Lipoma

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Lipoma

Pedunculated lipoma at the nape of the neck - S Das, Manual on Clinical Surgery

Definition

A lipoma is the most common benign soft-tissue tumor in adults. It is a cluster of mature adipocytes (fat cells) that become overactive, distend with fat, and form a palpable mass. It arises from mesenchymal tissue and can develop anywhere normal fat cells exist in the body.

Robbins & Kumar Basic Pathology, p. 153
S Das, Manual on Clinical Surgery, p. 52

Epidemiology

Most common benign mesenchymal neoplasm
Occurs predominantly in adults, with no strong gender predilection for classical lipoma
Rarely seen in children
Peak incidence in middle adulthood
Multiple lipomas are uncommon and may signal an underlying syndrome

Sites / Classification

Lipomas can be classified by location:

Type	Location	Key Feature
Encapsulated subcutaneous	Most common; trunk, neck, shoulder, upper limbs	Well-capsulated, mobile, lobulated
Intramuscular	Within muscle belly	Poorly circumscribed; higher recurrence rate (up to 20%)
Intermuscular	Between muscle groups	May reach considerable size
Subfascial	Below fascia	May be deeper than expected clinically
Parosteal	On bone surface
Subserous / Submucous	GI tract	Can cause obstruction or intussusception
Intra-articular / Subsynovial	Joints	"Lipoma arborescens" - villous synovial proliferation
Subdural / Extradural	Spine	Can cause neurological symptoms
Diffuse (Pseudolipoma)	Neck/face, subcutaneous/intermuscular	No capsule; associated with alcohol excess
Multiple (Lipomatosis)	Limbs and trunk	May be painful (neurolipomatosis); see Dercum's disease

The back of the neck, shoulder, and upper back are the most common overall sites.

Clinical Features

Symptoms:

Painless, soft, slowly growing swelling (the classic presentation)
Usually small (<5 cm for superficial types); deep-seated ones may grow very large
Overlying skin is normally colored
In very large lipomas: stretched skin with dilated veins may be visible (more characteristic of naevolipoma)

On examination:

Surface: Smooth and lobulated
Consistency: Soft, "semifluctuant" (fat is in near-liquid state at body temperature; does not truly fluctuate)
Transillumination: A large lipoma may transilluminate (does NOT contain fluid)
Mobility: Freely mobile over deeper structures (muscle, fascia)
"Slip sign": The edge of a lipoma slips away under the palpating finger - this distinguishes it from a cyst, where the edge yields rather than slips
Movement test: Can be moved over a taut underlying muscle both along its long axis and at right angles to it
A trapezius maneuver (shrugging shoulder against resistance) can help distinguish lipoma from the underlying muscle

Pathology

Gross: Yellow, lobulated, encapsulated mass of fatty tissue indistinguishable from normal fat.

Histology:

Well-circumscribed, encapsulated, lobulated lesion
Composed of mature adipocytes with only mild variation in size and shape
No nuclear atypia
Thin, hypocellular fibrous septa with thin-walled capillaries
Immunohistochemistry: Negative for p16, MDM2, and CDK4 (positive staining would suggest atypical lipomatous tumor/well-differentiated liposarcoma)

Variants:

Fibrolipoma - abundant fibrous tissue
Chondrolipoma - contains cartilage
Osteolipoma - contains bone
Adenolipoma - contains sweat glands/ducts (in purely dermal lipomas)
Angiolipoma - increased vascularity with fibrin thrombi; often painful and multiple
Spindle cell/Pleomorphic lipoma - distinct cytogenetic entity, mainly posterior neck/shoulder in elderly males; benign
Naevolipoma - contains dilated blood vessels
Hibernoma - brown fat tumor (synonym: lipoma of embryonic/immature fat)

Genetics / Pathogenesis

Lipomas are cytogenetically heterogeneous. Key abnormalities include:

Aberrations at 12q13-15 (including HMGA2 gene at 12q14.3, involved in pathogenesis)
Rearrangements at 6p21-23
Deletions at 13q

These are distinct from liposarcoma (which involves MDM2 amplification at 12q13-15 and other changes).

Varieties of Multiple Lipomas

Condition	Features
Lipomatosis	Multiple small-to-moderate lipomas; trunk and limbs
Neurolipomatosis	Multiple painful lipomas containing nerve tissue
Dercum's disease (Adiposis dolorosa)	Tender lipomatous swellings mainly on trunk; more common in women
Madelung disease (Symmetric lipomatosis)	Middle-aged Mediterranean men; neck/shoulder/proximal upper limbs; associated peripheral neuropathy

Associated Syndromes

Syndrome	Associated Features
Gardner syndrome	FAP, desmoid fibromatosis, lipomas, osteomas, cysts
Bannayan-Riley-Ruvalcaba	Macrocephaly, lipomas, lymphangiomas, hamartomas, intestinal polyposis
Cowden syndrome	Hamartomas, trichilemmomas, lipomas, oral papillomas
CLOVE syndrome	Congenital lipomatous overgrowth, vascular malformations, epidermal nevi
Goldenhar-Gorlin syndrome	Intracranial lipomas, hydrocephalus
MERRF syndrome	Symmetric lipomas, myoclonus epilepsy, ragged-red fibers

Differential Diagnosis

Sebaceous cyst - edge yields under palpation (no "slip sign"), has a punctum
Dermoid cyst - not mobile over deeper tissues
Angiolipoma - painful, multiple; contains fibrin thrombi on histology
Liposarcoma - atypical cells, MDM2/CDK4 positive on IHC; suspect if large, deep, or rapidly growing
Ganglion - related to joint/tendon, contains synovial fluid
Lymph node - regional location, firm
Abscess - fluctuant, signs of infection

For any lipoma in an atypical location, large size, or rapid growth - consider imaging (US, CT, or MRI) and biopsy to exclude liposarcoma.

Complications

Cosmetic disfigurement
Pressure effects (nerve compression - e.g., posterior interosseous nerve, brachial plexus)
Calcification
Infection
Ulceration (from repeated trauma)
Malignant transformation to liposarcoma - disputed; many authorities believe lipoma never undergoes malignant change and that liposarcoma arises de novo. However, this possibility (particularly for retroperitoneal lipomas) must always be kept in mind.

Investigations

Clinical diagnosis is sufficient for most superficial lipomas
Ultrasound: First-line if imaging needed - shows echogenic lesion with thin capsule
MRI: Best for deep, large, intramuscular lipomas; shows uniform fat signal (T1 bright, suppressed on fat-sat)
CT: Useful for retroperitoneal/intra-abdominal lipomas
Plasma D-dimer may help distinguish lipoma from well-differentiated liposarcoma (elevated in the latter)
Biopsy/histology: For atypical lesions; IHC negativity for MDM2 and CDK4 confirms benign lipoma

Treatment

Surgical excision is the treatment of choice:

Small superficial lipoma: squeeze tissue to make it bulge, incise, and enucleate ("shell out")
For large/multilobular lesions: establish a plane of cleavage, use blunt and sharp dissection
Ensure meticulous haemostasis; close dead space or insert a small suction drain
Intramuscular lipomas: simple excision, but counsel patient that local recurrence occurs in up to 20%
GI lipomas: endoscopic removal may be possible

Non-surgical options:

Observation for small asymptomatic lesions
Steroid injection (triamcinolone) can reduce size but does not eliminate the lipoma
Liposuction/aspiration - less effective for fibrous variants; higher recurrence

Key surgical principle: Benign lipomas are "shelled out" completely. This is INADEQUATE and inappropriate for liposarcoma, which requires wide excision.

Prognosis

Classical subcutaneous lipoma: excellent prognosis; very rarely recurs after complete excision
Intramuscular/intermuscular lipoma: recurrence rate up to 20%
Transition of a pre-existing lipoma to atypical lipomatous tumor: exceedingly rare

Sources:

Robbins & Kumar Basic Pathology, p. 153
Fitzpatrick's Dermatology, Vol. 1-2, pp. 2201-2204
S Das, A Manual on Clinical Surgery 13th Ed., pp. 52-54
Pye's Surgical Handicraft 22nd Ed., p. 65

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