Mass Shape — Irregular
Irregular is a mass shape descriptor indicating a mass that is neither oval, lobulated, nor round — characterized by acute-angle protruding portions.
Definition
A mass that is neither oval, lobulated, nor round. Has acute-angle protruding portions. Irregular shape often implies a suspicious finding.
Imaging Appearance
- Contour has angular, acute-angle protrusions (spikes or points)
- Lacks the smooth contour of oval/round or gentle undulations of lobulated
- Most commonly associated with spiculated or indistinct margins
Association with Malignancy
Irregular shape is frequently seen in:
- Invasive ductal carcinoma — most common association; often irregular + spiculated
- Invasive lobular carcinoma — can be irregular + spiculated
- Higher-grade invasive cancers
Board Pearl
Irregular shape is the most suspicious mass shape and often implies malignancy. Combined with spiculated margin, it is the most suspicious mass descriptor combination.
Common Combinations in Source Figures
- Irregular + spiculated + heterogeneous → IDC (multiple cases)
- Irregular + spiculated + thick rim enhancement → Invasive lobular carcinoma
- Irregular + indistinct + heterogeneous → IDC
Terminology Note
The term “irregular” as a shape descriptor should NOT be confused with the former margin descriptor “irregular margin,” which has been replaced by “indistinct” in v2025 to avoid this exact confusion.