In 1992, the American College of Radiology published a guiding document: Breast Imaging Reporting and Data System (BI-RADS). It has been revised three times, and by 2003, it not only guided mammography diagnosis (4th edition) but also included ultrasound and MRI diagnosis. It standardizes the diagnostic reports of all imaging findings of the breast as a whole organ, using unified professional terminology, standard diagnostic classification, and examination procedures, making radiologists' diagnoses systematic. It also enhances coordination and understanding between radiology and other clinical departments, so that clinical physicians know what to do next when they see the radiologist's report.

Category 0: Often used in screening situations, rarely used after complete imaging evaluation and comparison with previous images. Recommended additional imaging methods include spot compression, magnification, special positioning, ultrasound, etc.

Category 1: Mammography shows clear breast structure with no lesions. Note that the commonly used terms in China such as cystic hyperplasia, lobular hyperplasia, adenosis (collectively referred to as fibrocystic changes or structural dysplasia) are classified as Category 1 according to BI-RADS. If a clinical lump is palpable and there is localized asymmetry, even if the final diagnosis is sclerosing adenosis, it cannot be classified as Category 1, but may be classified as Category 3 or 4A. Intramammary lymph nodes and anterior axillary lymph nodes showing low-density hilum (lateral view) or central low density (axial view of the hilum) are considered normal lymph nodes and belong to Category 1.

Category 2: Definite benign breast masses (such as fibroadenoma, fibrolipoma, lipoma, simple cyst, milk cyst, oil cyst), definite benign calcifications (such as ring-like calcifications, clearly defined short strip-like calcifications, coarse punctate calcifications, sparse round dot-like calcifications of relatively uniform size, crescent-shaped deposition calcifications, etc.) are classified as Category 2. However, a clear boundary of the mass does not necessarily exclude malignancy. For women over 35 years old, palpation should be noted, and old films should be recalled for comparison, or follow-up should be observed for changes. Therefore, it may be evaluated as Category 0 or 3.

Category 3: High probability of benignity, expected to be stable or shrink in short-term follow-up (less than 1 year, generally 6 months) to confirm the judgment. The malignancy rate of this category is generally less than 2%. Non-calcified clear boundary masses, focal asymmetry, clustered round or/and dot-like calcifications are considered likely benign changes. For this category, the first step is short-term follow-up with X-ray (6 months), then 6 months, then 12 months follow-up to 2 years or even longer to confirm the judgment. Stability for 2 or 3 years can change the original Category 3 (probably benign) to Category 2 (benign). This category is used after complete imaging evaluation and is generally not recommended for initial screening; it is also not suitable for evaluating palpable lumps clinically; for possibly benign lesions that increase in follow-up, biopsy should be recommended instead of continued follow-up.

Category 4: This category includes a large number of lesions requiring clinical intervention. These lesions do not have characteristic mammographic changes of breast cancer but have the possibility of malignancy, with an overall malignancy rate of about 30%. It is further divided into 4A, 4B, 4C, and clinical doctors and patients can make the final decision on the treatment of the lesion based on their different malignancy possibilities.

Category 4A: The results of biopsy or cytology for benignity are relatively reliable, and routine follow-up or follow-up after six months can be performed. Palpable masses with clear X-ray boundaries but ultrasound suggesting possible fibroadenoma, palpable complex cysts, and palpable abscesses are classified in this subcategory.

Category 4B: It is important for radiologists and pathologists to reach a consensus on the reliability of biopsy results for this group of lesions. Biopsy results for masses with partially clear and partially infiltrative boundaries can be accepted and followed up. However, biopsy results for papillomas need further excisional biopsy to confirm.

Category 4C: Irregularly shaped, infiltrative boundary masses and clustered fine polymorphic calcifications can be classified in this subcategory. For imaging classified as Category 4C and biopsy results as benign, further evaluation of the pathology results is needed to clarify the diagnosis.

Category 5: This category has a high probability of malignancy. The probability of detecting malignancy is greater than or equal to 95%. Irregularly shaped high-density masses with spiculated margins, segmental and linear distribution of fine linear and branching calcifications, irregularly shaped masses with spiculated margins accompanied by polymorphic calcifications are classified in this category.

Category 6: This category is used for imaging evaluation of biopsy-proven malignancies that have not yet been treated. It mainly evaluates imaging changes after previous biopsy or monitors imaging changes before surgery or neoadjuvant

chemotherapy.