Molecular Breast Imaging Ups Detection in Dense Breast Tissue
An emerging breast cancer imaging technique called molecular breast imaging (MBI) has been shown to improve the detection of cancers when compared with mammography in women with dense breast tissue. The study comes from the Mayo Clinic in Rochester, Minnesota, where the new technique was developed.
The study, published in the American Journal of Roentgenology, showed that MBI could find an additional 8.8 cancers per 1000 women compared with mammography alone, and the authors say it represents a “major milestone.”
However, two independent radiology experts who were approached for comment by Medscape Medical News point to several problems with the research.
“This is an interesting paper with remarkable findings…that need to be repeated,” Daniel B. Kopans, MD, professor of radiology at Harvard Medical School, told Medscape Medical News. But he said, “a detailed analysis of the radiation risks [is] needed.”
Likewise, Debra Monticciolo, MD, professor of radiology at Texas A&M College of Medicine, in Bryan, said the study authors failed to fully address the issue of radiation dose, which is still “too high for routine screening,” she told Medcape Medical News. “[This] is a big issue with all breast nuclear medicine studies [such as this one], which the authors routinely fail to address.”
But Michael O’Connor, PhD, senior author of the study and inventor of the MBI technology, specifically emphasized its low radiation exposure to patients.
The study is “a major milestone for both safety and efficacy of the imaging device, largely because of the high detection rates achieved through low radiation exposure,” he confirmed in an email.
“This new study is important because it incorporates many of the advances in MBI pioneered here at Mayo Clinic and shows that studies can be performed safely, with low radiation exposure to the patient,” said Dr O’Connor. “This means MBI is safe and effective as a supplemental screening tool.”
Study in Women With Dense Breast Tissue
The study included 1585 women with mammographically dense breasts who underwent MBI at the time of their screening mammogram.
MBI is a form of breast nuclear medicine that uses injection of technetium-99m (Tc-99m) sestamibi, a radioactive substance that allows tumor visualization with a gamma camera.
The Mayo team say they used the modified MBI system that allows a reduced radiation dose because “prior studies of MBI and other nuclear medicine–based breast imaging have used radiation doses deemed too high for serial screening.”
The study found that the addition of MBI increased the cancer detection rate by 8.8 per 1000 women screened, from 3.2 for mammography alone to 12.0 for the combination (P < .001).
Sensitivity was also increased with the addition of MBI compared with mammography alone: from 23.8% to 90.5% overall (P < .001), and from 18.8% to 81.3% for invasive cancer (P = .006).
In addition to the improved cancer detection rate with supplemental MBI, the technique also came with an increased recall and biopsy rate ― although the authors note these rates compare favorably with other mammography alternatives, such as breast ultrasound and MRI.
Specifically, the recall rate rose from 11.0% with mammography alone to 17.6% with the addition of MBI (P < .001), and the biopsy rate increased from 1.3% for mammography alone to 4.2% with the combination (P < .001).
MBI fills an important gap for supplemental screening in women with dense breasts.
Dr Amy Conners
“While we endorse annual mammography for all women age 40 and over, and the addition of annual MRI for women at high risk, MBI fills an important gap for supplemental screening in women with dense breasts who are not otherwise at high risk,” noted Amy Conners, MD, chair of the Mayo Clinic’s Breast Imaging Division and a coauthor of this study.
The reduced radiation exposure of the study’s MBI technique corresponds to an effective dose of between 2.0 and 2.4 millisievert (mSv), noted the authors.
“Although this is higher than the average effective dose from digital mammography (~0.5 mSv) and the effective dose from digital mammography combined with tomosynthesis (1.2 mSv), which is not considered excessive, this effective dose is below natural background radiation levels (US annual average, 3 mSv) and is well below the threshold of 50-100 mSv at which current national and international recommendations indicate that radiation risk concern is warranted,” they wrote.
However, this is not reassuring to Dr Monticciolo, who is immediate past president of the Society of Breast Imaging and chair of the American College of Radiology Commission on Quality and Safety.
“The authors should be commended for trying to decrease the radiation dose, which they have done by enhancements in detection equipment and technique,” she said. “However, only the dose to the breast is discussed (and it is still 4-5 times higher than mammography). As with all nuclear medicine studies, the substance injected goes to the entire body, so the radiation goes to the entire body, too.”
Radiotracers tend to concentrate in certain areas, with those receiving the highest dose known as the “critical organs,” she explained.
“For sestamibi, the critical organ is the colon, not the breast. Even if the breast dose is acceptable (arguable), one must take into account the whole-body dose and critical-organ dose. For a typical 20-30 mCi injection, the dose to the colon is 5-6 rads/50-60 mSv (for reference, a typical mammogram at my institution is 0.09 rads, or 0.9mSV). In this study, the authors have decreased that to just around 10 mCi, which still leaves a dose to the colon of just under 2 rads/20 mSv. This is a high radiation dose and needs to be considered if MBI is intended for use in screening, which entails regular exams.”
Dr Kopans echoed this concern.
“Since the breast is relatively resistant to radiation carcinogenesis after the age of 30, and certainly by the age of 40, the dose to the breast is not the critical problem,” he noted. “Unlike mammography, the radioactive tracer goes all over the body, and other organs are exposed which may be more susceptible to radiation damage. This is likely a problem for annual, or even as the authors suggest, biennial screening with the agent.”
Dr Monticciolo and Dr Kopans had other concerns with the research, including the lack of explanation about why lesions were mammographically occult, how biopsies were obtained, and why the mammogram sensitivity rate was unusually low in the study.
“Mammography sensitivity is known to be lower in dense tissue, but this is far below what is typical of digital mammography. Obviously, this makes MBI look better in magnitude,” said Dr Monticciolo.
“Overall, I think their technique is very promising, but they still are avoiding discussion and addressing the whole-body and critical-organ dose, which are too high for routine screening, even with the lower Tc-99m amount,” she added.
The authors conclude by suggesting that the next step is a multicenter, prospective randomized trial.
“MBI can be interpreted rapidly and requires minimal radiologist training to achieve high interobserver agreement and diagnostic accuracy. The compact MBI machine could be readily incorporated into breast imaging practices with access to Tc-99m sestamibi,” the authors write.
They concede, however, that “ultimately, a multicenter, prospective randomized trial comparing MBI to other supplemental screening modalities is needed to determine which supplemental screening modality offers the best balance of benefit (supplemental detection rate of clinically important cancers) to harms (supplemental recall and biopsy rates for false-positive findings).”
AJR Am J Roentgenol. 2015;204:241-251.