The Breast Imaging-Reporting and Data System (BI-RADS) is a standardized quality assurance tool developed by the American College of Radiology (ACR). Radiologists use this system to classify findings on breast imaging tests, such as mammograms, ultrasounds, and MRIs. The primary purpose of BI-RADS is to ensure clear, consistent communication of results and follow-up recommendations among healthcare providers. Using a uniform lexicon and a scale of 0 to 6, the system standardizes the interpretation of breast images across different facilities.
The Meaning of Category 0
A BI-RADS Category 0 result signifies an “Incomplete Assessment” that requires additional imaging evaluation or comparison to previous studies before a final classification can be assigned. This designation is a temporary holding category, not a final diagnosis of a benign finding or malignancy. It means the radiologist did not gather enough information from the initial screening mammogram to confidently assign a score from Category 1 through 6. Category 0 is an indication for more information, not necessarily an indicator of a severe problem. This result is relatively common, prompting a recall in about one in ten screening mammograms.
Why an Assessment May Be Incomplete
A radiologist assigns a Category 0 classification when critical information needed for a definitive final assessment is missing. One frequent reason is the need for comparison with prior mammograms, especially if a subtle change is noted but older images are not immediately available. Comparing current images with past ones helps determine if a finding is new or has remained stable over time, which is a key factor in assessment.
Another common cause is the presence of dense breast tissue, which can obscure small masses or architectural distortions. Dense tissue appears white on a mammogram, similar to how potential masses or tumors appear, making it difficult to differentiate between normal and abnormal structures. In these cases, the radiologist may suspect a questionable mass that is most likely caused by overlapping normal tissue, requiring additional views to resolve.
Technical factors can also contribute to an incomplete assessment, although this is less frequent. Images that are technically inadequate, perhaps due to patient movement or poor positioning, may not be sharp enough for a proper evaluation. When the initial screening images are unclear, the radiologist requests additional views to ensure the entire breast is adequately visualized before making a final determination.
Required Follow-Up Procedures
Once a BI-RADS Category 0 is assigned, the patient is recalled for a diagnostic workup to obtain the missing information and assign a final BI-RADS category. The immediate next step is usually a diagnostic mammogram, which differs from the initial screening exam by focusing on a specific area of concern. This often involves specialized views such as spot compression, where a smaller paddle applies concentrated pressure to spread out overlapping tissue.
Magnification views are also used during the diagnostic mammogram to provide a clearer, enlarged image of subtle details like microcalcifications. These additional mammographic images are generally sufficient to resolve the vast majority of Category 0 findings by confirming the questionable area was overlapping tissue. The radiologist immediately reviews the new images to update the assessment.
If the additional mammography views do not fully clarify the finding, the next common procedure is a targeted breast ultrasound. Ultrasound uses sound waves to create images and is useful for determining if a questionable area is a solid mass or a simple fluid-filled cyst, which is typically benign. Ultrasound can also help evaluate findings in dense breast tissue where mammography may be limited.
In a small percentage of cases, if non-invasive imaging still suggests an abnormality, the radiologist may recommend more advanced steps. This could involve a breast MRI or a biopsy to obtain a tissue sample for definitive diagnosis. The goal of these follow-up procedures is to gather enough evidence to reclassify the finding into a final category, such as Category 1 (Negative), Category 2 (Benign), or a higher category suggesting the need for a biopsy.