BI-RADS Categories

April 10, 2025

This page was reviewed under our medical and editorial policy by Leah Naghi, M.D., assistant clinical professor, and Susan Yost, Ph.D., staff scientist, Department of Medical Oncology & Therapeutics Research, City of Hope^® Cancer Center Duarte

Women who get a mammogram may see a BI-RADS score listed in their mammography report and wonder what it means. Doctors use the BI-RADS system to help calculate breast cancer risk scores. The system includes categories that estimate how likely a finding is to be cancerous, ranging from no risk at all to a high likelihood of cancer.

BI-RADS stands for Breast Imaging Reporting and Data System, a classification system created by the American College of Radiology. It’s designed to standardize mammogram reports and make it easier for nonradiologists to understand the findings.

How Are BI-RADS Categories Assigned?

BI-RADS categories are assigned based on specific details found during a breast imaging test like a mammogram. The process starts with the reason for the exam, whether it is a routine breast screening, a diagnostic test for a specific issue or a follow-up test after a previous abnormal finding. The patient’s medical history is also noted, as it may influence the assessment.

Next, the radiologist looks at the overall composition and density of the breast tissue. This sometimes gets a separate grading.

The main part of the BI-RADS classification involves describing any findings seen on the mammogram. There are several categories of findings, including:

• Masses
• Asymmetries
• Architectural distortions
• Calcifications

Each of these categories has subcategories that further describe what is seen. For example, a mass might be oval, round or irregular in shape, and its edges (margins) might be smooth or jagged. These characteristics help determine how suspicious the finding is for cancer.

Asymmetries are areas where the tissue does not look the same on both sides of the breast, and these may range from mild to more concerning. Architectural distortion happens when the normal structure of the breast tissue looks disrupted, which might be a sign of breast cancer or other conditions like scar tissue.

Calcifications, or small deposits of calcium, are also examined closely because their shape and pattern may indicate if they are benign or suspicious.

Finally, the radiologist reviews all the findings and assigns a BI-RADS category, ranging from 0 (incomplete, more tests needed) to 6 (known cancer, often from a previous biopsy). These categories help guide next steps, whether it is continuing with routine screening or moving on to further testing or treatment.

What Each BI-RADS Score May Mean

Each BI-RADS category has a different meaning and comes with a different recommended follow-up plan to assist the care team. Category 0 means that the testing was incomplete and needs to be finished before an accurate score is able to be assigned.

BI-RADS 1

This is a negative, normal test result. It is given when the breast appears symmetrical and no lumps, abnormal structures or calcifications are noted. Patients with a BI-RADS 1 score should continue with their regular screening mammograms.

BI-RADS 2

A BI-RADS 2 score also means that there are no signs pointing to cancer; however, the radiologist noted one or more noncancerous findings. Examples include changes from a prior biopsy, an unusual lymph node or a benign calcification. These patients should also continue with their regular screening mammograms.

BI-RADS 3

Radiologists use this category to report abnormal mammogram findings with a very low chance of cancer (2% or less). Patients with these findings should be watched closely to make sure the questionable area does not change over time. Usually, this means follow-up imaging every six to 12 months for at least two years.

BI-RADS 4

This category means that a suspicious finding has been seen. The radiologist has noted an area that does not definitely look like cancer but still could be. The BI-RADS 4 category is often divided into subcategories based on how likely the finding is to be cancer, as listed below.

4A: More than 2% up to 10% likelihood.

4B: More than 10% up to 50% likelihood.

4C: More than 50% but less than 95%.

Radiologists suggest that patients in this category have a breast biopsy to test cells from the questionable area.

BI-RADS 5

A finding in this category means an area in the breast is highly suspicious for cancer. Typically, this means there is at least a 95% chance that the finding is cancer. Patients in this category are strongly urged to have a biopsy. This will allow doctors to take a sample of cells from the abnormal area, view them under a microscope and provide a diagnosis.

BI-RADS 6

The BI-RADS 6 category is only used when the patient already has known, biopsy-proven cancer. The patient may be having a mammogram to see how well they are responding to treatment. Patients in this category should continue on the breast cancer treatment plan developed with their care team.

BI-RADS and Breast Density

BI-RADS categories may also include a letter indicating breast density. Breast density refers to the amount of fibrous and glandular tissue compared to fat tissue in the breast. It is important to know that breast density is not related to breast size or firmness. The radiologist who reviews the mammogram assigns one of four categories to describe the breast density, as listed below.

Category A (almost entirely fatty): The breast is mostly made up of fat, making it easier to detect any abnormalities.

Category B (scattered areas of fibroglandular density): There are some dense areas, but most of the breast tissue is still fatty.

Category C (heterogeneously dense): More of the breast tissue is dense, which may make it harder to see small masses on a mammogram.

Category D (extremely dense): The breast is mostly dense tissue, which may obscure potential findings, making it more difficult to detect abnormalities.

Patients with Category C or D breast density are considered to have dense breast tissue, which may slightly increase the difficulty of spotting breast cancer on a mammogram. About half of the women who have mammograms in the United States are considered to have dense breasts, according to the National Cancer Institute.