TOMOSYNTHESIS FOR BREAST IMAGING

Tomosynthesis Enters the Breast-Imaging Field

More than a decade has passed since the advent of the first digital mammograms, time enough for more than 70% of US mammographers to adopt this technology. Practitioners maintain that digital imaging makes it easier to see signs of cancer earlier. Even so, approximately 10% of women who are screened with full-field digital mammography are recalled for more tests, according to practitioner reports.
A new device, recently approved by the US Food and Drug Administration (FDA), may improve cancer detection while reducing the number of patient recalls.
Selenia Dimensions® from Hologic® (Bedford, Massachusetts) is like no other breast-imaging platform on the market. It takes 15 successive images, each at a slightly different angle along an arc across the breast. The concept is simple: What is hidden behind fibroglandular tissue in one image might be visible in another if the angle is slightly different (Figure).

Figure. (a) A suspicious lesion seen on standard 2D digital mammography (far left). (b) After examining multiple slices generated using breast tomosynthesis (5 images), the lesion seen on 2D (far left) is determined to be a false positive. (Images courtesy of Hologic. Used with permission.)
"You can see cancer with tomosynthesis that you could not see with routine mammography," said Phil Evans, MD, Director of the University of Texas Southwestern Center for Breast Care in Dallas.
FDA reviewers considered data gathered by Dr. Evans in their early February approval of Hologic's new device. Although the overall value of tomosynthesis has been established, exactly where it fits in the arena of women's health has yet to be determined, Dr. Evans said.
"We know it to be useful for screening, particularly for women with dense breasts, but exactly how it fits into the routine protocols still has to be worked out," he said. Mary Hayes, MD, who is Medical Director of Women's Imaging at Memorial Healthcare in Hollywood, Florida, says the device has proved to be beneficial in evaluating women with dense or fatty breasts.
"When we implement tomosynthesis in our practice, we will implement it across the board for fatty- and dense-breast patients," said Dr. Hayes, who until now has used the system only within a research protocol. "Our goal is to roll it out for routine use initially in the screening population and then in the diagnostic setting on a case-by-case basis."

Will They Buy It?

Currently, Hologic is the only vendor with a breast tomosynthesis device approved by the FDA. However, other vendors, including General Electric and Siemens, already have similar devices in development. At this writing, Hologic has not yet set a price, but the system will almost certainly cost more than the unit configured with only 2D capabilities.
Will price prove a disincentive? Providers will be asked to invest in breast tomosynthesis just a few years after making an investment of $300,000 or more in its 2D counterpart, according to MD Buyline, a Dallas-based business-intelligence firm specializing in medical devices. Still, Rachael Bennett, a clinical analyst at the firm, expects that many will sign on.
"I have been practically bombarded with people wanting this technology," said Bennett, who focuses on mammography and oncology. "They want to know how much it costs and when they can get it."
In addition to the tomosynthesis unit, providers will also need a specially equipped workstation to view the data. Product rollout is scheduled for this spring.

Pros and Cons of Tomosynthesis

Tomosynthesis examinations take a few seconds longer than standard 2D examinations. As with standard mammography, the breast is compressed between a paddle and the detector housing. Instead of 2 views of the breast, 15 are taken across a 15° arc. During this scan, conventional 2D images are also taken.
The volumetric data are reconstructed into slices, similar in appearance to those created using CT. One breast may give rise to 60 or more slices. These slices, along with the 2D images, are transmitted for study to a diagnostic workstation.

More Accuracy, More Data, More Radiation

Although examination time will not be significantly longer, reading time will. Radiologists must, in essence, interpret 2 examinations -- one comprising dozens of image slices and the other comprising standard 2D images. Physicians will not be compensated more for their extra effort because there is no additional reimbursement for breast tomosynthesis, at least not yet. But that does not bother Dr. Evans.
"The main thing we try to do is find cancer and find it early," he said.
Preliminary evidence suggests that tomosynthesis may be able to detect cancer earlier and more accurately than standard mammography. In 2 studies cited by FDA reviewers, the ability of radiologists to distinguish between cancerous and benign lesions improved 5%-12% using a combination of 2D and 3D images as compared with 2D images alone.^[1] In addition, study results presented to the FDA indicate that the combination of 2D and 3D images is associated with a 30% reduction in patient recalls.^[1,2]
To gain the potential advantage offered by tomosynthesis, patients are exposed to about twice the radiation of a typical digital mammogram, according to the FDA. However, the increased risk for cancer as a result of the added radiation is still very small, a little more than 1% compared with the natural cancer incidence, and < 1% more than the risk associated with conventional 2D mammography.^[1]
"The study results suggest that the potential benefit outweighs the risk," said FDA spokesperson Erica Jefferson.
To put the tomo radiation dose in perspective, patients typically absorb about 0.5 mSv during 2D mammography, said Dr. Hayes, citing numbers calculated by Hologic and presented to the FDA. An exam combining 2D and tomosynthesis images exposes patients to about 1 mSv. Americans are annually exposed to about 3.0 mSv from background radiation (or 4 mSv if they live in the mile-high city of Denver).
"I have no reservations about tomosynthesis for myself, for my mother, for my sister, or whomever," Dr. Hayes said.

Interpreting Images Without a Safety Net

To reap the benefits of tomosynthesis, physicians must learn to interpret this new type of breast image. As a condition for approving Selenia Dimensions, the FDA required Hologic to develop an 8-hour training course in the new technique. Mammographers who are already experienced in reading standard 2D digital images should be proficient in this new modality after reading about 100 tomosynthesis cases, according to Dr. Evans.
Proficiency at tomosynthesis is important because mammographers will be reading images without the computer safety net that they have come to depend on when reading 2D images. Today, the vast majority of facilities making digital mammograms use some kind of computer-aided detection (CAD) software to highlight suspicious lesions. These CAD analyses, called "second readings" because they are done after the physician has examined the images, direct the physician back to suspicious lesions that may have initially escaped their detection.
No such back-up systems exist for tomo; the CAD algorithms that work in 2 dimensions do not work in 3 dimensions. Hologic and other developers of tomosynthesis are currently working on new algorithms.
Once algorithms are available, workstations will need the computing horsepower to run them. More than likely, this will not be an issue. Hologic has already equipped its SecurView™ workstation with special software and hardware to run tomosynthesis studies. It is also helping vendors of picture archiving and communication systems (PACS) and workstations to modify their systems to display tomosynthesis images. In addition, the company claims that it is close to having a universal digital imaging and communication in medicine (DICOM) standard for tomosynthesis.

Where Does Tomo Fit in Breast Evaluation?

Hologic's pioneering reach into the third dimension signals an advance for the field of mammography, but how much of an advance is yet to be seen. The need to interpret 3D and 2D images together raises the question of how to best evaluate the breast for disease and what role tomosynthesis should play amid an array of mammography improvements and emerging technologies.