Mass Enhancement — Dark Internal Septations
Dark internal septations is an internal enhancement pattern characterized by dark (non-enhancing) lines within an otherwise enhancing mass.
Definition
There are dark non-enhancing lines within the mass on post-contrast imaging.
Imaging Appearance
- Post-contrast T1W: dark (low signal) linear or curvilinear lines traversing an otherwise enhancing mass
- The septations do not enhance — they remain dark on post-contrast series
- Background mass may be heterogeneous or homogeneous between the dark lines
Pathological Basis
Dark internal septations correspond to fibrous bands or collagenous septa within the mass, commonly seen in fibroadenomas.
Association with Fibroadenoma
Board Pearl
Dark internal septations are suggestive of fibroadenoma, but ONLY if the other morphologic features of shape and margin support this diagnosis. Dark septations within a mass with suspicious shape (irregular) or margin (spiculated) do NOT confer benignity.
Role in Benign Combination Rule
Dark internal septations are an alternative to homogeneous enhancement in the ≤ 2% cancer risk combination:
All four of the following → ≤ 2% malignancy:
- Oval shape
- Circumscribed margin
- Dark internal septations (or homogeneous enhancement)
- T2 hyperintense signal
Pitfalls
- Dark septations must be accompanied by supportive morphology (oval/lobulated, circumscribed) to suggest fibroadenoma
- Do not diagnose fibroadenoma based on septations alone in a mass with any suspicious features