BI-RADS 4B Ultrasound Findings
A breast lesion is classified as BI-RADS 4B on ultrasound when it demonstrates moderate suspicion for malignancy with an intermediate probability of cancer (approximately 20-40% malignancy risk), typically characterized by one or more suspicious features that fall between low suspicion (4A) and high suspicion (4C) categories. 1, 2
Key Ultrasound Features That Define BI-RADS 4B
Primary Suspicious Characteristics
BI-RADS 4B lesions typically exhibit moderately suspicious morphologic features including:
- Irregular shape without the classic spiculated appearance of highly malignant lesions 2
- Microlobulated margins (positive predictive value for malignancy of approximately 41.5%) 2, 3
- Indistinct margins that are neither clearly circumscribed nor overtly spiculated 2
- Nonparallel orientation (where the lesion is taller than wide), though less pronounced than in 4C lesions 4
Secondary Features
Additional findings that support 4B classification include:
- Microcalcifications within the mass (significantly associated with malignancy but not definitive) 2, 5
- Posterior acoustic features that are neither clearly enhancing nor completely shadowing 2
- Hypoechoic echo pattern without the marked hypoechogenicity typical of higher suspicion lesions 5
Distinguishing 4B from Adjacent Categories
Differentiation from BI-RADS 4A (Lower Suspicion)
BI-RADS 4A lesions (malignancy risk ~10-20%) typically show:
- Oval shape with circumscribed margins 2
- Parallel orientation 2, 4
- Abrupt interface with surrounding tissue 2
The key distinction is that 4B lesions have at least one moderately suspicious feature (microlobulated or indistinct margins, irregular shape) that elevates concern beyond 4A. 2, 3
Differentiation from BI-RADS 4C (Higher Suspicion)
BI-RADS 4C lesions (malignancy risk ~50-75%) demonstrate:
- Spiculated margins (positive predictive value 80%) 2
- Marked irregular shape 2
- Nonparallel orientation with posterior shadowing 4
BI-RADS 4B lacks the classic highly suspicious features of 4C but has more concerning features than 4A. 2, 3
Clinical Context and Malignancy Risk
The positive predictive value for malignancy in BI-RADS 4B lesions ranges from 25% to 41.5% across studies, representing intermediate suspicion. 2, 3, 6 This contrasts with 4A lesions (6-19.5% malignancy rate) and 4C lesions (70-74.3% malignancy rate). 2, 3, 6
Management Implications
All BI-RADS 4B lesions require tissue diagnosis via core needle biopsy regardless of subcategorization, as recommended by NCCN guidelines. 7, 1 The subcategorization primarily helps patients and clinicians understand the intermediate level of concern and alerts pathologists to the degree of suspicion. 3
Important Caveats
The inter-reader reliability of BI-RADS descriptors and subcategorization is only fair-to-moderate, meaning different radiologists may classify the same lesion differently. 4 Despite this limitation, the subcategorization provides valuable risk stratification that is clinically meaningful. 2, 3
Pathology-imaging concordance must be verified after biopsy; discordant benign results in a 4B lesion warrant repeat imaging or additional tissue sampling. 8, 1