A mammogram uses low-dose X-rays to create detailed images of breast tissue, allowing radiologists to spot cancers, cysts, and calcifications that can’t be felt during a physical exam. The X-ray beam passes through the breast, and different types of tissue (fat, fibrous tissue, tumors) absorb different amounts of radiation. A digital detector on the other side captures what comes through, producing an image where dense structures appear white and fatty tissue appears dark.
What Happens During the Exam
You’ll stand in front of the mammography machine while a technologist positions one breast at a time on a flat platform. A clear plastic plate then presses down firmly to compress the breast. For a standard screening, the machine takes two X-ray images of each breast: one from top to bottom and one from side to side. The entire process typically takes about 20 minutes, though the actual X-ray exposure lasts only a few seconds per image.
The compression is the part most people notice. It can feel uncomfortable or even painful, but it serves several important purposes. Flattening the breast spreads the tissue out so structures don’t overlap and hide potential abnormalities. Thinner tissue also means less radiation is needed to get a clear picture, and it reduces blurring from movement. The difference between a readable mammogram and an unclear one often comes down to how well the breast was compressed.
2D vs. 3D Mammography
A conventional (2D) mammogram produces a single flat image of each view. Because all the tissue is compressed into one layer, structures can overlap and obscure small abnormalities. This is especially problematic in dense breasts, where both normal tissue and tumors show up white on the image.
A 3D mammogram, called digital breast tomosynthesis, works differently. The X-ray tube moves in an arc over the compressed breast, capturing many thin images from multiple angles. Software then reconstructs these into individual slices, similar to flipping through pages of a book. This dramatically reduces the overlap problem, making it easier to distinguish a real abnormality from a harmless crossing of normal tissue. The experience from the patient’s side is nearly identical to a 2D mammogram, though the machine may take a few extra seconds per view.
Why Deodorant Causes Problems
Screening centers ask you to skip deodorant, antiperspirant, powder, and lotion on the day of your appointment. The reason is surprisingly specific: aluminum-based compounds in antiperspirants create tiny bright spots on the X-ray image that look almost identical to microcalcifications, which are one of the earliest signs radiologists look for when screening for cancer. Solid stick antiperspirants are the worst offenders, producing particles that most closely mimic calcifications. Roll-on formulas cause fewer artifacts, but the safest approach is to avoid all of these products before your appointment to prevent unnecessary follow-up testing.
How Radiologists Read the Results
After the images are taken, a radiologist examines them and assigns a standardized score from 0 to 6. Understanding these categories can save you a lot of anxiety when your results letter arrives.
- Category 0: The images are incomplete. The radiologist spotted something that needs a closer look, so you’ll be called back for additional imaging like a diagnostic mammogram or ultrasound. This doesn’t mean cancer was found.
- Category 1: Negative. No significant abnormalities were seen.
- Category 2: A benign finding, such as a cyst or lymph node. It’s noted for future comparison but isn’t a concern.
- Category 3: Probably benign, with a greater than 98% chance of being noncancerous. You’ll typically have a follow-up imaging appointment in six months to confirm nothing has changed.
- Category 4: Suspicious. The finding has features that could indicate cancer (anywhere from a 2% to 95% likelihood), and a biopsy will be recommended.
- Category 5: Highly suggestive of cancer, with at least a 95% chance of malignancy. A biopsy is the next step.
- Category 6: Used only when cancer has already been confirmed by a previous biopsy. Imaging at this stage helps track the extent of disease or monitor treatment response.
The Challenge of Dense Breasts
About half of women who get mammograms have dense breast tissue, and density has a direct impact on how well the test works. Dense tissue and tumors both absorb X-rays and appear white on the image, which means a cancer can essentially hide in plain sight. Mammography is measurably less sensitive in women with dense breasts, meaning it’s more likely to miss a cancer that’s actually there. Fatty breast tissue, by contrast, appears dark on the image, so any white abnormality stands out clearly.
If you’re told you have dense breasts, your doctor may recommend supplemental screening with ultrasound or MRI. Many states now require that mammography facilities notify patients about their breast density for exactly this reason.
Radiation Exposure in Context
A standard screening mammogram (two views of each breast) delivers about 0.4 millisieverts of radiation. To put that in perspective, you absorb roughly 3 millisieverts per year just from natural background sources like the sun, soil, and building materials. The dose from a single mammogram is equivalent to about seven weeks of everyday background exposure. The amount is small enough that the benefit of detecting cancer early substantially outweighs the minimal radiation risk, which is why screening guidelines support regular mammography over decades.
Current Screening Recommendations
The U.S. Preventive Services Task Force recommends mammograms every two years starting at age 40 and continuing through age 74 for people at average risk of breast cancer. This applies to cisgender women and all other people assigned female at birth, including transgender men and nonbinary individuals. If you have a family history of breast cancer, a known genetic mutation, or other risk factors, your doctor may recommend starting earlier or screening more frequently.