July 27, 2009
Functional Breast Imaging Modalities — Breast Surgeons Explore Their Roles in Improving Cancer Diagnosis
By Kathy Hardy
Vol. 10 No. 14 P. 22
In the art world, complementary colors oppose each other on the color wheel but work together to enhance the composition. In the diagnosis of breast cancer, complementary imaging tools enhance the diagnosis of breast cancer. Modalities such as positron emission mammography (PEM), breast-specific gamma imaging (BSGI), and molecular breast imaging (MBI), are more often seen today as adjuncts to a woman’s annual mammogram. MBI is still in the clinical trial stage. They join breast MRI as tools to improve on screening mammography.
Experts in the various functional breast imaging modalities addressed questions from breast surgeons as part of the American Society of Breast Surgeons’ recent annual meeting. Attendees were curious about the different functional breast imaging techniques and which modalities they should incorporate into their standards of care. Differences among modalities vary, but the desired results are the same.
“Functional breast imaging is done for the same reasons as other forms of breast imaging: to look for signs of cancer,” says Lorraine Tafra, MD, FACS, director of the Anne Arundel Medical Center Breast Center in Annapolis, Md.
Both PEM and BSGI rely on the use of radiotracers to detect cancer cells. BSGI uses a gamma camera scaled and designed specifically for breast imaging. With PEM, the tracer contains glucose, which is metabolized by cells for energy. Since cancer cells use more energy than normal cells, they take up more of the tracer and are highlighted on the image. Similarities exist between BSGI and MBI, with both utilizing nuclear medicine technology. However, MBI adds a dual-head, small field-of-view detector and semiconductor technology to the modality.
“What’s unique about PEM is the ability of the image to represent the physiology of the area of interest as opposed to just its anatomic appearance,” Tafra says.
Where PEM and BSGI image cancer activity, MRI images its structure. MRI was approved by the FDA for use in breast cancer diagnosis in 1991. Per the 2007 American Cancer Society guidelines, MRI in conjunction with mammography is good for use in women with dense breast tissue and those at high risk for breast cancer. MRI is highly sensitive to small abnormalities but not as specific as some other modalities when it comes to diagnosing cancerous lesions in breast tissue.
According to one study presented earlier this year during the annual National Interdisciplinary Breast Center Conference, BSGI could be used to improve on the specificity of MRI, potentially as a follow-up to mammography. Study results show that with the addition of BSGI, doctors could eliminate the need for one half of all breast biopsies or possibly change a patient’s decision to have more radical surgery.
Tafra notes what many imaging professionals believe: Screening mammography is good but by no means perfect, specifically for women with dense breast tissue. While MRI has emerged as an alternative for screening high-risk women, the process is expensive and time consuming. But the biggest concern with breast MRI is its high percentage of false-positive results, she says.
“With MRI, you’re jumping out of the frying pan and into the fire,” Tafra says. “There’s a huge controversy with MRI in that perhaps surgeons are performing more unnecessary mastectomies. MRI has opened up a lot of controversial issues.”
However, with PEM, the imaging provides physiological information along with anatomical information. “MRI doesn’t normally tell us what the item is that’s being imaged,” she says.
PEM is an organ-specific, high-resolution PET scanner that provides functional imaging for breast cancer detection. This technology uses two detectors integrated into a conventional mammography system that allows for the dual registration of images gathered via mammography and FDG PET. Specifically, FDG will accumulate in inflammatory and cancerous areas that have higher metabolic rates than normal cells.
Once FDG is injected, imaging is done with dedicated breast PET in a manner similar to mammography, with slight compression to immobilize the breast.
Tafra says PEM is used to assist with evaluating patients diagnosed with breast cancer. This modality is helpful not only with the presurgical staging of breast cancer but also with biopsy guidance, once a course of action has been determined.
Some say PEM appears equivalent to MRI at showing cancer but may be more specific in that it may be less likely to show noncancerous lesions, Tafra says. Although the sensitivity of whole-body PET is not very high, she says, it is quite specific for the detection of metastatic disease and effective for tumor staging.
“Then PET technology was fused with CT scans,” Tafra says. “With the fused version showing physiology and the anatomy, the accuracy of the imaging obviated the need for large studies. The future fusion of PEM with MRI may follow the same path.”
However, cost could be a factor in the adoption of PEM technology in everyday practice, she says. She estimates that there are currently only about 15 to 20 PEM devices nationally.
“PEM studies and technology are quite expensive,” she says. “Imaging centers looking at MRI or PEM technology would most likely go with MRI.”
Tafra notes at least one ongoing multicenter trial of 500 to 600 patients in which MRI and PEM are being compared. The conclusion could bring more clarity to the appropriate role of each modality.
“I’ve worked with breast cancer patients for 15 years and, in that time, I’ve never had two patients with the same issue,” Tafra says. “Each patient is different, and the goal is to provide tailored care. Technology that fuses the anatomy with the physiology of the breast may provide the best route to tailored care.”
Handling False Positives
Elsie Levin, MD, medical director of the Faulkner-Sagoff Breast Imaging and Diagnostic Centre in Boston, notes that the sensitivity of MRI and PEM is about the same, but specificity is better with PEM.
“[But] when you put MRI in the hands of breast imagers, the specificity is greatly improved,” she says. There is concern in the imaging field that MRI’s lower specificity leads to false positives and unnecessary biopsies and worry for the patient.
It also depends on how you look at false positives, Levin adds. “If you read the MRI and see something, you can order an [ultrasound-guided biopsy],” she says. “The core biopsy results are back in 24 hours, and then you have specific answers. It might not be a cancer, but it’s something.”
Levin sees approximately 350 patients per month for breast MRI; 75% of those women are undergoing MRI for high-risk screening. She adds that MRI will show additional cancers in one third of newly diagnosed breast cancer patients.
In addition, a study presented at the last RSNA annual meeting showed that PEM’s ability to detect cancer did not appear to be adversely affected by breast density, hormone replacement therapy, or a woman’s menopausal status. Hormonal changes within a woman’s menstrual cycle can result in false positives in MRI studies.
MRI is also helpful for “mapping out” disease in the breast, and with that, Levin says, surgeons may be able to reduce their reexcision rates.
“We will use MRI to conduct preoperative staging,” she says. “Mammography will frequently underestimate the extent of the disease. We can help surgeons to scope marginal concerns.”
Postchemotherapy use of MRI is helpful as well, enabling physicians to see whether treatments have reduced the size of tumors or eliminated them altogether.
Any imaging technology comes with its own unique learning curve. For example, with MRI, it’s a matter of imaging women at the right time in their menstrual cycles or picking up on the nuance of interpreting the up to 1,000 images generated during an MRI scan. However, champions of MBI note that this modality requires fewer images—eight pictures in all—and could possibly be a simpler, less expensive alternative to imaging high-risk women with MRI.
“MBI has an emerging role as a way to image breasts for those who have breast cancer or are suspected to have cancer,” says Judy C. Boughey, MD, an assistant professor of surgery at the Mayo Clinic in Rochester, Minn. “There’s also potential application for screening high-risk women, those women with a family history of breast cancer, or those who have had an atypical biopsy.”
Boughey is part of a team at the Mayo Clinic that is conducting a study comparing MRI with MBI. What the group has found to date is that MBI offers the same benefits as MRI, with comparable sensitivity and specificity, but MBI is easier for the physician, easier on the patient, and less expensive than MRI, coming in at one fifth less the cost.
“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,” Boughey says.
At the Mayo Clinic, some additional study participants have come from a program where female patients visiting the clinic with heart issues are receiving free MBI screenings. In one instance, a 7-mm cancerous breast lesion was detected. Nothing was found in a screening mammogram the woman underwent earlier that year.
“In this case, we found early disease when it couldn’t be seen by standard methods,” she says.
MBI has also helped with diagnosis, enabling physicians to look at the entire breast in question, as well as the other breast. The dual-headed gamma camera with two opposing detectors used with MBI is more likely to pick up lesions that may be missed because they fall outside the scope of a single detector. In addition, recording two facing views provides a means to better localize a lesion’s depth and estimate its size. Ultimately, this modality can help patients and doctors make the decision between mastectomy and lumpectomy.
“With the dual-headed camera, you decrease the distance between the detector and the tumor by a factor of two,” Boughey says. “This allows us to pick up smaller tumors.”
As the range of modalities available as adjuncts to mammography continues to evolve, it seems that breast cancer surgeons and radiologists will seek to add methodologies to their palates that fit a variety of situations, all with the goals of earlier detection and improved treatment.
— Kathy Hardy is a freelance writer based in Phoenixville, Pa., and a frequent contributor to Radiology Today.