Lipoma
Subcutaneous lipoma; spindle-cell lipoma; pleomorphic lipoma; angiolipoma (vascular variant); "fatty tumour"
Lipoma is the commonest benign soft-tissue tumour in adults — a soft, mobile, slow-growing subcutaneous fatty nodule that is essentially universal as one ages. The skin-oncology importance lies in the critical differential: atypical lipomatous tumour (ALT) / well-differentiated liposarcoma, which can clinically mimic a benign lipoma, particularly in larger (>5 cm), deeper (subfascial / intramuscular), rapidly growing, or painful lesions. Sarcoma red flags — size >5 cm, depth (subfascial), rapid growth, pain, fixity to deeper structures, or unusual location (retroperitoneal, deep extremity) — should always prompt MRI and sarcoma MDT review with consideration of biopsy / excision and MDM2 / CDK4 amplification testing. Multiple lipomas may signal underlying syndromes including familial multiple lipomatosis, Madelung disease, Bannayan-Riley-Ruvalcaba and Cowden syndrome (PTEN).
Clinical features
- Soft, mobile, dome-shaped subcutaneous nodule, 1–10 cm, with ill-defined edges.
- "Slip sign" — when pressed at the edge, the lesion slips beneath the examining finger (characteristic).
- Distribution — neck, shoulders, back, abdomen, proximal extremities; less often distal limbs (which warrants more concern about ALT / liposarcoma).
- Median age — adults; both sexes.
- Asymptomatic; occasional pressure / cosmetic concern.
- Variants:
- Angiolipoma — often painful (one of the painful skin tumours); multiple in young adults.
- Spindle-cell lipoma / pleomorphic lipoma — back / posterior neck of older men; firmer; histologically dramatic but benign.
- Hibernoma — rare; brown-fat origin; warm to touch.
- Multiple lipomas — consider familial multiple lipomatosis, Madelung disease (symmetric upper body), Bannayan-Riley-Ruvalcaba (PTEN), Cowden syndrome (PTEN), Gardner syndrome (FAP).
Sarcoma red flags — when to worry
The following features should prompt MRI and sarcoma MDT review with biopsy of an apparent "lipoma":
- Size >5 cm.
- Depth — subfascial, intramuscular, retroperitoneal.
- Rapid growth over weeks to months.
- Pain, particularly at rest or at night.
- Fixity to deeper structures, including muscle and bone.
- Recurrence after previous excision.
- Unusual location (deep thigh, deep arm, retroperitoneum, atypical visceral site).
- Any of these features warrants "sarcoma until proven otherwise" approach — refer for MRI ± image-guided biopsy through a sarcoma MDT, not unplanned excision in primary care.
Histology & molecular
- Well-circumscribed lobular aggregate of mature univacuolated adipocytes with bland nuclei pushed to the periphery.
- No atypia, mitoses or necrosis in benign lipoma.
- Variants:
- Spindle-cell lipoma — bland CD34+ spindle cells in a myxoid / collagenous matrix admixed with adipocytes.
- Pleomorphic lipoma — "floret" multinucleated giant cells.
- Angiolipoma — adipocytes plus thin-walled vessels with fibrin thrombi (the painful component).
- Atypical lipomatous tumour / well-differentiated liposarcoma — has lipoblasts, atypical hyperchromatic stromal cells, and characteristic MDM2 / CDK4 amplification (12q13–15 ring chromosome) by FISH or IHC.
- The MDM2 / CDK4 distinction is the key tool for differentiating benign lipoma from ALT / liposarcoma in equivocal lesions.
Management
- Typical small superficial subcutaneous lipoma — no treatment required; observation with photographic surveillance is reasonable.
- Surgical excision indications — cosmetic concern, symptoms (pressure, pain), diagnostic uncertainty.
- Sarcoma red-flag lesions — refer to sarcoma MDT for MRI and biopsy before unplanned excision; unplanned "lipoma excision" of an unrecognised liposarcoma in primary care produces R1 / R2 resection and substantially worse outcomes.
- Multiple lipomas — clinical evaluation for familial / syndromic context; refer to clinical genetics if features of PTEN syndromes or Gardner syndrome.
References
- Salam GA. Lipoma excision. Am Fam Physician; 2002.
- Kransdorf MJ, Murphey MD. Imaging of soft tissue tumors — chapter on lipomatous tumours.
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