Lymph Node — Intramammary
Intramammary lymph nodes are lymph nodes located within the breast parenchyma, most frequently in the lateral and usually upper portions of the breast (upper outer quadrant), though they may occur anywhere including the medial breast. They are usually adjacent to a vein because lymphatic drainage parallels venous drainage. Intramammary nodes are part of the first echelon of axillary drainage and are classified as level I axillary nodes for staging purposes.
Imaging Appearance
T1-Weighted
- Isointense to slightly hypointense cortex relative to breast parenchyma
- Fat-containing hilum appears hyperintense on T1 (when visible)
- Fatty hilum may be difficult to discern when nodes are small (<5 mm)
T2-Weighted
- T2 hyperintense cortex — a hallmark of normal lymphoid tissue
- Fatty hilum follows fat signal (intermediate to high on T2 without fat sat, drops on fat-saturated sequences)
- Thin, uniform cortex in benign nodes
Post-Contrast (Dynamic Enhancement)
- Homogeneous enhancement of the cortex in normal nodes
- Enhancement is typically rapid and intense due to high vascularity of lymphoid tissue
- Non-enhancing fatty hilum creates the characteristic reniform (kidney-bean) morphology
- Abnormal nodes may show heterogeneous or rim enhancement
DWI / ADC
- Normal lymph nodes may show restricted diffusion due to high cellularity of lymphoid tissue — this is a known pitfall and does not indicate malignancy
- Markedly low ADC values in the setting of morphologic abnormality increase suspicion
Normal Features
- Circumscribed, reniform (kidney-bean) shape
- Homogeneously enhancing cortex
- T2 hyperintense cortex
- Preserved fatty hilum (may be difficult to discern in small nodes)
- Usually <1 cm in short axis
- Often multiple; bilateral intramammary nodes are common and typically benign
Abnormal Features
- Subjectively enlarged compared to expected size or compared to contralateral breast
- No established MRI size or cortical thickness threshold to distinguish benign from suspicious
- Suspicious features:
- Increase in size compared to prior exams
- Non-circumscribed margin (irregular or spiculated)
- Absence of fatty hilum (in larger nodes where hilum should be visible)
- Focal cortical thickening or eccentric cortical bulge
- Heterogeneous or rim enhancement
- Loss of reniform morphology (rounded shape)
- Associated ipsilateral malignancy increases suspicion
Board Pearl
There is no established size threshold for intramammary lymph nodes on breast MRI. Unlike axillary nodes where subjective asymmetry is key, intramammary node assessment relies on comparison to prior exams and morphologic features. Correlating with mammogram/ultrasound for stability can be helpful.
Board Pearl
Intramammary lymph nodes are classified as level I axillary nodes for AJCC staging. A positive intramammary sentinel node upstages breast cancer to at least pN1 — the same as a positive axillary node. Do not dismiss them as incidental.
Differential Diagnosis
| Entity | Key Distinguishing Features |
|---|---|
| Normal intramammary lymph node | Reniform, circumscribed, fatty hilum, T2 bright, homogeneous enhancement |
| Metastatic intramammary node | Loss of fatty hilum, rounded morphology, cortical thickening, heterogeneous enhancement, interval growth |
| Small circumscribed mass (fibroadenoma) | No fatty hilum, T2 bright but solid, may show non-enhancing septations; lacks reniform shape |
| Papilloma | Enhancing intraductal mass, associated duct, may mimic small node on MRI |
| Circumscribed carcinoma (mucinous, medullary) | Very T2 bright (mucinous), circumscribed, lacks fatty hilum and reniform shape |
Clinical Significance and Management
- Benign intramammary nodes are extremely common and represent the most frequent cause of circumscribed enhancing masses in the lateral breast
- In the setting of ipsilateral malignancy: morphologically abnormal intramammary nodes may indicate metastatic involvement and should be reported but do not receive a separate BI-RADS assessment
- As an isolated finding without ipsilateral malignancy: assign BI-RADS based on level of suspicion
- Intramammary nodes may serve as sentinel nodes and can be identified on preoperative lymphoscintigraphy
Reporting
- When abnormal in the setting of known ipsilateral malignancy: describe fully (location, size, morphology) but assign no separate BI-RADS assessment
- When isolated finding (no ipsilateral malignancy): assign BI-RADS based on suspicion level
- Always document comparison with prior exams when available — interval change is the single most reliable indicator
- Note laterality and number if multiple abnormal nodes are present
Pitfalls
- Small size masking hilum: The fatty hilum is often not visible in nodes <5 mm; absence of visible hilum alone does not indicate malignancy in small nodes
- DWI false positive: Normal lymphoid tissue is cellular and may show restricted diffusion — do not rely on DWI alone to call a node suspicious
- Reactive enlargement: Infection, recent biopsy, vaccination (especially COVID-19 or flu vaccine in ipsilateral arm), and inflammatory conditions cause reactive nodal enlargement that mimics metastatic disease
- Mistaking a node for a mass: Intramammary nodes in atypical locations (medial breast, inferior breast) may be misinterpreted as solid enhancing masses — look for the fatty hilum and reniform shape
- Ignoring intramammary nodes in staging: These are level I axillary nodes; a positive intramammary node changes the N stage
Board Pearl
Post-vaccination lymphadenopathy is a well-recognized cause of unilateral reactive intramammary and axillary nodes. Always check vaccination history (especially recent ipsilateral arm injection) before recommending biopsy for new or enlarged nodes.
v2025 Updates
BI-RADS v2025 maintains the approach of prior editions: intramammary lymph nodes are included in the lymph node section of the MRI lexicon with emphasis on morphologic assessment rather than size-based criteria. The key v2025 points are:
- Continued emphasis that no size threshold has been validated for intramammary nodes on MRI
- Morphologic features (margin, hilum, shape) and interval change remain the primary assessment criteria
- Reporting guidance reinforced: no separate BI-RADS when concurrent ipsilateral malignancy is present