The Future is in Motion
High-resolution radiation therapy techniques like IMRT enable doctors to plan and deliver radiation doses that conform tightly to the three-dimensional contours of a tumor. However, tumors are not stationary, unchanging targets; they move between and during daily treatments. For one thing, tumors are subject to changes in position due to unavoidable day-to-day variations in how patients are positioned for treatment. Even when patients are placed in precisely the same position for their daily treatments, some tumors can shift by as much as two to three centimeters over six to eight weeks of therapy. In addition, normal physiological processes like breathing cause some organs and tumors to move significantly during a daily treatment session.
         In a perfect world, all therapeutic X rays would be directed at tumor cells, with none directed at healthy cells. In the real world, to deal with tumor position uncertainties as well as tumor motion, doctors treat a margin of normal, healthy tissue around the tumor. This ensures that no part of the cancer is missed or under-dosed. However, doctors could shrink the margin of healthy tissue treated if they had a way to see and adjust for changes in tumor position at the moment of treatment.
         “Image-guided radiotherapy will be of major importance in providing us with treatment technology that is accurate enough to allow us more precise margins around the tumors and hence to have more limited volumes, sparing non-targeted tissues,” said Dr. Jean Bourhis, head of the Radiation Oncology Department at the Institut Gustave-Roussy, in Villejuif, France, a leading expert on treating cancers of the head and neck.
         In addition, image-guided radiation therapy “takes into account the motion of organs such as the lungs, heart, and bowel,” said Dr. Richard Hoppe, chair of the Radiation Oncology Department at Stanford University. “By enabling us to track critical organ motions, this new technique could allow us to safely use higher doses, which should improve our ability to control tumors.”
         At the Tohoku University School of Medicine in Japan, Dr. Yoshihiro Takai has been investigating ways of using X-ray fluoroscopy to visualize the target on a daily basis and make positioning adjustments. “Set-up error and organ motion interferes with the accuracy of radiotherapy,” Dr. Takai said. “The important goal of shrinking the treatment margin can only be achieved with better patient positioning techniques.”
         In a recent editorial for Varian’s Centerline magazine, Dr. James Cox, head of the University of Texas M. D. Anderson Cancer Center’s Division of Radiation Oncology, wrote that the evolution of image-guided radiation therapy would expand the realm of treatable tumors. “We will be able to visualize and treat for the first time the elusive small lesions of the liver and other metastatic sites, potentially improving patient outcomes.”

What is Dynamic Targeting IGRT?
“We are just at the beginning of implementing real image-guided therapy,” said Dr. Cox. “As we understand more about tumor motion, we have had to realize that we cannot position patients just on the basis of marks or tattoos on their external anatomy. As the treatments have become more conformal, and as we try to confine the high dose area much more strictly just to where the tumor is, we have to be all the more diligent in knowing exactly where the tumor is, every day.”
         Dynamic Targeting IGRT addresses this clinical challenge. It is an approach that uses patient positioning devices and imaging tools to target tumors more precisely. Dynamic Targeting IGRT helps clinicians in two important ways. It enables them to deal with the small set-up changes that are invariably introduced when a patient comes back and is positioned for daily treatments over a period of weeks. In addition, it provides doctors with ways of taking tumor motion into account during treatment planning, simulation, and most importantly, during radiation therapy treatment delivery.
         “Dynamic Targeting image-guided radiation therapy gives doctors a way to see exactly where a tumor is, and how it is moving, every day just prior to treatment and even during the treatment,” said Calvin Huntzinger, M.S., product manager for Varian’s Dynamic Targeting IGRT initiative. “This is certain to make it possible for doctors to use radiation therapy to treat tumors that were not readily treatable before.”
         “Varian’s current tools for Dynamic Targeting IGRT include the Exact™ Couch with Indexed Immobilization™ for patient positioning, the Acuity™ simulator, the PortalVision™ electronic digital imaging system, and RPM™ Respiratory Gating,” said Huntzinger. “Each of these tools is designed to help clinicians plan and compensate for tumor motion and day-to-day changes in tumor position.”

IGRT and Stereotactic Radiotherapy:
Shortening the Course of Treatment
Because Dynamic Targeting IGRT improves precision, it also raises the possibility of reducing the 30 to 40 daily treatment sessions or “fractions” normally needed for delivering a total dose of radiation. With improved imaging and delivery technology, some small lesions could be treated in a single session with “stereotactic radiosurgery.” Others could be treated in as few as three to five sessions with “stereotactic radiotherapy.” Doctors agree that the possibilities are enormous. According to Dr. Hoppe, an IGRT approach that enables doctors to apply higher radiation doses over a smaller number of fractions “could mean a better tumor response to individual doses and greater convenience for the patient because the number of treatment sessions is reduced.”