Image-Guided Radiation Therapy
Dynamic Targeting™ Image-Guided Radiation Therapy
Standard radiation therapy is often limited by normal shifts within the human anatomy. Tissues and organs can settle differently each time a patient climbs onto a treatment table. Weight fluctuations over the course of multiple treatments can also cause significant changes in tumor location. Even normal breathing can change a tumor's locale by several centimeters. Radiation oncologists have traditionally compensated for tumor movement by enlarging treatment areas, exposing more healthy tissue to the cell-killing effects of radiation. Doses are typically lowered to avoid complications.
New techniques are being developed to better control this uncertainty. For example, real-time imaging combined with real-time adjustment of the therapeutic beams capable of administering ultra-precise doses of radiation to previously unreachable tumors is now available. This next generation treatment technology - called image-guided radiation therapy (IGRT)—represents a major step in the fight against cancer.
IGRT allows the use of higher, more effective doses of radiation because there is less risk of damaging nearby healthy tissue. IGRT's onboard robotic imaging system is mounted on the treatment machine itself. The device produces high-resolution images using low-energy X-rays.
These real-time imaging techniques allow doctors specializing in radiation therapy to locate and target tumors with unprecedented accuracy.
How It Works
When patients are positioned on a treatment couch, an X-ray system mounted on a robotic arm is rotated around their body to gather images that help pinpoint a tumor's exact location. These images are then compared with existing images (MRI, CT or other kinds of scans) to determine if the tumor has moved since the last treatment. Because tissues and organs can settle differently each time a patient lies down on a treatment table, tumors may be in different positions from one treatment session to another. In addition, tumors can move several centimeters due to a patient's normal respiratory cycle.
For Dynamic Targeting IGRT, the linear accelerator is outfitted with a number of sophisticated imaging devices that provide the clinician with images that help to guide the treatment.
Central to this approach is Varian's On-Board Imager® kV imaging system, an imaging tool that is attached to the treatment machine on a pair of robotic arms, and produces low-dose, high-resolution kilovoltage X-ray images for pinpointing the position of the tumor immediately prior to treatment. The On-Board Imager can be operated in three distinct imaging modes to generate different types of images, including:
- Radiographic (two-dimensional)
- Fluoroscopic (moving, in real-time)
- Cone-beam CT (three-dimensional)
These distinct images types provide doctors with different information about the tumor and surrounding anatomy, and can reveal changes in tumor shape, size or position over a multi-week course of treatment. Fluoroscopic images can be used to track tumor motion for a clear indication of exactly how a tumor will move during treatment due to respiration or other normal physiological processes. This enables doctors to design optimal treatments for their patients.
Through more precise targeting of the beam, radiation dosage levels can be increased and target volumes (the three-dimensional areas to receive treatment) can be reduced--so tumors get a higher dose of radiation and healthy surrounding tissues get very little. Higher doses have been shown to enhance treatment effectiveness. In addition, better targeting helps reduce the possible side effects of radiotherapy.