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Advances in breast imaging

December 18, 2006

A new gamma camera technique for the detection of small breast tumors

SAN ANTONIO - A diagnostic device that resembles a mammography unit can detect breast tumors as tiny as one-fifth of an inch in diameter, which may make it a valuable complementary imaging technique to mammography, say researchers at Mayo Clinic, who helped develop the technology along with industry collaborators Gamma Medica and GE Healthcare.

This new technique, Molecular Breast Imaging, uses a new dual-head gamma camera system and is sensitive enough to detect tumors less than 10 millimeters (about two-fifths of an inch) in diameter in 88 percent of cases where it is used. Early findings from an ongoing comparison of the device with mammography show that it can detect small cancers that were not found with mammography, say the investigators. Mayo Clinic physicist Michael O'Connor, Ph.D., will present these results Saturday, Dec. 16, at the 2006 meeting of the San Antonio Breast Cancer Symposium.

"Our ultimate goal is to detect small cancers that may be inconspicuous or invisible on a mammogram for high-risk women with dense breasts," says Dr. O'Connor.

The investigators also say their device will likely be only slightly more expensive to use than mammography, and will be much more comfortable for women because much less pressure is needed to image a breast.

"We hope that our studies will eventually show our device to be almost as sensitive as magnetic resonance imaging (MRI), which is probably the best diagnostic test available to date, but is not widely used because of its expense," says Stephen Phillips, M.D., a Mayo radiologist and a study co-author. An MRI scan costs as much as ten times more than a traditional mammogram and involves injection of a contrast agent.

Mammography uses low-dose X-rays (ionizing radiation) to create images of the anatomy of breast tissue. If the breasts are very dense, it can only accurately help in tumor diagnosis in 30 to 50 percent of cases, says Deborah Rhodes, M.D., another study co-author. Yet women who have dense breasts are four to six times more likely to develop breast cancer, and more functioning breast tissue is available in which disease can occur, she says.

Conventional gamma cameras cannot be easily adapted for breast imaging. Instead, the investigators used new, small semiconductor-based gamma cameras and incorporated them into a new breast imaging system. Images obtained with these gamma cameras are not affected by dense or fatty tissue. In the procedure, women are injected with a small amount of the radioactive drug sestamibi that preferentially travels to tumors, which absorb the substance. These women then are seated in front of the device, which looks like "a strange mammography unit," Dr. O'Connor says. Each breast is lightly compressed between the gamma cameras with just enough pressure to keep it from moving for 5 to 10 minutes while several images are taken. "It is much more comfortable for women, because a force of only 15 pounds is used, compared to the 45-pound force compression needed to take a mammogram," he says.

The image usually shows low, but some, absorption of the sestamibi throughout the breast. In areas of cancer, the amount of drug absorption is significantly increased by the cancer. Although some benign conditions such as fibroadenomas will occasionally absorb the drug, creating a false-positive result, the researchers believe that the error rate is less than the approximately 10 percent rate found with traditional mammography.

The research team used this innovative dual-head gamma camera system to scan 100 patients who had suspicious breast lesions that were small, with a diameter of 2 centimeters (four-fifths of an inch) or less. Eighty-two cancers were later identified at surgery in 54 patients. The gamma camera detected 76 of the cancers, giving it a 93 percent success rate in these cases. Some were missed, either because the breast was not properly positioned in the device or because they were too small to detect with this technology (2 to 4 millimeters or about one-tenth to fifteen-one hundreths of an inch), says Dr. O'Connor. Still, the gamma camera was 88 percent accurate in picking up cancers less than 10 millimeters.

The researchers will also discuss preliminary findings from an ongoing, blinded clinical trial comparing the gamma camera with mammography in 2,000 women who have come to Mayo Clinic for routine screening. In the first 250 patients, the gamma camera detected four cancers, and three of these were not visible on a mammogram.

"Although these initial results are very exciting, we clearly need to image more patients to confirm the promise of the device," says Dr. Rhodes. She says the team hopes to complete the ongoing trial in the next two years.

Mayo Clinic