Lymph Node — Internal Mammary
Internal mammary lymph nodes (also called parasternal lymph nodes) lie along the internal mammary (internal thoracic) vessels within the intercostal spaces, typically 2–3 cm lateral to the sternal border. They receive lymphatic drainage primarily from the medial and central breast and are frequently visible on modern breast MRI due to improved spatial resolution and contrast techniques.
The fact that a node is visible does not make it abnormal — visibility reflects technical capability, not pathology.
Anatomy and Drainage
- Located in the parasternal fat between the internal intercostal and transversus thoracis muscles
- Most commonly identified in the first three intercostal spaces
- Receive afferent drainage from the medial quadrants of the breast, deep pectoral lymphatics, and upper abdominal wall (via diaphragmatic lymphatics)
- Efferent drainage passes superiorly to the subclavian (apical axillary) and supraclavicular nodes, or joins the bronchomediastinal trunk
- Cross-drainage between right and left chains exists, explaining occasional contralateral metastasis
Board Pearl
Internal mammary nodes drain the medial and central breast — medially located tumours have the highest probability of internal mammary metastasis. Approximately 5–10% of breast cancers metastasise to internal mammary nodes, often without concurrent axillary involvement.
Imaging Appearance
T1-Weighted (Pre-contrast)
- Isointense to slightly hypointense relative to pectoral muscle
- Normal fatty hilum may show T1 hyperintensity centrally
- Small nodes may be indistinguishable from surrounding parasternal fat
T2-Weighted / STIR
- Mildly hyperintense relative to muscle
- Reactive and metastatic nodes both show T2 hyperintensity — this sequence alone is non-specific
Post-contrast (Dynamic)
- Normal nodes show mild, homogeneous enhancement
- Metastatic nodes may demonstrate intense, early enhancement or heterogeneous enhancement
- Loss of the normal fatty hilum on contrast-enhanced sequences raises suspicion
DWI / ADC
- Metastatic nodes may show restricted diffusion (low ADC values)
- DWI is supplementary — not validated as a standalone criterion for internal mammary nodes
Normal Features
- Frequently visible on modern MRI — this alone is not pathologic
- In asymptomatic screening women (no cancer history): may measure up to 9–10 mm in long axis and still be normal
- Assessed by comparison with ipsilateral and contralateral internal mammary nodes
- Preserved fatty hilum and smooth cortex favour benignity
- Bilateral, symmetrically enlarged nodes are more likely reactive
Abnormal Features
- Subjectively asymmetric in size compared to other ipsilateral or contralateral internal mammary nodes
- Ipsilateral to current or prior breast cancer makes an asymmetric node more suspicious
- In patients with current breast cancer: size ≥5 mm was most helpful in predicting malignancy in one small study
- Loss of fatty hilum, cortical thickening, or rounded morphology (long-to-short axis ratio approaching 1) increases suspicion
- Heterogeneous or rim enhancement is concerning for metastatic replacement
Board Pearl
Normal internal mammary lymph nodes can be up to 9–10 mm in long axis in screening populations — do not overcall normal visible nodes as suspicious. The key is asymmetry and clinical context, not absolute size alone.
Differential Diagnosis
| Finding | Favours Benign / Reactive | Favours Metastatic |
|---|---|---|
| Size | ≤9–10 mm (screening population) | ≥5 mm with known ipsilateral cancer |
| Symmetry | Bilateral, symmetric | Unilateral asymmetric enlargement |
| Morphology | Oval, preserved fatty hilum | Round, effaced hilum |
| Cortex | Thin, uniform | Eccentric cortical thickening |
| Enhancement | Mild, homogeneous | Intense, heterogeneous, or rim-like |
| Context | No cancer history | Ipsilateral to known malignancy |
Other entities that may mimic an internal mammary lymph node:
- Internal mammary artery aneurysm — follows vascular signal on all sequences
- Sternal metastasis — bony destruction, irregular margins
- Parasternal soft tissue mass (e.g., lymphoma) — typically larger and more irregular
Clinical Significance and Management
- Staging impact: Internal mammary node metastasis upstages breast cancer to N3b (AJCC 8th edition) when not accompanied by axillary metastasis, or N3b when combined with axillary disease
- Treatment implications: Positive internal mammary nodes may prompt inclusion of the internal mammary chain in the radiation field, altering the radiotherapy plan significantly
- Surgical sampling: Internal mammary sentinel node biopsy is technically feasible but uncommonly performed due to operative risk and limited therapeutic benefit
- PET/CT correlation: FDG-PET may help characterise equivocal MRI findings; however, small nodes (<5 mm) are below PET spatial resolution
Board Pearl
Internal mammary node metastasis changes the radiation field — if identified, the radiation oncologist should be notified to consider parasternal nodal irradiation. This is one of the most actionable findings at breast MRI staging.
Reporting
- Describe side and intercostal space (define ribs above and below: e.g., “intercostal space between ribs 1 and 2”) — facilitates correlation and targeted radiation therapy planning
- Note size (long-axis and short-axis), morphology, and any asymmetry compared to the contralateral side
- Ipsilateral to known malignancy: describe fully, no separate BI-RADS category; management depends on multidisciplinary decision-making
- Isolated finding (no known cancer): assign BI-RADS based on degree of suspicion
- If suspicious, recommend correlation with PET/CT or short-interval follow-up depending on clinical scenario
Pitfalls
- Overcalling normal visible nodes — up to 9–10 mm is normal in screening; bilateral symmetric enlargement is almost always reactive
- Ignoring contralateral comparison — always compare with the opposite internal mammary chain; unilateral asymmetry is the key discriminator
- Forgetting cross-drainage — medial tumours may metastasise to internal mammary nodes without axillary involvement, so a negative axilla does not exclude internal mammary disease
- Omitting intercostal space localisation — vague reporting (“parasternal node”) hinders surgical and radiation planning; always specify the intercostal level
- Confusing with axillary Level III nodes — Level III (infraclavicular/apical) nodes are lateral to the pectoralis minor; internal mammary nodes are medial, along the sternal border
Board Pearl
Internal mammary node metastasis can occur without axillary metastasis — particularly with medially located tumours. A clear axilla does not rule out internal mammary spread.
v2025 Updates
BI-RADS v2025 emphasises that visibility alone is not abnormal and provides explicit size guidance for screening populations (up to 9–10 mm). The edition reinforces that asymmetry and clinical context (ipsilateral cancer) are the primary criteria for suspicion, moving away from rigid size-only thresholds used in older references.