Lipoma is the most common benign soft tissue tumor composed of mature adipocytes (fat cells). Superficial lipomas are located in subcutaneous fat and constitute approximately 50% of all soft tissue tumors. Most common between ages 40-60 with slight male predominance. Most frequently located on the trunk, upper extremity, and neck. Clinically presents as a painless, soft, mobile, slow-growing subcutaneous mass. Ultrasonography is the primary diagnostic modality — isoechoic/slightly hyperechoic structure, parallel echogenic lines (fibrous septa between fat lobules), compressibility, and avascular structure are characteristic findings. Bright T1 signal and fat-suppression signal loss on MRI confirm the diagnosis. Malignant transformation is extremely rare.
Age Range
25-70
Peak Age
45
Gender
Equal
Prevalence
Very Common
Lipoma is a benign neoplastic proliferation of mature adipocytes. Adipocytes are histologically identical to normal subcutaneous fat cells but karyotypic anomalies (especially HMGA2 gene rearrangement at 12q13-15) may be detected. The tumor is surrounded by a thin fibrous capsule and divided by thin fibrous septa similar to normal fat lobules. On ultrasonography, these septa appear as parallel echogenic lines: collagen content of septa (high acoustic impedance) differs from surrounding fat (low impedance) → acoustic reflection at septa-fat interfaces → echogenic lines. Overall echogenicity same or slightly different from subcutaneous fat → isoechoic or slightly hyper/hypoechoic. Compressibility from low elastic modulus of fat tissue. Avascular on Doppler due to low metabolic activity. On MRI, T1 hyperintensity from short T1 relaxation of lipid methyl (CH₃) and methylene (CH₂) groups. Fat suppression selectively nulls fat signal → signal loss confirms diagnosis.
Thin echogenic lines running parallel to the skin surface within the lipoma represent fibrous septa separating fat lobules and are a highly characteristic finding. These lines result from collagen-fat impedance difference and confirm the fat composition of the lipoma.
On B-mode ultrasonography, an isoechoic or slightly hyperechoic, oval, well-defined mass is seen with the same echogenicity as subcutaneous fat. Thin echogenic lines (fibrous septa) running parallel to the skin surface within the mass are characteristic. Long axis is parallel to skin surface. Thin hyperechoic capsule enables differentiation from surrounding fat. No or minimal posterior enhancement — solid fat tissue is not fluid. Size generally 1-10 cm.
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An isoechoic, oval, well-defined mass with parallel echogenic lines (fibrous septa) is seen in subcutaneous fat; consistent with lipoma.
The lipoma is prominently compressed with probe pressure and returns to original shape when pressure is released — elastic deformation. This compressibility is highly characteristic and helps differentiate from cystic lesions (compressible but different elastic rebound) and firm solid tumors (non-compressible). During dynamic evaluation, lateral spreading of the mass during compression is observed.
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The mass is prominently compressed with probe pressure and returns to original shape upon release; elastic deformation consistent with lipoma.
No vascular signal detected within the lipoma on Color and Power Doppler — completely avascular. Typical for benign lipoma and important for differentiation from vascular tumors (increased vascularity) and liposarcoma (may show internal vascularity). Minimal vascularity may be seen at the periphery of large lipomas (>5 cm) but no internal vascularity.
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No vascular signal detected within the mass on Color Doppler; avascular structure consistent with benign lipoma.
On T1-weighted MRI, lipoma shows homogeneous bright signal — isointense to subcutaneous fat. Thin fibrous septa may appear as low T1 signal lines. Capsule appears as thin hypointense line. Heterogeneous T1 signal or non-fat components (solid nodules) should raise suspicion for liposarcoma.
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The mass shows homogeneous hyperintense signal isointense to subcutaneous fat on T1-weighted images; consistent with lipoma.
Homogeneous signal loss on fat suppression sequences (STIR or frequency-selective fat-sat) — the mass loses signal along with subcutaneous fat. This confirms fat content and diagnosis of lipoma. No residual signal (homogeneous loss) — heterogeneous loss or non-suppressed solid components suggest liposarcoma.
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The mass shows homogeneous signal loss on fat suppression sequences; pure fat content confirming lipoma diagnosis.
A thin hyperechoic capsule clearly defines lipoma boundaries — enabling differentiation from surrounding subcutaneous fat. No or minimal posterior acoustic enhancement — solid fat is not fluid and attenuation is near normal soft tissue. Some lipomas may be difficult to distinguish from surrounding fat — panoramic or harmonic imaging may improve contrast.
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The mass is bordered by a thin hyperechoic capsule with no posterior acoustic enhancement.
Criteria
Most common type of mature adipocytes. Homogeneous fat signal.
Distinct Features
Isoechoic on US, parallel lines, compressible, avascular. Homogeneous T1 bright on MRI with complete fat suppression signal loss.
Criteria
Fat + vascular component. Usually painful. Multiple in young adults.
Distinct Features
Hyperechoic areas (fat) + hypoechoic nodular areas (vascular) on US, shows internal vascularity. Pain differentiates from conventional lipoma.
Criteria
Fat + prominent fibrous component. Firmer consistency.
Distinct Features
Thicker echogenic septa on US, less compressible. More prominent hypointense septa on MRI T2.
Distinguishing Feature
Epidermoid cyst is hypoechoic, cystic with laminated pattern and posterior enhancement — lipoma is isoechoic, solid, compressible, no enhancement.
Distinguishing Feature
Neurofibroma: hypoechoic solid lesion along nerve with target sign. Lipoma: isoechoic with parallel lines, no nerve association.
Distinguishing Feature
Dermatofibroma: small (<2 cm) hypoechoic dermal nodule — dimple sign pathognomonic. Lipoma: subcutaneous, isoechoic, larger.
Urgency
routineManagement
conservativeBiopsy
Not NeededFollow-up
no-follow-upLipoma is completely benign with no malignancy potential. Diagnosis is confident with typical US and MRI findings, no biopsy needed. Asymptomatic lipomas need no treatment. Surgical excision is curative for symptomatic/cosmetic cases (1-3% recurrence). For atypical features (>5 cm, rapid growth, heterogeneous signal, internal vascularity, non-fat components), MRI and/or biopsy are needed to exclude liposarcoma.
Lipomas are benign lesions with extremely low risk of malignant transformation. Asymptomatic lipomas require no treatment. Surgical excision is performed for cosmetic or symptomatic indications. MRI and/or biopsy are recommended for lipomas >5 cm, deep (subfascial), rapidly growing, or painful due to suspicion of liposarcoma. Lipomatosis (multiple lipomas) may be associated with Dercum disease, Madelung disease, or familial multiple lipomatosis.