DEAR MAYO CLINIC: Why is a mammogram the standard screening tool used to look for breast cancer? Wouldn't MRI catch the disease earlier?

ANSWER: For most women, mammograms work well as a screening test for breast cancer. Magnetic resonance imaging of the breast, or breast MRI, can be useful in some cases along with a mammogram. But MRI is not intended to be used in place of a mammogram. Although it is a sensitive test, breast MRI may miss some breast cancers that mammography can detect.

A mammogram is an X-ray image of the breast. Most mammograms are digital images that appear on a computer screen. Breast MRI is an imaging technique that captures multiple images of the breast. Those images are combined using a computer to generate detailed pictures. Both tests can reveal tumors and other breast abnormalities.

Mammography remains the recommended first test in breast cancer screening for several reasons. First, mammography is widely available. Most women have access to a clinic or hospital that offers mammograms. Second, the test is well-established and standardized among health care facilities. That means the way mammography is done is very similar from one place to another throughout the United States.

Breast MRI, on the other hand, is a newer technology, and is not as widely available as mammography. In addition, breast MRI is not well standardized. Large differences exist in the way the procedure is performed, as well as in the kind of machine and other equipment used. So the same test is not necessarily going to provide the same results from one place to another. Breast MRI studies also are much more expensive than mammograms. Finally, there are some abnormalities that can be found using a mammogram that may not be evident on MRI, such as calcium deposits in breast ducts and other tissues. These calcifications may be associated with cancer in some cases.

Having breast MRI after a mammogram can be a useful way to screen for breast cancer in certain situations. For example, women who are at high risk for breast cancer due to a family history of the disease or other factors may benefit from both a mammogram and MRI.

Breast MRI also may be recommended for women who have very dense breasts. Dense breast tissue can make it harder for abnormalities to be found on a mammogram because dense tissue can look similar to an abnormality on the mammogram image. In those cases, breast MRI may be able to detect problems that could go unnoticed on a mammogram.

No matter what the individual situation, all women should be screened for breast cancer on a regular basis. Not all organizations agree on how often that needs to happen. For instance, the U.S. Preventive Services Task Force mammogram guidelines recommend women begin breast cancer screening at age 50 and then have a mammogram every two years after that. The American Cancer Society and other organizations recommend screening begin at 40 and continue annually.

At Mayo Clinic, doctors offer mammograms to women annually beginning at age 40. When to begin mammogram screening and how often to repeat it is a personal decision that should be based on a woman's needs and preferences. Talk to your doctor to discuss the benefits, risks and limitations of mammograms and decide together what is best for you.

If you do not have health insurance, or if your insurance does not cover mammograms, check with your state's department of health. Most states offer programs that can help women get access to this important cancer screening tool. -- Stephanie Hines, M.D., Breast Clinic, Mayo Clinic, Jacksonville, Fla.

(Medical Edge from Mayo Clinic is an educational resource and doesn't replace regular medical care. To submit a question, write to: medicaledge@mayo.edu. For health information, visit http://www.mayoclinic.com.)

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