A screening mammogram is a routine breast imaging test performed to detect signs of breast cancer in women who don’t have any obvious symptoms. A screening mammogram can detect early-stage cancer years before any physical symptoms appear.
Screening mammograms are performed for a different reason than diagnostic mammograms are. Diagnostic mammograms are done to find out what’s causing any type of unusual sign or symptom in the breast, such as a lump, nipple discharge, or skin change.
While the reason to have screening mammograms is clear to most women, the specifics of how and when to get tested are murkier. What’s more, important concepts like baseline mammograms and follow-up screening aren’t familiar to everyone who needs this test.
What is a baseline mammogram?
A baseline mammogram is typically a woman’s first screening mammogram. Radiologists use it as a benchmark to compare with future mammograms so they can more easily distinguish normal from abnormal areas of the breast. Studies show that comparisons with one or more prior mammograms reduce by about half the chances of call-backs for additional testing, often due to a false-positive result (a suggestion of breast cancer when none exists).
If you’re screened at the same facility each time, your radiologist should have past mammograms on record for comparison. If you go elsewhere for a mammogram later on, you can ask your former facility to send digital copies of your prior mammograms to your new facility.
False positives and overdiagnosis
About one in 10 women are called back for follow-up testing after a screening mammogram. Of the women recalled, about one in nine are diagnosed with cancer. In other words, each time a woman has a mammogram, there’s a more than 98 percent chance that it will not be cancer. More than half of all women screened annually experience at least one false positive over 10 years.
False positives are more likely with younger age, dense breasts, menopausal hormone therapy use (which can stimulate breast tissue), previous breast biopsies, no comparison to prior mammograms, a three-year interval between screenings, and a family or personal history of breast cancer. The radiologist’s training and experience also play a role in interpreting images.
False-positive results can have significant, and sometimes harmful, consequences. For instance, you might be recalled for another mammogram, which exposes you to more radiation. Or, you might be advised to undergo a procedure like a breast biopsy.
False positives cause significant emotional, physical, and sometimes even financial stress. Some women experience anxiety about breast cancer and mammography for several months after a false-positive result.
Another potential harm is “overdiagnosis.” Overdiagnosis occurs when the cancer that’s found is clinically insignificant and poses no future threat to your health. While most people assume all cancers are dangerous and will grow, this is actually not the case. Unfortunately, doctors can’t easily predict whether cancer will grow, so nearly all breast cancers detected are treated. As a result, if you’re overdiagnosed, you might undergo cancer treatment such as chemotherapy or surgery that you would never have needed.
Breast Cancer Screening Recommendations
It can be hard to know when and how often to get mammograms, since guidelines from different organizations vary. These recommendations from the U.S. Preventive Services Task Force and the American Cancer Society are both reasonable options.
Types of mammograms
Mammography is currently the best screening tool we have, but it still misses up to 20 percent of underlying breast cancers, most often in younger women and women with dense breasts. Dense breasts contain more glandular and connective tissue and are more common in younger women; breasts tend to become less dense with age. Radiologists often have difficulty distinguishing between dense tissue and tumors. Also, dense tissue can obscure a radiologist’s view and hide existing tumors.
During a mammogram, the breast is compressed between two panels while the image is taken. Typically, the technician takes two views of each breast. In the past, images were produced on film. Now, nearly all images are digital, which produces a higher-quality image than film. The images are sent directly to a computer, where the radiologist can enlarge the image for a more accurate view.
A digital mammogram uses low-dose X-rays to produce an image. The average radiation dose during a digital mammogram is the same as the amount we’re exposed to over seven weeks of natural background radiation from our surroundings.
Another type of screening mammogram is a three-dimensional (3-D) digital mammogram, called digital breast tomosynthesis (DBT). This technology isn’t currently recommended by medical societies, such as the American Cancer Society, because of a lack of evidence comparing it to standard mammography. A DBT image is taken from multiple angles and synthesized by a computer into a 3-D image of the breast. DBT modestly increases the likelihood of accurately detecting abnormalities and decreases false positives in women. However, radiation exposure from 3-D mammograms can be up to twice that of a regular digital mammogram, though that’s still within acceptable FDA limits. Newer DBT technologies, though not yet widely available, use significantly less radiation than current DBT systems.
The Affordable Care Act requires health insurers to fully cover screening mammograms for women 40 and older every one to two years. Medicare covers the cost of one screening mammogram a year. It will also pay for computer-aided detection (CAD), which uses software to analyze digital mammogram images and identify suspicious areas. CAD improves the likelihood of detecting cancer, but it also leads to more false positives and overdiagnosis.
Self-Compression During Mammograms
Allowing women to control the degree of breast compression themselves during mammograms can reduce discomfort or pain without compromising the test’s accuracy, according to a recent French study.
A standalone screening mammogram is usually sufficient for women at average risk. However, your doctor may recommend one of the following additional tests if you’re at elevated risk (for example, you have a family history of breast cancer or a BRCA gene mutation) or if findings on your mammogram require further evaluation:
- Breast ultrasound. An ultrasound uses sound waves to create images of the breasts and is most often used as a diagnostic tool after a lump is found. Some doctors recommend an ultrasound combined with a mammogram as a routine screening tool for women with dense breasts. However, ultrasound hasn’t been established as a tool for screening dense breasts and has a substantial rate of false positives.
- Breast MRI. This test uses powerful magnets, radio waves, and contrast dye to create detailed images of the breasts. MRIs may be able to pick up more cancers than mammography, but they’re more apt to produce false-positive results among women with dense breasts. An annual MRI and mammogram, each six months apart, is often recommended for high-risk women, which is determined by your history and family history. An MRI’s screening benefits haven’t been established for average-risk women, and depending on your insurance coverage, you may have to pay a deductible or co-pay.
Some new tests may be more sensitive at picking up breast cancer than mammograms. Their role in breast cancer screening is still being studied, so they’re not routinely used or covered by insurance. New technologies include molecular breast imaging, which uses a small amount of radioactive tracer that attaches to any existing breast cancer cells; fast breast MRI, which takes two to three minutes compared with a traditional MRI, which can take up to 50 minutes; and stereoscopic digital mammography, which generates two images at slightly different angles to view the internal structure of the breast directly and in depth.
Thermography: Stay Away
Some health spas, mobile screening units, and other facilities market this noninvasive test as a screening tool for breast cancer, even though there’s no evidence that it’s helpful and it hasn’t been approved for this use.
Getting the results
The length of time it takes to receive the written results of a screening mammogram can vary among facilities, but it can be as long as 30 days. If you don’t receive a letter or hear from your doctor, don’t assume that the results were normal—contact your doctor’s office. Moreover, some states require that patients be notified of their breast density along with their results, and the FDA is currently developing federal standards to require that all U.S. mammogram screening facilities report breast density in their mammography notifications.
This article first appeared in the June 2019 issue of UC Berkeley Health After 50.
Also see When to Have Mammograms: No Consensus.
Published June 27, 2019