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For other articles and previous issues click here. May 2, 2005 Think
Small — Mayo Clinic Researchers Retool Scintimammography to
Target Small Lesions Researchers at the Mayo Clinic College of Medicine in Rochester, Minn., have developed a breast imaging technique that is proving to be highly sensitive in detecting small lesions in high-risk women. The technique, which they are calling molecular breast imaging (MBI), uses a high-resolution gamma camera specifically designed to image the breast. The results of the first 100 patients are “very promising,” says Deborah J. Rhodes, MD, a Mayo Clinic physician who specializes in the detection of breast cancer. “Not only did we find almost all the lesions we knew about from mammography, but we also saw additional lesions that you couldn’t see on mammograms.” “By optimizing the camera to detect smaller breast lesions, this technique should aid in the detection of early-stage breast cancer, something that was not possible with conventional gamma cameras,” says Michael O’Connor, PhD, a medical physicist at the Mayo Clinic. Early detection dramatically improves survival. The idea of dedicated nuclear medicine breast imagers is not new. The FDA has approved three imagers produced and sold by Gamma Medica, Naviscan PET Systems, and Dilon Technologies. Small Lesions, Dense Breasts Roughly 25% to 40% of women have dense breast tissue. Breast cancer itself is made up of dense tissue. On a mammogram, a tumor is harder to spot in dense tissue than in fatty tissue because the tumor looks much like the tissue surrounding it. “It’s like looking for a [specific] tree in a forest,” Rhodes says. “Mammography is extremely accurate for women with fatty breasts, but the sensitivity is low in women with very dense breasts,” she adds. “We need a complementary technology so if we see a woman with very dense breasts, we have the ability to offer her an alternative imaging technology that will give us a greater assurance that she does not have breast cancer.” The Mayo researchers call their new gamma camera system MBI to distinguish it from conventional scintimammography, which has limitations in breast imaging and performs poorly for small lesions. The need to be able to detect small cancers of the breast is critical as up to one-third of screening mammography-detected breast cancers are smaller than 1 centimeter, Rhodes says. Sestambi Tracer “It probably can be any number of tracer materials, but we use technetium 99m-sestamibi,” Rhodes says. The gamma camera localizes areas of tracer uptake in the breast. A higher rate of sestamibi uptake indicates the presence of cancer. The advantage of the gamma camera is that the visibility of the tumor is not influenced by the density of the surrounding tissue. It relies on differences in metabolic behavior of tumors vs. normal breast tissue. “Mammography is really an anatomic tool,” Rhodes says. “It finds abnormalities by subtle differences between the appearance of normal and the appearance of tumor. Our camera does not rely on anatomic difference, but rather on the difference in the molecular behavior of normal tissue vs. tumor.” The concept behind their camera is not new. “Scintimammography has been around for years,” Rhodes says. “It started in the 1990s and arose and died.” The problem with the conventional gamma camera was that “you couldn’t get it close enough to the breast and you couldn’t compress the breast, so the resolution was really quite poor and the technology was not very good for finding small lesions in the breast,” Rhodes explains. The idea was all but abandoned. Approximately four years ago, however, O’Connor began adapting the technology for breast imaging. He had been visiting the GE Healthcare research facility in Haifa, Israel, where they were working on a small prototype semiconductor-based gamma camera. “Their initial goal was to develop a detector for cardiac imaging, but we looked at it and said, ‘Hey, can we use it for breast imaging?’” he recalls. Moving Closer “When we had the woman lie prone and would bring the camera in from the side, it didn’t work well,” O’Connor says. “We were looking through the full thickness of the breast, 4 to 6 inches of tissue, and the more tissue you go through, the poorer the image quality.” To improve the images’ resolution, O’Connor mounted the camera on a gantry similar to that of a mammography system and designed paddles to compress the breast and get close to the camera. “In scintimammography,” Rhodes says, “there was a large dead space between the chest wall and the edge of the detector. Essentially, our camera has no dead space so you can image the entire breast.” The Mayo MBI system uses lighter compression than employed by mammography systems, making it more tolerable to patients, Rhodes says. One problem with their gamma camera is the time it takes to image a breast—40 to 50 minutes. The researchers are working on developing a dual-headed camera that will cut imaging time in half. “When we have two detectors, it will cut our imaging time to 20 minutes, which is very similar to mammogram,” Rhodes says. The amount of tracer used for a study exposes the patient to approximately the same amount of radiation as mammography, Rhodes says. MBI has several advantages over MRI, which is sometimes used to find breast cancer in high-risk women, Rhodes says. One is cost. The cost of a bilateral breast MRI is several thousand dollars. “The cost of our technology will be an order of magnitude less than that,” Rhodes says. Clear Result To accurately read an MRI of the breast requires a high level of training, Rhodes says. “Most people at academic institutions who read breast MRI have specialty training in breast MRI. The high cost and complexity of interpretation limit the adaptation of MRI for screening purposes.” MBI also has advantages over ultrasound for screening, Rhodes says. While ultrasound has an increasingly important role in evaluating palpable or mammographically visible lumps, it, too, is very user-dependent and labor-intensive, thus limiting its use for screening. In the January issue of Mayo Clinic Proceedings, the researchers described their findings using MBI for the first 40 patients. All the women who underwent MBI had suspicious findings on mammograms and were scheduled for surgical biopsy. “We were already selecting a pool of women whom we know to have an abnormality on their mammogram,” Rhodes says. The purpose of their pilot study was to see whether the new camera could detect those lesions. “If the camera couldn’t detect small breast lesions, then you’re done. That’s the first question that needed to be answered,” Rhodes says. The answer was a resounding yes. Twenty-six women had 36 malignant lesions confirmed at surgery. MBI detected 33 of the 36 lesions. In addition, four cancers were detected that were not seen on mammogram. In a release, Stephen Phillips, MD, a Mayo Clinic radiologist involved in the study, said the technique yielded the highest sensitivity yet reported for a gamma camera in detecting small breast tumors—less than 1 centimeter—reporting an 86% rate of detection (19 of 22 cancers). The researchers realized that the lesions they missed had to do primarily with the placement of the breast on the gantry. “This is a new technology and there is definitely a learning curve in terms of optimal placement of the breast on the detector,” Rhodes says. “With very few exceptions, the only ones we missed were the ones that were outside the field of view because the breast hadn’t been placed on the detector properly.” The Right Techs The gamma camera produced some false-positive readings; however, mammogram did as well. “Both mammogram and our camera picked up lesions that did not prove to be cancer with biopsy,” Rhodes says, “but they were biopsied or excised because they looked suspicious. The false positives were generally lesions difficult to distinguish on mammography as well.” Rhodes believes any breast imaging technique is going to have some false positives. The goal of the pilot study was not to reduce false positives, but to demonstrate that the camera can detect small breast lesions in women for whom mammograms don’t find them. The group is about to update its findings to 100 patients. “Results from our 100 patients look as promising as those from our initial 40 patients,” Rhodes says. Rhodes believes that someday soon, MBI could become mainstream technology for women with dense breasts and other risk factors. Next on the researchers’ agenda is to study the technology in high-risk women with dense breasts. “We’re going to start that study this year,” Rhodes says. Once it is completed and published, “if we can demonstrate, which we are confident we can, that this technology is better than mammography in detecting breast cancer in this population, at that point, it is very likely that it could become a more standard option.” Rhodes does not believe MRI will become obsolete for breast cancer detection. “It certainly won’t,” she says. “As a diagnostic tool, it’s fantastic. If you have a woman with a known abnormality and mammography is difficult to interpret, it’s very appropriate to get an MRI to evaluate that further, but I personally feel we need another option for screening women at increased risk for breast cancer because of the cost factor.” Rhodes believes dedicated nuclear medicine breast imaging cameras are exciting. “I don’t think it’s one of those flash-in-the-pan technologies. I think it’s here to stay,” she says. Other Gamma Contenders “I think it has an important role in both evaluating indeterminate lesions and potentially has a very important role in terms of screening high-risk women for breast cancer,” she says. “That’s especially true among women who had previous breast cancer or a lumpectomy where mammography and ultrasound can be very difficult to interpret due to scarring.” Brem has worked with the high-resolution, small–field-of-view gamma camera manufactured and distributed by Dilon Technologies of Newport News, Va., first at Johns Hopkins University in Baltimore and now at George Washington. “We have used it at my breast center for nearly six years. There have been over 500 patients imaged and some of the early data was reported with me as the first author in the Journal of Nuclear Medicine several years ago,” she says. Her research group has a study that will appear in Radiology this summer using the breast-specific gamma imaging approach for screening high-risk women. — Beth W. Orenstein is a freelance medical writer based in Northampton, Pa. She is a regular contributor to Radiology Today. |
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