Fat necrosis (Breast)

Overview & pathophysiology

Definition: Sterile, non-suppurative inflammatory process of adipose tissue resulting from ischemic or traumatic insult.

• Mechanism: Adipocyte death $\to$ release of fatty acids $\to$ saponification with calcium $\to$ inflammatory response (macrophages/giant cells) $\to$ fibrosis & scarring.
• Key exam relevance: Great mimic of carcinoma (clinical and radiological). Can present as a hard, fixed mass with skin tethering.

Etiology

• Trauma: Accidental (seatbelt injury most common blunt cause) or Iatrogenic (biopsy, lumpectomy, reduction mammoplasty, reconstruction with flaps/implants).
• Radiation Therapy: Common in post-BCT breast (differentiate from recurrence).
• Systemic: Warfarin therapy (rare, extensive necrosis), Polyarteritis Nodosa, Weber-Christian disease.
• Idiopathic: Spontaneous in large pendulous breasts.

Clinical Presentation

• Asymptomatic: Incidental screening finding.
• Symptomatic: Palpable lump (often hard), skin thickening/retraction, nipple inversion.
• Timing: Variable; can appear weeks to years after insult.

Imaging findings

Mammography

Appearance evolves with time.

• Classic (Benign):
  • Oil cyst: Round/oval radiolucent lesion with smooth walls.
  • Calcification: "Eggshell" or rim calcification (pathognomonic). Dystrophic coarse calcifications.
  • Capsule: Thin, radiodense rim.
• Indeterminate/Suspicious (Mimics malignancy):
  • Spiculated mass: Due to dense fibrotic reaction (looks like invasive ductal carcinoma).
  • Focal asymmetry: Ill-defined density.
  • Microcalcifications: Can be pleomorphic or clustered (early stage), prompting biopsy.

Ultrasound (US)

Notoriously variable ("The Chameleon").

• Location: Subcutaneous (most common).
• Echogenicity:
  • Hyperechoic halo: Surrounding edema/inflammation (acute).
  • Hyperechoic mass: Solid, mimics lipoma but with shadowing.
  • Anechoic/Complex Cystic: Oil cyst content.
• Shadowing: Common due to calcification or dense fibrosis (mimics malignancy).
• Specific sign: "Shifting echogenic bands" (internal echoes that change orientation with patient position) $\to$ High specificity for liquefied fat/serum interface.

MRI

Used for problem-solving in post-surgical breasts.

• T1: High signal (fat intensity).
  • Differentiation: Must match fat signal on all sequences.
• T2: Variable; often high signal (cystic oil) or low (fibrosis).
• Fat Sat / STIR: "Black hole" appearance (signal drop out of the fatty center).
• Enhancement (Post-contrast):
  • No enhancement: Mature oil cyst.
  • Thin rim enhancement: Common, benign inflammation.
  • Thick/Irregular/Nodular enhancement: Suspicious. Can mimic recurrence.
  • Kinetics: Can show washout in inflammatory phase (false positive).

Diagnostic algorithm & management

BIRADS classification

• BIRADS 2: Classic findings (oil cyst, eggshell calcification, lucent-centered mass).
• BIRADS 3: Indeterminate findings (e.g., solid mass with some fatty features but not classic).
• BIRADS 4: Spiculated mass without central fat, increasing size, or suspicious microcalcifications.

Differential diagnosis

1. Carcinoma (Recurrence vs. De novo):
   • Key discriminator: Central fat density (MG) or "Black hole" (MRI) favors necrosis. Developing density favors Ca.
2. Post-surgical scar:
   • Should decrease in size/density over time. Fat necrosis can wax/wane or calcify.
3. Abscess:
   • Clinical signs of infection (redness, heat) usually more acute.
4. Galactocele:
   • Fat-fluid level (milk), lactation history.

Exam Pearls

• Triple assessment: If clinical exam is suspicious (hard/fixed) but imaging is classic benign fat necrosis $\to$ Believe the imaging (Discordance rule exception).
• Biopsy pitfall: Avoid biopsy of obvious oil cysts (risk of infection/non-healing). However, biopsy is mandatory if radiological features are indeterminate to exclude malignancy.
• Warfarin: Associated with extensive breast necrosis; looks dramatic on imaging.