implementing IGRT at the Henry Ford Health System.

After receiving the fifth of 25 planned radiation treatments, prostate cancer patient Jan-Olov Carlsson says his experience at Karolinska has been problem free. “To be able to adjust position before beaming the radiation is just great,” says Carlsson. “My hope is that it results in fewer side effects. I am happy to get treated with the latest technology, especially when it means that the radiation should hit me exactly where it is supposed to.”

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