Mammogram Interpretation: Categories and the ACR/BI-RADS

The American College of Radiology (ACR) has established the Breast Imaging Reporting and Database System (BI-RADS^TM) to guide the breast cancer diagnostic routine. Radiologists sometimes refer to each BI-RADS^TM category as a "level."

Category 0 is a temporary category that means additional imaging is needed before assigning a permanent BI-RADS^TM assessment category. Most category 0 findings are shown to be benign after the additional imaging is completed.

The BI-RADS^TM categories or levels are used to standardize interpretation of mammograms among radiologists. They are useful for statistical analysis of mammography practice and BI-RADS^TM results are compiled on a nationwide basis in the U.S. to help refine mammography procedures everywhere.

The ACR’s BI-RADS^TM system is the product of a collaborative effort between members of various committees of the ACR with cooperation from the National Cancer Institute, the Center for Disease Control and Prevention, the U.S. Food and Drug Administration (FDA), the American Medical Association, the American College of Surgeons, and the American College of Pathologists.  According to the ACR, the BI-RADS^TM system is intended to guide radiologists and referring physicians in the breast cancer decision making process that facilitates the management of patients.   

Each BI-RADS^TM level has an appropriate management or follow-up plan associated with it. For example, if a referring doctor sees a mammogram report with a "category 3" assigned to it, he or she knows the recommendation is for the woman to receive a follow-up mammogram in six months (see below).

If used correctly and consistently, each BI-RADS^TM category has the following risk of malignancy and the associated plan of management or follow-up:

• Category 1: Negative, there is a 5/10,000 chance of cancer being present. Continue annual screening mammography (for women 40 and older).
• Category 2: Benign (non-cancerous) finding, same statistics and plan of follow-up as level 1. This category is for cases that have a finding that is characteristically benign such as cyst or fibroadenoma (see below for more detail).
• Category 3: Probably benign finding, there is less than 2% chance of cancer. Usually receives a 6 month follow-up mammogram; most level 3 abnormalities do not receive biopsy. 
• Category 4: Suspicious abnormality. Most category 4 abnormalities are benign but may require biopsy since this category can be malignant in 25-50% of cases.
• Category 5: Highly suggestive of malignancy. Classic signs of cancer are seen on the mammogram. All category 5 abnormalities typically receive biopsy and if the biopsy results are benign, the abnormality usually receives re-biopsy since the first biopsy may not have sampled the correct area. Depending on how individual radiologists differentiate between category 4 and 5, the percentage of category 5 abnormalities that will be cancer may vary between 75% and 99%.

Note: Category 2 indicates a non-cancerous area such as a cyst or fibroadenoma may be present.

• Cysts are harmless accumulations of fluid in the breast (or other tissue or organs). The exact causes of cysts are not known, but cysts are known to change with hormonal variations, either during normal menstrual cycles or from post-menopausal hormone replacement therapy. Cysts do not become cancer or increase the risk of cancer. Most of the time, cysts may be left alone, but sometimes a physician may drain them with a small needle (fine needle aspiration).
• Fibroadenoma is the most common benign, solid growth in the breasts. Fibroadenoma is round, movable, and firm. Fibroadenoma has no significant risk of becoming cancer and does not put a patient at increased risk of breast cancer.

Additional Resources and References

• The American College of Radiology (ACR) provides detailed information on the BI-RADS^TM system at http://www.acr.org/
• To learn more about the breast cancer diagnostic process, please visit http://www.imaginis.com/breasthealth/diagnosis.asp
• To learn more about mammography, please visit http://www.imaginis.com/breasthealth/mammography.asp

Updated: May 4, 2008