Any score of three, four, or five will require follow-up studies.
Category 0 -- Incomplete
Your mammogram or ultrasound didn't give the radiologist enough information to make a clear diagnosis. Don't worry, this may mean that scar tissue from a previous surgery or biopsy may have shown up, or that a breast lump that is big enough to be easily felt isn't imaging clearly. You will need to have a follow-up image done.
Category 1 -- Normal
There are no suspicious masses or calcifications to report; tissue looks healthy.
Category 2 -- Benign or Negative
Breasts are same size and shape and tissue looks normal. Any cysts, fibroadenomas, or other masses appear benign.
Category 3 -- Probably Benign
There are no suspicious lesions, masses, or calcifications to report, but follow-up to confirm that no cancer exists is recommended. This may be needed if your radiologist does not have your baseline or previous mammogram for comparison.
Category 4 -- Possibly Malignant
There are some suspicious lesions, masses, or calcifications to report; a breast biopsy is recommended to check the suspicious area.
Category 5 -- Malignant
There are masses with an appearance of cancer. A biopsy is recommended to make an accurate diagnosis.
Category 6 -- Malignant Tissue from a biopsy has been examined and found to be cancerous, and treatment such as surgery, chemotherapy, and/or radiation is required.Here is a table of the BIRAD scores.
American College of Radiology. BI-RADS US (PDF document) Copyright 2004. BI-RADS US.
breastscore.com. Breast Ultrasound Computer-Aided Scoring For Level Of Suspicion To Cancer. Copyright 2004. Breast Ultrasound Computer-Aided Scoring Overview.
Christopher Comstock MD, Associate Clinical Professor, Director of Breast Imaging, UCSD Dept of Radiology, La Jolla CA. 8/31/07 interview.