Soft tissue lipoma is the most common benign soft tissue tumor of mesenchymal origin. It is composed of mature adipocytes and accounts for approximately 50% of all soft tissue tumors. It is most common between ages 40-60 with a slight male predominance. Subcutaneous location is most frequent; deep-seated (subfascial/intermuscular) lipomas are less common but more important for differential diagnosis with liposarcoma. Intraosseous and paraosteal (periosteal) locations are rare. On imaging, it appears as a homogeneous, well-defined mass following fat signal on all sequences.
Age Range
30-70
Peak Age
50
Gender
Equal
Prevalence
Very Common
Lipoma is a well-encapsulated benign neoplasm of mature white adipose tissue. The tumor consists of mature fat cells histologically identical to normal adipocytes; however, neoplastic cells show clonal proliferation and chromosome 12q13-15 translocation (HMGA2 gene) is the most common cytogenetic abnormality. This homogeneous adipose composition is the fundamental reason it shows the same signal/density characteristics as normal subcutaneous fat on all imaging sequences — bright on MRI T1, complete signal loss on fat suppression, negative HU values on CT. Thin fibrovascular septae (<2mm) separate tumor lobules and may show minimal enhancement; as long as these septae are not thickened and contain no nodular component, benignity is supported. Deep-seated lipomas (subfascial, intermuscular, intramuscular) can be larger than subcutaneous types, and in the differential diagnosis with liposarcoma, size (>10cm), thick septae (>2mm), and presence of non-adipose component are critical parameters.
Lipoma shows signal identical to subcutaneous fat on all MRI sequences: T1 hyperintense, T2 intermediate-hyperintense, complete signal loss on fat-suppressed sequences (STIR, fat-sat), no enhancement. This 'fat on all sequences' pattern is the most reliable diagnostic criterion for lipoma.
On T1-weighted sequences, lipoma shows high signal identical to subcutaneous fat. The signal is homogeneous and may show continuity with surrounding subcutaneous fat at the same intensity. Thin hypointense septae may reveal the lobulated structure of the mass. Presence of non-adipose component (T1 hypointense areas) raises suspicion for liposarcoma.
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On T1-weighted sequences, the mass demonstrates homogeneous hyperintense signal identical to subcutaneous fat, consistent with lipoma.
On T2-weighted sequences, lipoma shows intermediate-to-high signal identical to subcutaneous fat. On standard T2 sequences, fat is still bright. Homogeneous signal architecture is maintained. On conventional spin-echo T2, fat has high signal; on fast spin-echo (FSE/TSE) T2, fat signal is even more pronounced due to J-coupling effect.
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On T2-weighted sequences, the mass demonstrates homogeneous intermediate-to-high signal consistent with subcutaneous fat.
On STIR sequences, lipoma shows complete and homogeneous signal loss — suppressed identically to subcutaneous fat. This confirms the entire lesion consists of mature adipose tissue. Heterogeneous or incomplete suppression (residual bright areas) suggests presence of non-adipose component and raises the differential diagnosis of liposarcoma.
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On STIR sequences, the mass shows complete and homogeneous signal loss identical to subcutaneous fat, consistent with lipoma composed of mature adipose tissue.
On chemical shift imaging (in-phase/opposed-phase), 'India ink' artifact is seen at the lipoma-muscle interface. A line of signal loss forms at the fat-water boundary on opposed-phase. Since the internal structure of lipoma is homogeneous fat, intratumoral signal drop is not expected; however, the prominent India ink effect at the border with surrounding muscle confirms the fat content of lipoma.
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On chemical shift imaging, India ink artifact at the mass-muscle interface confirms the fat content of the mass.
On post-contrast images, lipoma shows no enhancement. Minimal enhancement may be seen in thin fibrovascular septae, but no nodular or mass-like enhancement is present. Enhancement within non-adipose areas strongly raises suspicion for liposarcoma and is an indication for biopsy.
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On post-contrast fat-suppressed sequences, no significant enhancement is seen within the mass, consistent with benign lipoma.
On CT, lipoma shows homogeneous low density; measured HU values range from -65 to -120 HU and are identical to normal subcutaneous fat. Thin septae may be visible at soft tissue density. Calcification is rare and seen in chondrolipoma or osteolipoma variants. Presence of non-adipose soft tissue component raises suspicion for liposarcoma.
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On CT, the mass demonstrates homogeneous low density (-65 to -120 HU) identical to subcutaneous fat, consistent with lipoma.
On US, lipoma typically appears as an oval, well-defined, compressible mass that is isoechoic or slightly hyperechoic to subcutaneous fat. Thin echogenic lines (fibrovascular septae) may be seen within the mass. Deep-seated lipomas may have less well-defined borders. No posterior acoustic shadowing is present. Internal vascularity is minimal or absent.
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On US, a well-defined, compressible mass isoechoic to subcutaneous fat is seen, consistent with lipoma.
On color and power Doppler, lipoma shows no internal vascularity or minimal peripheral flow. This avascular structure supports benignity. Presence of internal vascularity increases the possibility of atypical lipoma or liposarcoma.
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On color Doppler examination, no vascularity is identified within the mass, consistent with benign lipoma.
Criteria
Located in subcutaneous fat, superficial to fascia, soft and mobile on palpation
Distinct Features
Most common type. Usually <5cm. Very low malignancy risk. Treated by excision for cosmetic reasons or if symptomatic. Imaging usually unnecessary but US or MRI indicated for atypical findings.
Criteria
Deep to fascia, located between or within muscle fibers, may be >5cm, MRI required for differentiation from liposarcoma
Distinct Features
High clinical importance due to liposarcoma differential diagnosis. Homogeneous fat signal and thin septae on MRI support benignity. Biopsy recommended for >10cm, thick septae (>2mm), or presence of nodular non-adipose component. Intramuscular type may infiltrate between muscle fibers and borders may appear indistinct.
Criteria
Fat + prominent vascular component, may be painful (unlike classic lipoma), usually subcutaneous and multiple
Distinct Features
May show enhancement on MRI (vascular component). Vascular areas hyperintense on T2. Common in young adults. Tends to be multiple. Pain is a distinguishing feature from classic lipoma. Spinal angiolipomas may be found in the epidural space.
Criteria
Adjacent to bone surface, related to periosteum, may cause pressure erosion on cortical bone, osseous proliferation at base
Distinct Features
Cortical scalloping or reactive bone formation at the bone surface may be seen. CT best evaluates bone changes. Fat signal on MRI confirms lipoma diagnosis. Must be differentiated from periosteal chondroma and periosteal sarcomas.
Distinguishing Feature
Liposarcoma contains non-adipose component (thick septae >2mm, nodular solid areas, enhancing component); in lipoma the entire mass follows homogeneous fat signal. Liposarcoma shows incomplete/heterogeneous suppression on fat suppression, lipoma shows complete and homogeneous suppression
Distinguishing Feature
Soft tissue sarcoma is heterogeneous, T1 intermediate-to-low signal, marked enhancement, contains areas of necrosis/hemorrhage; lipoma has homogeneous fat signal and no enhancement
Distinguishing Feature
Hemangioma contains fat + vascular component (residual hyperintensity on STIR); lipoma shows complete signal loss on fat suppression. Hemangioma shows enhancement; lipoma shows no enhancement
Distinguishing Feature
Desmoid tumor shows T1 low-to-intermediate signal (no fat signal), variable T2 signal (depending on collagen content), shows enhancement; lipoma follows fat signal on all sequences
Distinguishing Feature
Schwannoma shows target sign on T2, is associated with nerve, split-fat sign with fat surrounding the mass; lipoma has homogeneous fat signal and is not nerve-associated
Urgency
non-urgentManagement
observation; surgical excision if symptomatic or cosmetically bothersome; MRI characterization for deep lesionsBiopsy
Not NeededFollow-up
none for typical subcutaneous lipoma; MRI follow-up for deep/large (>5cm) or atypical lesions; biopsy for non-adipose component or rapid growthLipoma is a benign tumor with negligible risk of malignant transformation. Imaging is usually unnecessary for subcutaneous lipomas; physical examination is sufficient. MRI characterization is mandatory for deep-seated, >5cm, or atypical lesions (heterogeneous, thick septae, non-adipose component). Liposarcoma differential diagnosis is the most important clinical concern; >10cm size, thick septae (>2mm), non-adipose component enhancement, and rapid growth are indications for biopsy. Recurrence rate after surgical excision is low (1-2%) but deep intramuscular lipomas may show higher recurrence.
Lipomas are benign with no risk of malignant transformation. Treatment is usually unnecessary; surgical excision may be performed for symptomatic or cosmetic reasons. Biopsy should be considered to exclude well-differentiated liposarcoma in lesions >10 cm, deep-seated, or with thick septa.