Mass Margin — Circumscribed
Circumscribed margin indicates the mass edge is smooth and sharply demarcated with an abrupt transition between the lesion and surrounding breast tissue.
Definition
The margin is smooth and sharply demarcated with an abrupt transition between the lesion and the surrounding tissue.
Critical MRI Rule (v2025)
Board Pearl
On MRI (unlike mammography), the entire margin must be well defined for a mass to be called circumscribed. If ANY portion of the margin is non-circumscribed, classify using the more suspicious margin descriptor (indistinct or spiculated).
This is stricter than mammographic circumscribed criteria, where partial obscuration by overlying tissue is acceptable.
Imaging Appearance
- Sharp, well-defined border visible in its entirety on MRI
- Abrupt transition between mass and surrounding parenchyma or fat
- No blurring, infiltration, or spiculation of any margin segment
Differential Diagnosis
Circumscribed masses are typically benign:
- Fibroadenoma — most common circumscribed enhancing mass
- Cyst — circumscribed, no true enhancement (only peri-cystic thin rim possible)
- Lymph node — circumscribed, fatty hilum
- Hamartoma — circumscribed, mixed signal
Malignant exceptions (circumscribed malignancy):
- IDC — can be circumscribed; represents approximately 10–20% of cancers
- Mucinous carcinoma — often oval, circumscribed, T2 hyperintense
- Medullary/metaplastic carcinoma — can be circumscribed
Pitfall
Circumscribed margin does NOT exclude malignancy. A circumscribed mass can represent IDC or other cancers. Always consider the full constellation of features.