BI-RADS categories convert a breast ultrasound examination into a single statement of malignancy risk and recommended management. The ACR BI-RADS Atlas standardizes lexicon descriptors and assessments 0 through 6 so reports remain consistent across practices. Assigned correctly, a BI-RADS category tells the referring clinician whether the study is incomplete, negative, benign, probably benign, suspicious, highly suggestive of malignancy, or already proven cancer.
The guidance below is for clinicians who scan or interpret breast ultrasound. It emphasizes incomplete, negative, highly suspicious, and known-malignancy assessments. Categories 2, 3, and 4 are summarized, with links to deeper companion articles.
BI-RADS Categories on Ultrasound: Assessment Rules
Give the examination one overall assessment. When several findings are present, the most suspicious lesion determines the final category. Describe each significant finding in the report body. Background echotexture is reported separately. It is not an assessment category and is not the same as mammographic density letters A through D.
Choose lexicon descriptors first, then the category that matches the features and the intended management:
- Category 0: Incomplete. Additional imaging needed.
- Category 1: Negative. No lesion. Essentially 0% likelihood of malignancy. Routine screening.
- Category 2: Benign. Essentially 0% likelihood of malignancy. Routine screening.
- Category 3: Probably benign. Likelihood of malignancy 2% or less. Short-interval follow-up.
- Category 4: Suspicious. Likelihood of malignancy greater than 2% and less than 95%. Tissue diagnosis.
- Category 5: Highly suggestive of malignancy. Likelihood of malignancy 95% or higher. Tissue diagnosis.
- Category 6: Known biopsy-proven malignancy.
These ranges are ACR-defined expectations for audit and communication. They are not patient-specific probabilities.
BI-RADS Category 0: Incomplete
BI-RADS category 0 means the examination does not yet support a final assessment. The code is common as a screening mammography recall and should be uncommon on diagnostic ultrasound, which often completes the workup.
Typical reasons on ultrasound include:
- Technical limitation that prevents adequate evaluation of the region of interest
- Incomplete correlation with mammography, MRI, or a palpable finding
- A finding that cannot be characterized without another modality
- Prior images required before a final category can be issued
Category 0 is not a diagnosis and has no ACR-defined likelihood of malignancy. Management is additional imaging, not biopsy by default. Complete the workup the same day when possible so the patient leaves with a final category 1 through 6. Do not quote a single cancer rate for category 0. Yield varies with why the study was incomplete.
BI-RADS Category 1: Negative
Use BI-RADS 1 only when no lesion is identified and the study is complete. Simple cysts, intramammary nodes, and other typically benign findings belong in category 2. Negative means nothing to describe. Management is routine screening. Likelihood of malignancy is essentially 0% at the sites evaluated. On targeted diagnostic examinations, state the indication so a negative field is not read as a negative whole breast.
Categories 2, 3, and 4 in Brief
Category 2 is for characteristically benign findings such as simple cysts, intramammary lymph nodes, and stable postsurgical change. See BI-RADS category 2 benign findings on breast ultrasound.
Category 3 is limited to findings with a likelihood of malignancy of 2% or less that can be followed rather than biopsied. A classic ultrasound example is a solitary, circumscribed, oval, parallel, hypoechoic mass. Irregular or not-parallel masses do not qualify. Short-interval follow-up is typically 6 months, then routine screening after documented stability, usually over 2 years. Details are in BI-RADS 3 criteria and follow-up on breast ultrasound.
Category 4 covers suspicious findings that need tissue diagnosis but lack classic category 5 morphology. Subcategories stratify risk: 4A (greater than 2% up to 10%), 4B (greater than 10% up to 50%), and 4C (greater than 50% but less than 95%). They aid communication and audit. They do not replace biopsy. See BI-RADS category 4 subcategories and pearls.
BI-RADS 5: Highly Suggestive of Malignancy
BI-RADS 5 is for classic malignant morphology with an expected likelihood of malignancy of 95% or higher. It is not a tissue diagnosis. Rare benign mimics exist, so biopsy remains required.
Features that together often support category 5 include:
- Irregular shape
- Not-parallel orientation
- Spiculated or angular margins
- Hypoechoic or complex cystic and solid echo pattern
- Posterior shadowing
- Associated architectural distortion, skin or nipple retraction, or duct extension
A single suspicious feature more often belongs in category 4. Reserve category 5 for a constellation that would make a benign result unexpected. Discordant benign histology does not erase the imaging assessment until correlation is resolved. Category 5 does not encode tumor type, stage, or survival.
Category 6: Known Biopsy-Proven Malignancy
Use category 6 when ultrasound evaluates a cancer already proven by biopsy, including extent of disease, second-look after MRI, pretreatment planning, and monitoring during neoadjuvant therapy. Do not assign category 6 to a highly suspicious but unsampled mass. That lesion is category 5 or 4 until histology is available. After successful excision, later examinations of that site are no longer category 6 unless residual or new biopsy-proven disease is being imaged.
Reporting Pearls and Common Pitfalls
- Describe first, then assess. Do not reverse-engineer lexicon terms to fit a score.
- Issue one final assessment for the study.
- Prefer same-day completion of diagnostic ultrasound over category 0.
- Do not use category 3 as a compromise for uncertainty. If follow-up is not safe, use category 4.
- Do not upgrade 4C to category 5 solely to ensure biopsy. Category 4 already means tissue diagnosis.
- Density letters A through D are not ultrasound assessment codes.
- State targeted versus whole-breast technique so a negative study is not over-interpreted.
When local workflow differs from ACR guidance, document why. Follow current ACR BI-RADS recommendations and your facility audit standards.
Consistent reporting starts with a complete examination, precise lexicon terms, and an assessment that matches both the features and the intended management. Use category 0 only when more imaging is required, keep category 1 truly negative, reserve category 5 for classic malignant morphology, and save category 6 for proven cancer. IAME offers accredited ultrasound CME courses on breast imaging and reporting for clinicians who want structured review of assessment decisions.
Frequently Asked Questions
What are the different BI-RADS classifications for breast ultrasound results?
Ultrasound uses the same seven assessment codes as mammography and breast MRI: 0 incomplete, 1 negative, 2 benign, 3 probably benign, 4 suspicious, 5 highly suggestive of malignancy, and 6 known biopsy-proven malignancy. Lexicon descriptors differ by modality, but the final category still communicates risk and next step. Subcategories 4A, 4B, and 4C further stratify suspicious lesions.
What is BI-RADS category 0?
Category 0 means the study is incomplete and additional imaging or comparison is needed before a final assessment. It is not a diagnosis and it does not carry an ACR-defined likelihood of malignancy. On ultrasound, use it sparingly and complete the workup the same day when possible so the patient receives a category 1 through 6.
Is BI-RADS 5 always cancer?
No. Category 5 means the features are classic for malignancy and the expected likelihood of malignancy is 95% or higher, not 100%. Benign lesions can rarely mimic this appearance, which is why tissue diagnosis is still required. The code does not predict histologic type or survival.
What is BI-RADS category C?
Category C is not an ultrasound assessment code. On mammography, BI-RADS density category C means the breasts are heterogeneously dense, which can obscure small masses. Assessment categories remain 0 through 6. On ultrasound, report background echotexture separately from the final assessment.
