June 30, 2008
Two Heads Better Than One?
By Kathy Hardy
Vol. 9 No. 13 P. 16
Dual-head gamma cameras seek better perspective on breast cancer detection
Are two heads better than one? Researchers at the Mayo Clinic in Rochester, Minn., think that may be the case as they investigate a method of nuclear medicine imaging technology that shows promise in finding additional lesions in women with breast cancer—particularly small, early-stage cancers less than 10 millimeters in size.
This technique for breast cancer detection, called molecular breast imaging (MBI), visualizes cancerous lesions through the functional uptake of the radioactive tracer sestamibi using specialized breast imaging protocols and new high-resolution semiconductor gamma cameras specifically engineered for breast applications. Mounted on equipment much like that used in mammography, these cameras incorporate a dual-head gamma camera system, an innovation over the currently used single-camera system.
Judy C. Boughey, MD, an assistant professor of surgery at the Mayo Clinic and a researcher on the project, presented study findings at the American Society of Breast Surgeons’ (ASBS) annual meeting in May that show the switch from a single-head to a dual-head gamma camera is responsible, in part, for recent advances in lesion-detection sensitivity. Current study results suggest MBI may be a useful modality in surgical treatment planning, she says, by locating multifocal cancers in women diagnosed with only a single tumor through mammography.
With the dual-head gamma camera, two opposing detectors are more likely to pick up lesions that may be missed because they fall out of the scope of a single detector, she explains. In addition, recording two facing views provides a means to better localize a lesion’s depth and estimate its size.
“With dual-head cameras, you decrease the distance between a cancer detector, which better enables radiologists to find cancerous lesions in the breast,“ she says. “This is key when you consider the importance of early detection in breast cancer survival rates.”
According to Boughey, MBI provides significantly higher resolution images now than when the radiotracer was first used in breast applications more than 10 years ago. Initially, sestamibi was used with gamma cameras for cardiac imaging. Scintimammography, a nuclear medicine test used as a supplement to traditional mammography, soon followed. However, this type of whole body imaging became problematic for breast screening. It did not allow the gamma camera to be positioned close to the breast and was not conducive to providing clear resolution of images taken through noncompressed breast tissue. “This method had difficulty detecting lesions less than 1 centimeter in size,” she says.
Mounting the single-headed camera onto equipment similar to that of a traditional mammography system was effective but still had its drawbacks, in that small lesions could be missed. That’s where the dual-head approach came into the picture.
Mayo Clinic physicists Michael K. O’Connor, PhD, and Carrie B. Hruska, PhD, developed the compact semiconductor-based, dual-head gamma cameras incorporated into a newly designed breast imaging system. With this revised system, the breast is lightly compressed between the gamma cameras, using less pressure than that experienced with mammography. Mammography requires 45 pounds of pressure, while MBI requires only 15 pounds of pressure—just enough to stop movement during imaging process.
“These new gamma cameras offer improvement in resolution by a factor of two,” said O’Connor via a press release. “That, coupled with the light breast compression, has resulted in dramatic improvement in resolution compared to older, conventional gamma cameras.”
Boughey adds, “With the dual-headed camera, you decrease the distance between the detector and the tumor by a factor of two. This allows us to pick up smaller tumors.”
Six Years’ Work
In a presentation before a packed audience at the ASBS meeting, Boughey discussed study findings gathered during six years of research that involved 900 patients. The first study involved 100 patients, and imaging was done using only the single-head gamma camera, a 20-centimeter X 20-centimeter detector made with cadmium-zinc-telluride semiconductor. In that study, MBI found 57 of 67 cancers in 53 women diagnosed with breast cancer, a sensitivity rate of 85%. A subsequent study, in which the dual-head camera was used, involved 150 women suspected of having breast cancer on mammography or ultrasound. This more advanced process resulted in a 92% sensitivity overall and showed improved ability to locate smaller cancers.
A more recent study involved 650 asymptomatic women with dense breast tissue at high risk for cancer, each screened with the dual-head camera. MBI detected seven cancers, while conventional mammography located only two of those seven. Mammography did locate one cancer that was not found by MBI.
Combining the data gathered from all studies, MBI detected 187 cancers in 146 patients. Twenty-one of these lesions were not found earlier using mammography.
The gamma camera presents an advantage for imaging dense breast tissue, Boughey says, in that the radioactive tracer detects lesions by their metabolic behavior. While small amounts of the tracer can be absorbed throughout the breast, seeing a higher concentration of tracer uptake means a greater possibility of the presence of cancer, regardless of the size of the lesion. Some benign conditions, such as fibroadenomas, can absorb high concentrations of the tracer, creating a false-positive result, but researchers believe that rate is less than the 10% found with mammography.
Mammography relies on the differences in the anatomic appearance of tumors as opposed to the appearance of normal breast tissue. Those differences can often be subtle or obscured by densities in the surrounding breast tissue.
While mammography remains the first line of screening defense in the battle against breast cancer, breast MRI has been used as an adjunct to mammography for the past 15 years. MRI is also used in surgical treatment planning, evaluating the extent of disease and multifocal disease. Used more prevalently in the past five years, advanced breast MRI enables radiologists to image both breasts simultaneously. Overall, breast MRI is considered more sensitive than mammography, but that increased sensitivity leads to the potential for false-positive results.
“The American Cancer Society recommends MRI screening for high risk women,” Boughey says. “If we can show MBI is as accurate as MRI, is less costly than MRI, and is less stressful for women to undergo, we really have something good.”
The cost factor definitely comes into play when considering breast imaging options. Based on current Medicare reimbursement rates, an MBI study is estimated to cost approximately one third of the cost of an MRI study.
Another issue is the learning curve. MRI generates several hundred images of each breast, which the radiologist must then review. MBI, however, only produces eight images.
“We have found that the learning curve for MBI is much easier than for MRI,” Boughey says. “With MBI, you have eight images to review. With MRI, you have hundreds of images. Reviewing that many images can take a significant amount of time and can create a greater chance that something could be missed.”
For the actual MBI process, researchers on this study have found that it is important that technologists are properly trained in breast positioning and are familiar with mammography positioning techniques. Improper placement can affect results, potentially causing lesions to be missed.
Single-head scintillating gamma cameras are already in clinical use, but the dual-head semiconductor gamma camera system is still in the research protocol stage. Boughey says future considerations could involve using new molecular tracers to see how they would affect results. The radiotracer currently used in this study involves a higher radiation dose to the breast itself than a mammogram, although the dose to the entire body is less than a mammogram, she says. Boughey adds that they are working on ways to significantly reduce the overall radiation dosage for MBI.
“Any research like this involves years of study and clinical trials,” she says. “As you do more work, you discover more things. More time allows for fine-tuning. We’re always cautious about putting something out too early. Once it’s out there, you can’t bring it back.”
And the work continues. Boughey mentions a possible opportunity for advancement of this technology, as well as a chance to offer additional health screening benefits to women who come to the Mayo Clinic.
“We’re considering a study that includes a free MBI breast screening with a scheduled cardiac screening,” she says. “The patient is already there for the heart screening and receiving a dose of sestamibi, so why not look at the breast at the same time?”— Kathy Hardy is a freelance writer and editor based in Phoenixville, Pa., and a frequent contributor to Radiology Today.