By: Janice Sung, MD, FSBI
Mammography is the main test used to find breast cancer in women who don’t have symptoms and is the only screening test that has been shown to reduce the number of women who die from breast cancer by about 30%. (A screening test is a test used to find a disease in people who don’t have signs or symptoms of the disease.) However, breast MRI is the most sensitive method of finding breast cancer, especially in women who are at a very high risk for breast cancer, like women who have a ≥20-25% chance of developing breast cancer in their life. So women at very high risk are recommended to have both a yearly mammogram and breast MRI as part of their annual breast cancer screening. However, because of its high cost and potential for false alarms which are discussed below, MRI is not generally recommended for women who are at normal or moderate risk for breast cancer.
What is the difference between a mammogram and breast MRI?
Mammography and breast MRI use different ways to find breast cancer. Mammography uses X-rays to take a picture (shadow) of the breast; cancers are usually seen as either bright dots (of calcium) or as masses that are denser or whiter than the normal breast tissue on the mammogram. However, because both cancers and dense breast tissue are white on a mammogram, some cancers will be hidden, especially in women who have dense breasts.
During a breast MRI, a contrast dye (gadolinium) is injected into a vein (also called IV) and magnetic fields and radiowaves are used to make 3D-images of the breast. Unlike a mammogram, there is no radiation. Breast cancers usually have increased blood vessels feeding the cancer and therefore will look brighter than the normal breast tissue. Because cancers are not usually hidden by normal breast tissue on MRI, many research studies have shown that breast MRI will find more breast cancers compared to a mammogram. For example, screening mammography finds about 5 cancers per 1000 women screened. If a MRI is done in women with a normal mammogram, an additional 10-15 cancers will be detected on the MRI. Without a doubt, breast MRI is the most sensitive test that we have to detect breast cancer.
Problems with breast MRI
Even though it finds many more breast cancers than regular mammograms, the use of screening MRI has been limited to a small group of women at very high risk for breast cancer because of its cost. A breast MRI can cost several thousands of dollars, compared to several hundred dollars or less for a mammogram. The major national organizations like the American Cancer Society, the National Comprehensive Cancer Network and the Society of Breast Imaging, among others, recommend extra screening with MRI for women who have a ≥20-25% (high) lifetime risk of getting breast cancer based on their family history. Most insurance companies follow these national guidelines and will only cover the cost of the MRI in women who are at this high level of risk. Talk to your doctor if you have a strong family history of breast cancer or if you think you may have a breast cancer gene.
All screening tests have false alarms, and breast MRI is no exception. False positives (false alarms) are when the screening test is abnormal, but there is no real problem. A false alarm may lead to an unnecessary biopsy. On a breast MRI there are many benign conditions that may look like cancer. Therefore in order to be sure the abnormality is not cancer, a needle biopsy may be recommended. Like mammography, about 60-75% of biopsies recommended from a breast MRI turn out to be benign and not cancer.
Recently, deposits of gadolinium (the contrast injected for breast MRI) have been discovered in areas of the brain in people who have had many MRI exams. The consequences of this are unknown. Gadolinium based contrast agents were approved by the FDA in 1988. After almost 30 years of use and more than 400 million doses of gadolinium, there has been no evidence of problems resulting from gadolinium injections.
Potential Expanded Role of Screening Breast MRI
Abbreviated (or Fast) MRI
Abbreviated (or fast) breast MRI (AB-MR) is a variation of breast MRI that could change the way we look for breast cancer in women. A regular breast MRI takes approximately 30-45 minutes to complete, adding to its high cost. In a fast MRI, the MRI study is shortened and only uses the most important images, so that the scan takes less than 10 minutes to complete. Early research has found that fast MRI is similar in finding breast cancer to a regular breast MRI. By reducing the time of the exam, the cost can be lowered from several thousand dollars to a few hundred. In addition, because the exams take less time, more openings on the MRI schedule would become available and allow for more women to be screened using MRI.
Dense breast screening
Because mammography is less accurate in women with dense breasts, 27 states have passed legislation that require women be notified of their breast density and to be made aware that they may benefit from added (supplemental) screening tests. Ultrasound is the most common test added to mammography for breast cancer screening. AB-MR may be a better option for added screening in women with dense breasts, detecting more cancers with fewer false alarms. In one study, an additional 11 cancers were found in 443 women after a negative screening mammogram. All 11 cancers were found in women who had also had a negative screening breast ultrasound. A NCI-approved research trial comparing AB-MR to digital breast tomosynthesis (3D mammogram) is now open for enrollment (https://www.cancer.gov/about-cancer/treatment/clinical-trials/search/view?cdrid=785101&version=HealthProfessional&protocolsearchid=6199578).
Breast MRI is the most powerful tool we now have for breast cancer screening. However, because of its high cost, risk of false alarms and limited availability, breast MRI is generally recommended only for women who are at high risk for breast cancer. Talk to your doctor to learn more about your risk for breast cancer and see if you may be suitable for breast MRI screening.
Dr. Sung is a Radiologist at Memorial Sloan Kettering Cancer Center.