Mass Margin — Indistinct
Indistinct margin (formerly “irregular margin”) indicates the mass edges are uneven but not spiculated — a non-circumscribed margin without radiating lines.
Definition
The margin is composed of edges that are uneven but not spiculated.
v2025 Change (Critical)
Board Pearl
“Irregular” margin has been replaced by “Indistinct” in v2025. The change was made to:
- Avoid duplication with “irregular” as a shape descriptor
- Harmonize with margin descriptors for other modalities (mammography, ultrasound)
Historically “irregular” → now “indistinct” in all BI-RADS v2025 documentation.
Imaging Appearance
- Border is poorly defined, blurred, or fuzzy
- Does not have the smooth sharpness of circumscribed margins
- Does not have the radiating lines characteristic of spiculated margins
- Edges are uneven and merge with surrounding tissue without a clear interface
Suspicion Level
Non-circumscribed / Suspicious. Indistinct margin indicates uncertainty about the boundary — the mass is infiltrating, not displacing, surrounding tissue.
Association with Malignancy
Most commonly associated with:
- Invasive ductal carcinoma — most frequent cause of indistinct margins
- Invasive lobular carcinoma — classic for poorly defined margins
- Other invasive carcinomas
Pitfalls
- BPE around a lesion can simulate indistinct margins — use subtraction images and kinetics to help
- Indistinct should be applied when ANY portion of the margin is non-circumscribed and non-spiculated