If radiation oncology had a Moore's Law, it would be that radiation therapy is destined to get progressively more accurate as new technologies enable more precise targeting of radiation beams to cancerous tissue. One example is intensity-modulated radiation therapy (IMRT), in which the shape and intensity of the radiation treatment beam is adjusted depending on the size and location of the target tumor.
The latest development that's sweeping the field is image-guided radiation therapy (IGRT), in which imaging technologies are brought straight into the treatment suite instead of just being used for treatment planning and simulation. Often used in conjunction with IMRT, IGRT systems are giving radiation oncologists new flexibility and power in treating patients.
IGRT systems target a common problem in radiation therapy: the fact that tumors do not remain still. They are subject to physiological processes that cause them to move between and during daily radiotherapy treatments. Interfraction motion -- motion between treatment sessions -- occurs with normal variations in anatomy, such as the amount of food in the stomach or bowel, or how patients are positioned for each treatment. Tumors responding to treatment sometimes shrink, as in the prostate. Motion during treatment can be due to respiration or digestive processes.
IGRT systems provide real-time confirmation of tumor location immediately before or during treatment, offering an improvement on external markers such as tattoos. They can be based on various modalities, including ultrasound, CT, and digital radiography. Automated software provides rapid registration against the treatment plan image, allowing on-the-fly correction for target shifts due to organ motion or change in shape.
The current IGRT systems offer an improvement over initial attempts, in which IGRT consisted of verifying that the treatment beam was correctly aligned by comparing images generated by portal radiographs using the treatment beam. These megavoltage (MV) images are high dose and low contrast, and are compared with high-resolution kilovoltage (kV) images acquired before treatment.
Usually, the lower the energy of the beam (kV), the better the image quality, although in certain cases such as imaging a pelvic region that has a hip prosthesis, the MV beam may better compensate for metal and produce better image quality than kV. In other cases, the MV radiograph may appear washed out, like an overexposed photograph.
Types of IGRT systems
Recently, sonography, fluoroscopy, and cone-beam CT imaging have been adapted to imaging during the treatment delivery process. There are a number of other approaches to IGRT, including a new generation of electronic portal imaging devices (EPID) that use amorphous silicon flat-panel technology, which allows for higher image quality with lower amounts of radiation.
Ultrasonic IGRT
Ultrasound-based systems hold the lion's share of the IGRT market. According to a 2004/2005 survey by market research and consulting firm IMV Medical Information Division of Des Plaines, IL, two-thirds of installed IGRT units used ultrasound for image guidance. This is changing slowly, as other technologies are developed. Ultrasound-based IGRT vendors currently include Resonant Medical of Montreal, Varian Medical Systems of Palo Alto, CA, North American Scientific of Chatsworth, CA, and CMS of St. Louis.
North American Scientific launched its BAT (B-Mode Acquisition and Targeting) localization system in 1998 through its Nomos radiation oncology division, and the unit is in its fourth generation of hardware.
Resonant, founded in 2000, has an installed base of its Restitu 3D ultrasound image-guided radiation therapy platform of more than 30 systems, all in North America. Restitu received clearance in December 2005 from the U.S. Food and Drug Administration (FDA). It acquires 3D ultrasound patient data and compares it to pretreatment 3D ultrasound data to quantify changes in organ position and morphology.
CT
CT-based IGRT systems are called "conebeam" or "slit-beam" for the shape of the MV beam as it exits the linac. While CT-based IGRT does not have the market share of ultrasound, there are signs that momentum is beginning to shift. The IMV survey found that of those planning to purchase an IGRT system, 50% anticipated purchasing a CT system, 24% ultrasound, and 23% x-ray.
The FDA recently granted 510(k) clearance to Siemens Medical Solutions of Malvern, PA, for its MVision package, an MV application producing a conebeam CT using the treatment tube. MVision will be a standard package on Siemens' new Oncor Expression linear accelerator and offered as an upgrade to current Primus and Oncor users.
With MVision, a 2D dataset of images is acquired as the gantry rotates around the patient, taking images every 1° for 200° of rotation. A conebeam CT image is then produced by a reconstruction algorithm using the 2D images. The reconstruction is not of the same image quality as a routine multislice CT, but does provide good localization of the treatment area. Proprietary software automatically registers the conebeam CT image with the reference CT.
Another system that uses the MV treatment tube to acquire IGRT images for localization -- thus obviating the need for an additional generator -- is the Hi-Art system from TomoTherapy of Madison, WI. TomoTherapy's slit-beam approach is based on its work in the 1990s using helical tomotherapy for IMRT. The Hi-Art system received FDA clearance in 2002 and has an installed base of around 70 systems.
Another vendor in the CT field is Elekta of Norcross, GA. The company was one of the first to add a kV conebeam capability by attaching an x-ray tube perpendicular to the treatment beam on the company's Synergy digital linear accelerator, and rotating the gantry around the patient. The design produces 400 to 500 projections to create a 3D image.
Varian has also gotten on board the IGRT bandwagon, offering its On-Board Imager (OBI) conebeam imaging system on the company's Clinac or Trilogy linear accelerators.
On-Board Imager employs kV x-ray imaging and a megavoltage (MV) digital portal image detector, allowing the machine to support several imaging modalities, including kV and MV planar radiographic imaging and kV volumetric conebeam CT imaging.
The Varian system incorporates features that allow for automated detection of tumor markers, as well as for respiratory gating to turn the radiation beam on and off to account for tumor motion. This is important for precise treatment of tumors in the lung or other parts of the body subject to respiratory motion.
Radiography
Radiography-based imaging is used by a number of IGRT vendors, including BrainLab of Heimstetten, Germany. BrainLab's ExacTrac X-Ray 6D is an automated IGRT system that utilizes high-resolution x-rays to pinpoint tumor sites seconds before treatment. It robotically corrects patient setup errors and tracks patient movement throughout the treatment.
A number of other companies have developed IGRT techniques. The CyberKnife system, marketed by Accuray of Sunnyvale, CA, uses image guidance technology with computer-controlled robotics to continuously track, detect, and correct for tumor and patient movement throughout the treatment.
CyberKnife doesn't image the tumor directly, but rather bony anatomy near the tumor, and adjusts to its movement. CyberKnife uses a reconstruction of the bony anatomy based on the treatment planning CT, which defines the tumor's positional relationship to the bones. Its respiratory tracking system allows patients to breathe normally throughout the treatment without breath-holding or gating techniques.
Although it was originally designed for cranial radiosurgery, Accuray says that more than 50% of CyberKnife procedures in the U.S. now are of the spine, lung, prostate, liver, and pancreas. The company was founded in 1990, and has a CyberKnife installed base of 140.
Electromagnetic localization
Among newer IGRT technologies is electromagnetic localization. A pioneer of this technique, Calypso Medical Technologies in Seattle, recently received FDA approval for its 4D Localization System, based on implanted electromagnetic transponders. The system is approved for use in prostate tumors.
The transponders, smaller than grains of rice, are implanted in or near the treatment site. The transponders send signals that generate location instructions to the radiation therapist to register the patient's target to isocenter prior to treatment.
The three transponders are implanted into the prostate in an outpatient procedure under transrectal ultrasound guidance. Under local anesthesia, the physician implants the transponders in the prostate, a procedure that takes about 10 minutes, according to Eric R. Meier, Calypso president, CEO, and one of the company's founders.
The transponders are imaged during the treatment planning CT, and their positions are identified and entered into the Calypso system. This information is used to guide patient alignment and monitoring during treatment. Meier said clinical studies have shown that the transponders remain stable in the prostate throughout the course of treatment.
By Robert Bruce
AuntMinnie.com contributing writer
October 20, 2006
Related Reading
Calypso receives 510(k), August 8, 2006
Varian crosses On-Board Imager milestone, July 27, 2006
NAS nets FDA nod, July 25, 2006
Digital imaging grows at rad therapy sites, October 10, 2005
Resonant gets 510(k), December 21, 2004
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