Definition/General
Introduction:
Lipoma represents the most common benign soft tissue tumor
It arises from mature adipose tissue
It constitutes 40-50% of all soft tissue tumors
FNAC shows characteristic mature adipocytes without atypia.
Origin:
Arises from subcutaneous fat or deep soft tissue
Originates from mesenchymal cells with adipocytic differentiation
Most commonly located in subcutaneous tissue of trunk and extremities
Can occur in intramuscular or intermuscular locations.
Classification:
WHO classification: Benign adipocytic tumor
Variants include conventional lipoma
Angiolipoma
Spindle cell lipoma
Pleomorphic lipoma
Intramuscular lipoma
Parosteal lipoma.
Epidemiology:
Peak incidence in 4th-6th decades
Male to female ratio 1:1
Most common soft tissue tumor in adults
Accounts for 16% of soft tissue FNAs
Multiple lipomas in 5-15% cases.
Clinical Features
Presentation:
Painless, soft mass (most common)
Slow-growing over years
Well-circumscribed, mobile mass
Fluctuant consistency
Size varies from 1-20 cm
Multiple lesions in familial lipomatosis.
Symptoms:
Usually asymptomatic
Pain in angiolipoma variant
Cosmetic concern (large lesions)
Functional impairment (deep-seated)
Nerve compression (rare)
Mass effect symptoms.
Risk Factors:
Family history (familial lipomatosis)
Age >40 years
Genetic syndromes (Gardner syndrome)
Previous trauma (rare)
Metabolic disorders
Hereditary multiple lipomatosis.
Screening:
Physical examination for soft tissue masses
Imaging for deep-seated lesions
FNAC for diagnosis
Family screening in hereditary cases
Clinical correlation essential.
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Gross Description
Appearance:
Well-circumscribed, encapsulated yellow mass
Soft, greasy consistency
Cut surface shows yellow, lobulated appearance
Size ranges from 1-20 cm
No necrosis or hemorrhage.
Characteristics:
Encapsulated by thin fibrous capsule
Lobulated architecture
Yellow to orange color
Homogeneous consistency
Well-demarcated from surrounding tissue
Fluctuant on palpation.
Size Location:
Most common: subcutaneous tissue (80%)
Trunk and extremities (most frequent)
Intramuscular (15%)
Intermuscular (5%)
Size: 2-10 cm (average)
Giant lipomas >10 cm (rare).
Multifocality:
Solitary in 85-90% cases
Multiple lipomas in 10-15%
Familial lipomatosis (autosomal dominant)
Symmetric lipomatosis (Madelung disease)
Gardner syndrome association.
Microscopic Description
Histological Features:
FNAC shows mature adipocytes with single, peripheral nucleus
Clear cytoplasm with lipid vacuoles
Uniform cell size
Capillary blood vessels
Minimal stromal elements
No cellular atypia.
Cellular Characteristics:
Mature adipocytes with single, small, dark nucleus
Peripheral nuclear location
Clear, vacuolated cytoplasm
Cell borders well-defined
Uniform cell morphology
No pleomorphism.
Architectural Patterns:
Scattered individual adipocytes
Loose tissue fragments
Background of lipid material
Minimal fibrovascular septa
No cohesive clusters
Oil red O positive (frozen sections).
Grading Criteria:
Benign cytological features
No grading system applicable
Assessment criteria: Cell uniformity
Nuclear morphology
Absence of atypia
Mature adipocytic morphology.
Immunohistochemistry
Positive Markers:
S-100 protein (positive in adipocytes)
Vimentin (positive)
Perilipin (adipocyte marker)
Adipophilin (lipid marker)
FABP4 (fatty acid binding protein)
PPAR-gamma (adipogenic marker).
Negative Markers:
Cytokeratin (negative)
EMA (negative)
CD68 (negative)
Smooth muscle actin (negative)
Desmin (negative)
CD34 (negative in conventional lipoma).
Diagnostic Utility:
S-100 confirms adipocytic differentiation
Differentiates from other soft tissue tumors
CD34 helps distinguish spindle cell lipoma variant
MDM2/CDK4 negative (vs atypical lipomatous tumor)
Ki-67 low (<2%).
Molecular Subtypes:
Conventional lipoma: No specific markers
Spindle cell lipoma: CD34 positive
Hibernoma: UCP1 positive
Angiolipoma: CD31/CD34 in vessels
Well-differentiated liposarcoma: MDM2/CDK4 positive.
Molecular/Genetic
Genetic Mutations:
HMGA2 rearrangements (12q14-15)
HMGA1 rearrangements (6p21)
LPP gene translocations
Chromosome 12 abnormalities (50-60% cases)
Simple karyotype in most cases
PLAG1 rearrangements (rare).
Molecular Markers:
HMGA2-LPP fusion gene
Chromosome 12q13-15 rearrangements
MDM2/CDK4 not amplified
TP53 wild type
RB pathway intact
Low mutational burden.
Prognostic Significance:
Excellent prognosis
No malignant potential
Local recurrence <5% after complete excision
No metastatic risk
Good response to surgical excision
Long-term cure expected.
Therapeutic Targets:
Surgical excision is treatment of choice
Liposuction for suitable cases
No adjuvant therapy required
Observation for small, asymptomatic lesions
Complete excision prevents recurrence
No targeted therapy needed.
Differential Diagnosis
Similar Entities:
Atypical lipomatous tumor (well-differentiated liposarcoma)
Hibernoma (brown fat tumor)
Spindle cell lipoma
Angiolipoma
Lipoblastoma (pediatric)
Myxoid liposarcoma.
Distinguishing Features:
Lipoma: Mature adipocytes, no atypia, S-100+, MDM2-
Atypical lipomatous tumor: Atypical cells, enlarged nuclei, MDM2+/CDK4+
Hibernoma: Multivacuolated brown adipocytes, UCP1+
Spindle cell lipoma: CD34+ spindle cells
Angiolipoma: Prominent vessels, pain
Liposarcoma: Cellular atypia, mitoses, MDM2+.
Diagnostic Challenges:
Distinguishing from atypical lipomatous tumor (requires tissue)
Lipoma vs normal fat in FNAC
Spindle cell lipoma variants
Angiolipoma with minimal vessels
Deep-seated location interpretation
Sampling adequacy in FNAC.
Rare Variants:
Chondroid lipoma (chondroid matrix)
Myolipoma (smooth muscle component)
Osteolipoma (bone formation)
Fibrolipoma (fibrous component)
Neural fibrolipoma (nerve elements)
Pleomorphic lipoma (atypical cells).
Sample Pathology Report
Template Format
Sample Pathology Report
Complete Report: This is an example of how the final pathology report should be structured for this condition.
Specimen Information
Fine needle aspiration from [site], [size] cm soft tissue mass
Clinical History
[age]/[sex] with [duration] history of soft, mobile mass at [site]
Gross Examination
Aspirated material: [amount] of yellow, greasy material
Microscopic Examination
Smears show scattered mature adipocytes with single, peripheral nuclei and clear cytoplasm. Background shows lipid material. No cellular atypia or mitotic activity seen.
Cytological Diagnosis
Cytological features consistent with LIPOMA
Comments
The cytological features are diagnostic of benign lipoma. Clinical correlation recommended. Complete excision if symptomatic.