2D vs 3D Mammograms: What Is the Difference?

This page is for anyone choosing between a 2D and a 3D mammogram. Many people now face that choice at screening time, often on a sign-up form. Both are X-ray tests of the breast, done on similar machines. This page covers how each test works, what the evidence shows, and what to ask before you book.

A standard digital mammogram is two-dimensional. It captures X-ray images of the breast. Those are stored and read on a computer as flat pictures.

3D mammography has a formal name: digital breast tomosynthesis, or DBT. NCI explains that the DBT machine takes multiple pictures of thin slices across the breast. Computer software then assembles them into a 3D picture. Instead of one flat image, the radiologist can page through the breast layer by layer.

Either technology can be used two ways. A screening mammogram looks for cancer before symptoms. A diagnostic mammogram investigates a lump or other change. NCI notes that diagnostic mammography takes images from more angles, so its radiation dose is higher.

NCI's mammogram fact sheet states that DBT combined with standard mammography is better at finding tumors than standard mammography alone. That is a real advantage. It is the main reason 3D machines have spread.

Finding more tumors is not automatically the same as saving more lives, though. The same NCI fact sheet notes that it is not yet known whether DBT cuts breast cancer deaths more than standard mammography does. NCI describes a possible advantage of fewer false positives, and TMIST is also measuring false-positive rates.

There are also trade-offs to weigh. NCI noted several when the federal TMIST study launched:

• The tomosynthesis radiation dose can be more than two times the dose of standard digital mammography.
• The machines cost more and take longer to use.
• Some women who ask for tomosynthesis do not get it, because insurance will not cover it.

Equipment and coverage have kept changing since then. Ask your own facility and insurer where things stand.

TMIST is the Tomosynthesis Mammographic Imaging Screening Trial. It is an NCI-sponsored study comparing the two technologies head to head. It set out to enroll about 165,000 women ages 45 to 74, at about 100 sites in the United States and Canada.

Its design answers a practical problem. Proving an effect on deaths would take 10 to 20 years. So TMIST instead measures advanced, life-threatening breast cancers over 5 years. If 3D screening leads to fewer of those, that would suggest it prevents deaths. Until results are in, the honest summary is this: 3D finds more, and the size of its life-saving benefit is still being measured.

Density matters for many people here. NCI reports that nearly half of women 40 and older who have mammograms have dense breasts. That means more glandular and fibrous tissue, and less fat. Dense tissue appears white on a mammogram, and so do tumors. That makes mammography more likely to miss cancer in dense breasts. Density also raises breast cancer risk on its own.

FDA updated its mammography rules, and facilities must now tell you if your mammogram shows dense breasts. So your report will tell you if your breasts are dense. What to do with that is individual. The USPSTF has said there is not yet enough evidence to recommend for or against extra imaging, such as ultrasound or MRI, for dense breasts. Our pages on dense breast notification and supplemental screening walk through that conversation.

Whichever test you have, the facility operates under the same federal law. FDA's Mammography Quality Standards Act program requires every U.S. mammography facility to be certified through an approved accreditation body. Each must also be inspected at least once a year. Reviews cover equipment, personnel, clinical image quality, and practices.

The practical differences often come down to logistics. Which machines does your facility have? What does your insurance cover? Does your breast density or history change which test makes sense? Bring those questions to the scheduler and your care team. Whichever test you have, the bigger point stands: keep screening on the schedule your team recommends. See mammograms and when to get a mammogram for the basics.

• Mammograms — National Cancer Institute
• Dense Breasts: Answers to Commonly Asked Questions — National Cancer Institute
• TMIST: Putting Advanced Mammography Technology to the Test — National Cancer Institute
• Mammography Quality Standards Act and Program — U.S. Food and Drug Administration