A mammogram report arrives with a number attached and almost no explanation of what the number means. People then look it up, find percentages out of context, and spend a week assuming the worst about a result that is, in most cases, routine.

BI-RADS is a shorthand written by radiologists for other clinicians. It is not designed to be read cold by the person it describes. Here is what each category actually means and, more usefully, what happens next.

The short answer

BI-RADS runs from 0 to 6. Category 0 means the study is incomplete and more imaging is needed, which is common and not alarming. Categories 1 and 2 are both normal results. Category 3 means probably benign, with a 2% or lower chance of cancer, and leads to short-interval follow-up. Category 4 means suspicious enough to biopsy, with subcategories spanning a wide range from just over 2% to under 95%. Category 5 is highly suggestive of cancer at 95% or above. Category 6 is used only when cancer has already been proven by biopsy.

What is BI-RADS and why does it exist?

BI-RADS stands for Breast Imaging Reporting and Data System. The American College of Radiology introduced it in 1993 to solve a specific problem: radiologists were describing findings in inconsistent prose, which made reports hard to act on and hard to compare across facilities and over time.

The system standardizes two things: the vocabulary used to describe findings, and a final assessment category that maps to a specific management recommendation. That second part is the useful bit for patients. Each category carries a defined next step, so the number is really an instruction rather than a diagnosis.

BI-RADS 0: incomplete, not abnormal

This is the category that causes the most unnecessary alarm, because it usually arrives with a phone call asking you to come back.

Category 0 means the radiologist cannot yet assign a final assessment and needs more information. The American Cancer Society describes it as additional imaging evaluation or comparison to prior mammograms being needed. In practice that means extra mammogram views such as spot compression or magnification, an ultrasound, or simply retrieving your images from a previous facility.

Two things worth knowing:

  • It is especially common on a first mammogram, because there is nothing to compare against. Overlapping normal tissue can look like a finding on a single view and resolve entirely on additional views.
  • Most callbacks do not end in a cancer diagnosis. The great majority resolve to a category 1 or 2 after the additional imaging. mammogram.md covers what to expect at a callback appointment here.

One practical warning: the follow-up imaging is usually billed as diagnostic rather than screening, which means it may not carry the same no-cost preventive coverage your screening mammogram did. That billing distinction catches people out across all types of screening, as colonoscopy.md explains in the context of colon screening. It is worth asking before the appointment.

BI-RADS 1 and 2: the two normal results

Both mean your mammogram is normal. The difference is only whether the radiologist found something worth describing.

Category 1, negative. Nothing new or abnormal was found. No masses, no suspicious calcifications, no architectural distortion. Return to routine screening.

Category 2, benign. Something was seen and identified as definitively not cancer. Typical examples include simple cysts, calcified fibroadenomas, fat-containing lesions such as lipomas or hamartomas, intramammary lymph nodes, secretory calcifications, and implants. Return to routine screening.

Category 2 exists mainly so the finding is on record. If a future radiologist sees the same stable cyst, the documentation prevents an unnecessary workup. Getting a 2 rather than a 1 does not mean your result was worse.

BI-RADS 3: probably benign

This is where the numbers become genuinely useful, because "probably benign" sounds far more ambiguous than it is.

Category 3 is defined as a finding with no more than a 2% chance of being cancer. Put the other way, at least 98 out of 100 findings in this category are not cancer.

The management is short-interval follow-up rather than biopsy: repeat imaging at around 6 months, then periodically until the finding has been stable for roughly two years. The logic is that cancers grow and benign findings generally do not, so stability over time is itself diagnostic information, obtained without a needle.

The one thing that undermines this category is not showing up. A BI-RADS 3 without the follow-up imaging is an unfinished assessment. Book the 6-month appointment before you leave.

BI-RADS 4: suspicious, and its three subcategories

Category 4 means a biopsy should be considered. It also spans an enormous range of risk, which is exactly why the subcategories exist and why you should look for the letter alongside the number.

  • 4A: more than 2% but no more than 10% likelihood of cancer
  • 4B: more than 10% but no more than 50%
  • 4C: more than 50% but less than 95%

All three lead to the same recommendation, a biopsy, but they describe very different situations. A 4A finding is still around nine times more likely to be benign than malignant. Reading "suspicious" and assuming a diagnosis is the most common misreading of the whole system.

The biopsy itself is typically an image-guided needle procedure done under local anesthetic, not surgery. Results usually take a few days. If your report says category 4 without a letter, it is reasonable to ask the radiology practice which subcategory applies, since it materially changes what you should expect.

BI-RADS 5 and 6

Category 5, highly suggestive of malignancy. At least a 95% likelihood of cancer based on imaging appearance. Biopsy is strongly recommended and the process typically moves quickly, sometimes with surgical and oncology consultations arranged in parallel.

Even here, the distinction between imaging and diagnosis matters. Cancer is diagnosed by pathology, not by a radiologist's assessment, and a small share of category 5 findings return benign. The category signals urgency, not certainty.

Category 6, known biopsy-proven malignancy. This one is not a screening result at all. It is applied to imaging performed on a cancer that has already been confirmed by biopsy, usually to monitor response to treatment before surgery. Nobody receives a category 6 as news.

What the density category means

Your report also contains a separate breast density assessment, graded a through d, running from almost entirely fatty to extremely dense. This is not a risk score and it is not a finding. It describes the composition of your tissue.

Since September 10, 2024, the FDA's amended Mammography Quality Standards Act regulations require every mammography report to include a density assessment, and every patient summary to include one of two federal notification statements telling you whether your tissue is dense or not dense. Roughly half of women aged 40 and over have dense breasts, so receiving that notification is unremarkable.

Density matters for two reasons. Dense tissue appears white on a mammogram, and so do many cancers, which reduces sensitivity. And dense tissue is itself associated with somewhat higher breast cancer risk. Together those may prompt a conversation about supplemental imaging such as ultrasound or MRI, depending on your other risk factors. mammogram.md covers dense breasts and supplemental screening options in detail.

What to do with your report

A short checklist that covers most situations:

  • Find the final assessment category. It is usually near the end, phrased as "ASSESSMENT: BI-RADS Category N." Descriptive language earlier in the report can sound alarming without changing the category.
  • Look for the subcategory letter if you have a category 4. Ask if it is missing.
  • Note the recommendation. Every category carries one, and it is the part that determines what you do next.
  • Book the follow-up before leaving. This applies particularly to category 0 and category 3, where the assessment is only completed by the next study.
  • Bring prior images to a new facility. Comparison images prevent a meaningful number of unnecessary callbacks.
  • Ask about billing if you are being called back, since diagnostic imaging is covered differently from screening.
  • Report new symptoms separately. A normal mammogram does not override a new lump, skin change, or nipple discharge. Those warrant clinical evaluation regardless of the BI-RADS category.

If you are not yet in a regular screening routine, this guide covers current screening age guidance and how to think about when to start.