More than half of patients with cancer require radiation in the course of managing their disease, according to the National Cancer Institute. Treatment plans have increased in complexity to meet the challenge of irradiating malignant tissues while sparing adjacent normal anatomy. And because the human body is dynamic and in constant motion, a radiation beam may or may not precisely match the target tumor.
That’s where image guided radiation treatment (IGRT) comes in — to offer sophisticated dose distribution plans carefully mapped before treatment begins and are verified on a daily basis. Equipment manufacturers and clinical thought-leaders have developed innovative approaches to the challenges presented by treating a moving target and propelled radiation therapy into new capabilities.
So when it comes to IGRT, the essential ingredients are daily targeting for the most accurate treatment, cone-beam CT to improve workflow, precise respiratory gating to track tumor movement during therapy, multimodality imaging that provides image guidance and precision in guiding radiosurgery.
1. Daily targeting with low-dose, hi-res images
“We are now doing real-time image guidance, or online image guidance,” explains Tim Fox, PhD, director of medical physics in the Emory University department of radiation oncology. In the past, once the initial planning images were completed and a treatment plan devised, technologists would obtain weekly megavoltage (mV) portal images to verify where treatment was delivered. Megavoltage energy, used for treatment purposes, produces cloudy images, where kilovoltage (kV) images are clear and show contrast between soft tissue and bone. Additionally, kV images generate a lower radiation dose to the patient.
Linear accelerator manufacturers have integrated kV x-ray units onto their basic treatment machines so that clinicians are able to determine exactly where the tumor is located each day when the patient is positioned for treatment.
For example, both of the Varian Medical Systems linacs — Clinac and Trilogy —feature the On-Board Imager (OBI) automated system to obtain high-resolution x-ray images to identify precise tumor location while automatically adjusting patient position when necessary. By combining low-dose, high-resolution kV x-ray imaging with integrated software control of treatment parameters, this system in intended to increase accuracy while facilitating efficient patient throughput. The OBI dynamic targeting IGRT is available as an upgrade for most Clinac systems currently installed.
“I think IGRT is most useful when daily targeting is critical,” asserts Hoke Han, MD, radiation oncologist at Hollywood Radiation Oncology in Florida. He explains that studies have shown that when the dose is escalated, the cure rate increases in prostate cancer treatment; however, complication rates can increase as well without careful targeting. Clinicians must target daily to insure that the prostate is exactly in the center of the field and therefore minimize radiation dose to the bladder and rectum.
Using the BrainLAB ExacTrac X-ray 6D automated IGRT system coupled with im-planted fiducial markers, high-resolution x-rays pinpoint internal tumor position immediately prior to treatment. The system will robotically correct patient set-up discrepancies, and track patient movement throughout the treatment. Han reports that BrainLAB is working on a respiratory gating capability for this system that can be installed on all existing linacs.
James Rubenstein, MD, medical director of Radiation Therapy Services headquartered in Ft. Myers, Fla., uses several different radiation treatment systems throughout their 66 centers in 14 states. Clinicians in their centers use an array of imaging modalities for initial targeting activities, and they have used implanted radio-opaque fiducials to guide treatment.
Rubenstein expects development of radiofrequency emitters to be implanted as fiducials so that a linac could be set to track the radiofrequency beam. “You could have those fiducials with dosimeters in them, so that if a fiducial lies outside the beam field, you would know that.” He believes there are prototypes for RF emitters and diodes in development, some awaiting FDA approval.
2. Cone-beam CT
In addition to planar x-rays, many treatment centers perform cone-beam CT images. Fox describes the process as taking the OBI system, rotating it around the patient and using the generated images to reconstruct a tomographic