Breast tomosynthesis faces small trials but big dreams

The clinical trial numbers may be small, but the ambitions of one new technology-breast tomosynthesis-were nevertheless larger than life at the RSNA meeting. While many of the breast imaging papers showcased refinements to relatively new techniques, breast tomosynthesis stood out by promising a revolution.

The title of one special focus session said it all: "Will tomosynthesis replace conventional mammography?" Panelist Dr. Daniel Kopans, director of breast imaging at Massachusetts General Hospital, answered that provocative question in the affirmative. He described positive experiences with tomosynthesis in 400 volunteers at MGH, one of just four academic sites in the U.S. testing the new technology.

Following the positive results of the Digital Mammographic Imaging Screening Trial in September, breast imagers have wondered whether digital mammography will overtake film-based systems. After seeing initial results at this special focus session, RSNA attendees were asking whether they should even bother buying a digital mammography system or simply wait for tomosynthesis to hit the market in a few years.

Panelists responded that FDA approval is expected in one or two years. Mammography vendors that are planning to introduce tomosynthesis systems are expected to provide upgrades for buyers of today's digital mammography systems upon request. The new systems will allow users to perform conventional mammography as well as tomosynthesis.

The potential advantages are easy to see. Conventional mammography superimposes layers of breast tissue in a 2D image, a key drawback. This superimposed tissue can hide cancers yet can also give the false appearance of cancer, leading to needless recalls and biopsies.

Tomosynthesis is a 3D digital technique that removes the effect of superimposed tissue. The principle is to reduce noise of overlapping normal breast tissue, improving detection of breast cancer.

"Looking at a 2D mammogram is like trying to look through a book with clear pages-you can see the letters but it is very hard to read them. It is much better if you can read each page individually," Kopans said.

BACK TO THE FUTURE

Kopans set the stage for the coming of breast tomosynthesis by reviewing the history of breast imaging, starting with industrial film and then xeroradiography. Breast cancer death rates in the U.S. that were constant for 50 years suddenly started to decline in the early 1990s, as the benefits of mass screening became evident.

Such benefits are now well accepted, but there is still room for improvement, Kopans said. Out of 100 women with breast cancer today, 80 cases will be detected. Of the cancers that are identified, 68 (85%) will be seen with mammography, while 12 will be detected in clinical breast exams. Yet 20 will go unobserved. Mammography screening is good for detecting cancers that grow moderately quickly, but it misses very fast-growing cancers.

"Mammography is excellent and beneficial, but we need to continue to work at getting better in finding breast cancers," Kopans said.

In a pilot study of 40 women at MGH (a subgroup of the 400 volunteers), breast tomosynthesis helped improve lesion visibility compared with conventional mammography. With breast tomosynthesis, radiologists were able to see masses and architectural distortion better than with conventional mammography, said Dr. Elizabeth Rafferty, an MGH radiologist.

In the pilot study, six out of seven malignant lesions identified on ultrasound that were occult on mammography were identified by tomosynthesis. At the special focus session, Rafferty presented an image of a young woman with very dense breast tissue and a palpable mass. A malignancy was not visible on the mammogram but was evident on ultrasound as well as on a breast tomosynthesis image.

A second malignancy in the same breast was occult on mammography and ultrasound but was demonstrated by tomosynthesis. The reduction of noise from overlapping tissue (using tomosynthesis) also proved to be beneficial in imaging women with fatty tissue, Rafferty said.

REDUCING RECALLS

Up to one-quarter of women recalled for follow-up after a mammogram ultimately turn out to have no abnormality. Researchers at Dartmouth Hitchcock Medical Center found that tomosynthesis could dramatically reduce the rate of false-positive mammograms.

In a study of 98 patients with abnormal mammograms in 99 breasts, tomosynthesis as a screening follow-up tool was equivalent or superior to diagnostic mammography in nearly 87% of cases, said Dr. Steven Poplack, an associate professor of diagnostic radiology at Dartmouth. There were 112 screening abnormalities in the study group. If tomosynthesis had been used as an adjunct to mammographic screening, the recall rate could have been cut by 40%, Poplack said. But he was cautiously optimistic in releasing the results, given that the study findings need to be validated in larger trials.

"On the basis of this small study, tomosynthesis is equivalent or superior [to mammography] for diagnosis. It will offer recall reduction and better reflects the reality of the breast, translating into a more accurate diagnosis," he said.

It took 20 seconds to acquire a tomosynthesis image and one minute to reconstruct the image. Two views were taken in most cases. Tomosynthesis proved superior to diagnostic mammography in 37 cases, equivalent in 49 cases, and inferior in 13 cases. Many of the cases where tomosynthesis was deemed inferior involved calcifications. In addition, quality may have been affected by the lengthy acquisition time and motion artifact, Poplack said.

Of the total number of potential abnormalities, five were cancers. Three cancers were detected by both mammography and tomosynthesis, and two cases were occult for both techniques.

POOR MAN'S MRI?

In the same session, University of Pennsylvania researchers demonstrated contrast-enhanced digital breast tomosynthesis, a new technique that exploits the different vascular characteristics of malignant and benign breast tissue. Researchers have documented these differences with gadolinium-enhanced breast MR, but drawbacks of MR for this purpose include poor specificity and high expense.

Contrast-enhanced digital mammography has also demonstrated vascular aspects of breast cancer biology, as malignant lesion conspicuity increases with intravenous contrast administration. But the technique relies on projection mammographic views, which are inherently limited as they are 2D images of a 3D anatomic structure, said Dr. Sara Chen, who presented the group's results.

The work is promising, Chen said, but superimposition of nonadjacent overlying breast tissue may have an unfavorable impact on lesion conspicuity, even with lesion contrast administration.

"If tomosynthesis and contrast-enhanced digital mammography could be combined into a single technique, the benefits of both might potentially be integrated," she said.

The group performed a pilot study, using modified commercial equipment, to test this theory. The pilot involved 13 women with breast lesions identified as BI-RADS category 4 or 5. All patients underwent multimodality breast imaging, including contrast- enhanced digital breast tomosynthesis. As researchers expected, lesions became more apparent with the combination of enhanced digital mammography and tomosynthesis.

"Our initial experience found contrast-enhanced digital breast tomosynthesis to be a clinically feasible technique that provides us with vascular information about breast lesions qualitatively concordant with that of breast MR. As such, the technique may have potential to visualize vascular characteristics of breast lesions," Chen said.

The group now plans to optimize technical factors, mainly to address problems with motion artifact. Researchers are also developing a dedicated system for contrast-enhanced digital breast tomosynthesis.