Definition/Introduction

The breast imaging reporting and data system (BI-RADS) is a system for the standardization of mammogram reports. Developed by the American College of Radiology in 1993, its goal is to provide information to referring clinicians and patients in a language that is clear, meaningful and standardized across facilities. Based on scientific data and developed by the world’s leaders in breast imaging, the BI-RADS system describes key mammographic findings and outlines appropriate follow-up and management.[1] The Mammography Quality Standards Act (MQSA) of 1997 stipulates that all mammograms in the United States must be reported using one of the BI-RADS assessment categories.[2]

Issues of Concern

The BI-RADS mammographic classification system begins with the indication for the study. The patient history is included in this section. The mammographer must also describe whether the study is a screening exam, a diagnostic exam or a follow-up exam. Breast composition is the next component of the standardized reporting system, which falls into four categories. These are 1. entirely fatty, 2. scattered areas of fibroglandular density, 3. heterogeneously dense which may obscure small masses and 4. extremely dense, which lowers the sensitivity of mammography.[3]

The most detailed component of the BI-RADS classification system is the description of all pertinent findings. There is a standard lexicon for the myriad of findings which may be seen by mammography, divided into five main categories.[4] These include mass, asymmetry, architectural distortion, calcifications, and associated features. Under each of these main categories are additional subcategories which further describe the finding.[1] For example, the description of masses is by their shape, margin, and density. A mass may be oval, round or irregular in shape. It may have circumscribed, obscured, microlobulated, indistinct or spiculated margins. The density of a mass may be high, equal, low or fat containing. The various descriptors correlate with varying levels of suspicion for malignancy. Microlobulated and indistinct margins are suspicious findings. Spiculated margins are highly suspicious for malignancy.[4] Similarly, low-density masses have low suspicion for malignancy whereas high-density masses should generate a high suspicion for malignancy.[5]

Asymmetries, the next category of BI-RADS findings, are areas of focal fibroglandular tissue lacking the discrete borders of a mass. There are four subcategories of asymmetries. A focal asymmetry is visible in two projections whereas an asymmetry is merely visible in one projection. Global asymmetries occupy at least one-quarter of the breast. Developing asymmetries are often the most worrisome in the category as they are new, larger or more conspicuous than on the prior study.[4]  Architectural distortion refers to the distortion of normal breast architecture without the identification of a discrete mass. Architectural distortion raises suspicion for malignancy but can also present in the setting of benign scar tissue.[6]

Calcifications are described by both morphology and distribution. Morphologic descriptors include amorphous, coarse heterogeneous, fine pleomorphic or fine linear/fine linear branching calcifications. There are five categories of calcification distribution as well. These are diffuse (randomly distributed), regional (occupying greater than 2 cm of breast tissue), grouped (a few calcifications in a small area of breast tissue), linear, and segmental (appear to be deposited in a duct(s)).[7]

The last two categories of pertinent findings on mammography are associated features and special cases. Associated features are findings seen in conjunction with the previously noted findings. Skin retraction, nipple retraction, skin thickening, and axillary adenopathy are all examples. Special cases are those findings that are so commonly seen they do not require detailed descriptions. Intramammary lymph nodes and skin tags are examples of special cases.

After the description of the findings, the radiologist must issue a final assessment.  The assessment categories are divisible into seven subcategories. The lowest is an incomplete assessment (category 0) which indicates the need for additional imaging. The remaining six assessment categories are for completed assessments (categories 1, 2, 3, 4, 5, 6). BI-RADS 1 and 2 indicate a negative and benign screening mammogram respectively. BI-RADS 3 assessment is for those diagnostic mammograms classified as probably benign.[8] BI-RADS 4 indicates a mammogram which is suspicious for malignancy and BI-RADS 5 suggests that the mammogram is highly suggestive of malignancy.[4] BI-RADS 6 assessment is for those with biopsy-proven breast cancer. Only BI-RADS 0, 1 or 2 assessment categories can be assigned to screening mammograms. BI-RADS 3, 4, 5 and 6 are for diagnostic mammograms after performing a complete imaging workup.[2]

The last component of a mammography report under the BI-RADS classification system is management recommendations. There are four options for management under the BI-RADS system. These recommendation options are (1) additional imaging studies, (2) routine interval mammography, (3) short-term follow-up, and (4) biopsy. All categories reflect the radiologist’s increasing level of suspicion for malignancy and have also been shown to have correlations with an increased risk of malignancy.[9]

The overwhelming majority of screening mammograms will end up classified as BI-RADS 1 and 2. A small percentage of mammograms (approximately 5 to 9%) will need additional imaging for further evaluation, short interval follow-up or possibly a biopsy. Approximately 7% of diagnostic mammograms will achieve a BI-RADS 3 assessment.[10] Only 2% of diagnostic mammograms will receive a BI-RADS 4 or 5 assessment and will require biopsy.[2]

Clinical Significance

The implementation of the BI-RADS lexicon has had a positive impact on education, quality assurance, and research.[11] The most valuable contributions of BI-RADS, however, have been the improved quality of mammographic interpretation and the enhanced communication between radiologists, referring and treating clinicians, and patients.