Lymph Node — Axillary
Axillary lymph nodes are categorized by anatomic level relative to the pectoralis minor muscle.
Anatomic Levels
| Level | Location | Surgical Relevance |
|---|---|---|
| Level I | Lateral to pectoralis minor | SLNB + ALND |
| Level II | Posterior/between pectoralis muscles (includes interpectoral/Rotter’s nodes) | ALND |
| Level III | Medial to pectoralis minor (infraclavicular) | Not routinely sampled |
Metastatic disease most frequently progresses sequentially: I → II → III (with exceptions).
Normal Features
- Circumscribed, reniform, homogeneously enhancing, T2 hyperintense
- Symmetric in size and cortical thickness vs. ipsilateral/contralateral nodes
- Lobulation of a thin cortex is normal
- Fatty hila typically visible (may not be in small nodes)
- Fast early/washout delayed kinetics are normal in lymph nodes and NOT suspicious
Board Pearl
The cortical thickness > 3 mm threshold used on ultrasound is NOT applicable to MRI. Many normal lymph nodes appear to have cortical thickness > 3 mm on MRI due to partial volume averaging. Use subjective asymmetry as the key criterion.
Abnormal Features
- Subjectively asymmetric cortical thickness vs. other ipsilateral/contralateral nodes
- Significantly increased in size compared to prior exam
- Cortical thickening may be focal or diffuse
- Loss of reniform shape (rounding) and absence of fatty hila may be abnormal, but can occur in normal small nodes — should not be sole criteria
Board Pearl
Abnormal-appearing axillary nodes in a patient with recent vaccination in the ipsilateral arm can be benign and reactive — always check clinical history. This is a common pitfall on boards and in practice.
Reporting
- Describe location/level(s) and approximate number of abnormal nodes
- Ipsilateral to known malignancy: describe fully, no separate BI-RADS
- Contralateral to known malignancy with no suspicious breast findings in that breast: assign BI-RADS
- Isolated finding: assign BI-RADS (e.g., “BI-RADS 4B — moderate suspicion”)