Fat necrosis is a benign inflammatory process resulting from ischemic necrosis of breast fat tissue. Accounts for 0.6-2.75% of all breast lesions. Most commonly develops after trauma (blunt or penetrating), surgery (lumpectomy, reduction, reconstruction), radiotherapy, and biopsy. Fat necrosis shows highly variable imaging features — may present as oil cyst, rim calcification, spiculated mass, architectural distortion, or focal asymmetry and can mimic malignancy. The most characteristic mammographic finding is an oil cyst with fat-density (radiolucent) center and peripheral thin rim calcification (eggshell type) — this appearance is pathognomonic. However, early-stage fat necrosis may present as spiculated mass or architectural distortion mimicking invasive carcinoma (BI-RADS 4-5). On MRI, lesion containing fat signal (bright on T1, signal loss on fat suppression) and rim enhancement are typical. On US, variable echogenicity — anechoic cyst, hyperechoic fat mass, or mixed echotexture may be seen. Clinical history (trauma/surgery) is critically important in diagnosis.
Age Range
30-70
Peak Age
50
Gender
Female predominant
Prevalence
Common
Fat necrosis is an aseptic inflammatory process that develops following ischemic or traumatic damage to adipocytes in breast fat tissue. Damaged adipocytes break down and release free fatty acids — these fatty acids trigger an inflammatory response in surrounding tissue. The inflammatory process progresses through four stages: (1) Acute inflammation — edema, hemorrhage, and neutrophil infiltration. At this stage, irregularly marginated mass or focal consolidation may be seen on imaging — can mimic malignancy. (2) Lipid phagocytosis — macrophages (foam cells/lipophages) phagocytose necrotic fat. Saponification (soap formation) of fatty acids begins. (3) Fibrosis and calcification — granulation tissue forms, fibrotic capsule develops. Binding of saponified fatty acids with calcium leads to peripheral dystrophic calcification (rim calcification). (4) Oil cyst formation — liquid fat (oleate) accumulates within the fibrotic capsule forming an oil cyst. The fat content of the oil cyst produces negative density (radiolucent center) on mammography and bright signal on T1 MRI — signal loss on fat suppression confirms diagnosis. Rim enhancement results from granulation tissue and neovascularization of the fibrotic capsule. Spiculated margin reflects fibrotic tissue retraction — this finding mimics desmoplastic reaction of invasive carcinoma and may raise malignancy suspicion.
Fat-density (radiolucent/dark) center with surrounding thin peripheral calcification (eggshell type rim) on mammography. This appearance is pathognomonic for fat necrosis — no other breast lesion produces this specific combination. When oil cyst diagnosis is made, no further workup or biopsy is needed (BI-RADS 2).
Fat-density (radiolucent) center surrounded by thin peripheral calcification — 'eggshell' type rim calcification. Oil cyst is usually round or oval, well-circumscribed. Size variable (from few mm to several cm). This appearance is pathognomonic for fat necrosis and requires no further workup.
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An oil cyst with fat-density center and peripheral rim calcification is observed at the surgical scar site on mammography, a benign finding consistent with fat necrosis; no further workup is needed (BI-RADS 2).
In early or active inflammatory phase, fat necrosis may appear as a spiculated or irregularly marginated mass — mimicking invasive carcinoma. Architectural distortion may accompany. Fat-density center may not yet have formed. This presentation is assessed as BI-RADS 4-5 and requires biopsy.
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A spiculated mass is observed at the surgical/trauma site; invasive carcinoma and fat necrosis should be considered in the differential diagnosis; core biopsy is recommended (BI-RADS 4B).
High signal on T1-weighted series (isointense with subcutaneous fat). Signal loss on fat suppression (fat-sat or STIR) sequences — confirms fat content. Oil cyst is round or oval, well-circumscribed with bright center on T1.
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A lesion showing bright signal on T1-weighted series with signal loss on fat suppression sequence is observed on MRI, consistent with oil cyst due to fat necrosis.
Thin rim (peripheral ring) enhancement around the oil cyst on contrast-enhanced MRI. Center does not enhance (liquid fat is avascular). Rim enhancement reflects granulation tissue and neovascularization of the fibrotic capsule. Rim thickness is usually thin and smooth — thick or irregular rim increases malignancy suspicion.
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Thin smooth rim enhancement around the T1-hyperintense lesion is observed on contrast-enhanced MRI, consistent with oil cyst and surrounding fibrotic capsule due to fat necrosis.
Fat necrosis shows extremely variable appearance on ultrasound: (1) Anechoic cyst (oil cyst), (2) Hyperechoic mass (fat-containing solid lesion), (3) Mixed cystic-solid structure, (4) Hypoechoic mass (early inflammatory phase), (5) Architectural distortion. Posterior acoustic enhancement (in cystic components) or acoustic shadowing (in calcified rim) may be seen.
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An anechoic cystic lesion with peripheral hyperechoic rim and posterior acoustic enhancement is observed at the surgical scar site on ultrasound, consistent with oil cyst due to fat necrosis; mammographic correlation is recommended.
Fat-density (-20 to -100 HU) lesion on CT. Peripheral calcified rim may be visible. Oil cyst appears as a well-circumscribed, round structure. Although CT is not the primary modality for breast, it may be incidentally detected on chest CT.
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A well-circumscribed fat-density (-60 HU) lesion with peripheral calcification is observed in the breast parenchyma on CT, consistent with oil cyst due to fat necrosis.
Criteria
Weeks to months after trauma/surgery. Active inflammation, edema, hemorrhage dominant. Oil cyst not yet formed. Calcification absent or minimal.
Distinct Features
Spiculated or irregularly marginated mass on mammography — mimics malignancy, biopsy required. Heterogeneous enhancement on MRI. Hypoechoic irregular mass on US. At this stage imaging alone cannot make diagnosis — clinical history and biopsy needed.
Criteria
Months to years later. Fibrotic capsule matured. Oil cyst formed. Rim calcification developed or developing. Inflammation minimal.
Distinct Features
Pathognomonic oil cyst on mammography (radiolucent center + rim calcification). T1 bright, fat-sat dropout on MRI. Anechoic cyst on US. Diagnosis definitive — biopsy not needed (BI-RADS 2).
Criteria
Fat necrosis developing after breast surgery (lumpectomy, mastectomy+reconstruction, reduction, augmentation). Multiple oil cysts may be seen along the surgical field. Common in TRAM/DIEP flap reconstruction.
Distinct Features
Oil cysts in linear alignment along surgical scar line. Scar retraction and architectural distortion may accompany. After surgery for breast cancer, differentiation from malignancy recurrence is important — fat signal (MRI) or oil cyst (mammography) indicates not recurrence.
Distinguishing Feature
IDC forms spiculated mass but does not show fat-density center — fat necrosis at oil cyst stage has radiolucent center. IDC does not contain fat signal on MRI, fat necrosis is T1 bright and loses signal on fat-sat. History of trauma/surgery supports fat necrosis.
Distinguishing Feature
DCIS shows fine pleomorphic/linear/branching intraductal calcifications — fat necrosis shows rim (peripheral) calcification. DCIS calcifications are segmentally distributed and duct-shaped. In fat necrosis, calcification is ring-shaped around the lesion.
Distinguishing Feature
Simple cyst shows low (water) signal on T1 — oil cyst shows high (fat) signal on T1. Simple cyst is bright on T2, oil cyst shows intermediate signal on T2. Simple cyst does not lose signal on fat suppression, oil cyst does. On mammography, simple cyst is water/soft tissue density, oil cyst is fat density.
Distinguishing Feature
Hamartoma is a well-circumscribed mass containing mix of fat and glandular tissue ('breast within a breast' appearance) — fat necrosis typically shows pure fat content and rim calcification. Hamartoma is not associated with trauma history and does not grow. Oil cyst may shrink over time or completely calcify.
Urgency
routineManagement
conservativeBiopsy
Not NeededFollow-up
no-follow-upFat necrosis at the oil cyst stage is a benign finding requiring no further workup or treatment (BI-RADS 2). Clinical history (trauma, surgery, radiotherapy) is critically important in diagnosis. However, early-stage fat necrosis (spiculated mass, architectural distortion) can mimic malignancy — core biopsy is needed in this situation (BI-RADS 4). After breast cancer surgery, fat necrosis must be differentiated from recurrence — fat signal on MRI (T1 bright, fat-sat dropout) confirms not recurrence. Symptomatic oil cysts (pain, palpable swelling) can be treated with aspiration. Over time, oil cysts may shrink or completely calcify — stable calcified oil cysts require no follow-up.
Fat necrosis is a benign lesion requiring no treatment when typical. Biopsy may be needed for atypical appearance (spiculated mass, enhancement). Should not be confused with carcinoma in post-surgical follow-up. Clinical-radiological correlation is important.