Breast cancer screening is a set of tests used to find breast cancer early, before symptoms appear. The most common screening tool is a mammogram, a low-dose X-ray of the breast. Current guidelines from the U.S. Preventive Services Task Force recommend that all women get a screening mammogram every two years starting at age 40 and continuing through age 74.
How a Mammogram Works
A mammogram uses X-rays to create images of the internal structures of your breast. Different tissues absorb X-rays differently: fatty tissue appears gray, while denser fibrous and glandular tissue appears lighter. Mineral deposits like calcifications show up bright white, which is one way radiologists spot abnormal growths.
During the exam, your breast is placed on a flat detector plate and compressed with a paddle from above. The compression spreads the tissue so overlapping structures don’t hide potential problems. Two images are taken of each breast from different angles. The whole process typically takes about 20 minutes, though the actual compression lasts only seconds per image.
A newer version called 3D mammography (digital breast tomosynthesis) takes multiple X-ray images along an arc and reconstructs them into thin slices, similar to a CT scan. This lets radiologists examine breast tissue layer by layer instead of looking at everything flattened into a single image. In women with a family history of breast cancer, 3D mammography reduces the number of false alarms compared to standard 2D imaging and catches a higher proportion of early-stage cancers with favorable outcomes.
Screening vs. Diagnostic Mammograms
A screening mammogram is what you get when you have no symptoms. It’s routine, uses two standard views per breast, and you can often schedule one directly without a referral. A diagnostic mammogram is ordered when something has already been found, whether that’s a lump you noticed, an unusual result on a screening mammogram, or another concerning change. Diagnostic mammograms take images from more angles to get a closer look at the area in question, which means they involve a slightly higher radiation dose and take longer.
How Effective Is Screening?
A large UK trial found that women offered regular screening mammograms starting at age 40 were 25% less likely to die from breast cancer within 10 years compared to those who weren’t screened. Over a longer follow-up period of 23 years, that benefit narrowed to about a 12% reduction, partly because cancers caught later in life were included in the overall numbers. The core benefit is straightforward: mammograms find tumors when they’re smaller and easier to treat, before they’ve had a chance to spread.
What a Callback Means
About 10% of screening mammograms lead to a callback for additional testing. That sounds alarming, but the vast majority of these turn out to be nothing. In one analysis of 3.5 million screening mammograms, roughly 345,000 resulted in false positives. A callback usually means the radiologist saw something on the image that needs a closer look, not that cancer was found.
If you’re called back, you’ll typically get a diagnostic mammogram, an ultrasound, or both. Most women are cleared after this extra imaging. A small percentage will need a biopsy, where a tiny sample of tissue is removed and examined under a microscope.
Understanding Your Results
Mammogram results are reported using a standardized scoring system called BI-RADS, with categories from 0 to 6. Here’s what each one means in practical terms:
- Category 0: The images were incomplete. You’ll need to come back for additional views or bring in prior mammograms for comparison.
- Category 1: Completely normal. No abnormalities found.
- Category 2: Benign findings. The radiologist may have noted something like a cyst or calcification, but it’s clearly not cancer. No follow-up needed beyond your regular screening schedule.
- Category 3: Probably benign, with less than a 2% chance of cancer. A follow-up mammogram in six months is typically recommended to make sure nothing changes.
- Category 4: Suspicious. The likelihood of cancer ranges from 2% to just under 95% depending on what was seen. A biopsy is recommended.
- Category 5: Highly suggestive of cancer, with a 95% or greater likelihood. A biopsy is strongly recommended.
- Category 6: Cancer has already been confirmed by a previous biopsy. This category is used when mammograms are tracking how a known cancer responds to treatment.
Categories 1 and 2 are the most common results. If you receive either, nothing further needs to happen until your next routine screening.
Dense Breast Tissue and Supplemental Screening
More than 40% of women between ages 40 and 74 have dense breast tissue, meaning they have more fibrous and glandular tissue relative to fatty tissue. Density is especially common in younger women. This matters for two reasons: dense tissue appears white on a mammogram, which can mask tumors that also appear white, and women with extremely dense breasts have more than four times the breast cancer risk of women with mostly fatty breasts.
Because density affects both risk and the accuracy of mammograms, most states now require that your mammogram report tell you about your breast density. As of 2019, 38 states and Washington, D.C. had passed notification laws, and federal legislation followed. If you’re told you have dense breasts, your doctor may recommend supplemental screening with breast ultrasound or MRI to catch cancers that mammography alone could miss.
Who Needs More Than a Mammogram
Women at high risk for breast cancer are recommended to get an annual breast MRI in addition to mammography. High risk includes carrying a BRCA gene mutation (or being an untested first-degree relative of someone who does), having a genetic syndrome like Li-Fraumeni that predisposes to cancer, having received radiation therapy to the chest between ages 10 and 30, or having a lifetime breast cancer risk of 20% or higher based on formal risk assessment tools. If any of these apply to you, talk to your doctor about adding MRI to your screening plan.
How to Prepare for Your Appointment
Skip deodorant, antiperspirant, powders, lotions, and perfumes on your underarms and breasts on the day of your mammogram. Metallic particles in these products can show up on the images and be mistaken for calcifications. If you menstruate, scheduling your appointment for the week after your period can help, since your breasts tend to be less tender at that point in your cycle.
If you’re going to a new facility, request that your previous mammograms be transferred on a disc and bring it with you. The radiologist compares your current images to older ones to spot subtle changes that might not look concerning on a single exam but stand out when viewed side by side.