Lipoma

Epidemiology and Etiology:

• Most common soft tissue tumour of adulthood
• mid-adulthood
• angiolipoma
• young adults (rare in children and in adults over 50)
• myolipoma:
• adults (40s-50s most commonly)
• intramuscular and intermuscular (infiltrating) lipoma:
• adults (30-60 years)
• children occasionally (distinguish from diffuse lipomatosis and lipoblastomatosis)
• intramuscular lipoma is more common than intermuscular lipoma
• but many lesions involve both muscular and intermuscular tissues
• chondroid lipoma:
• adult women
• spindle cell lipoma and pleomorphic lipoma:
• 45-60y
• men (striking predilection)
• familial cases with multiple lesions have been reported

Common sites:

• subcutis of proximal extremities and trunk
• intramuscular infrequently
• myolipoma:
• retroperitoneum
• abdomen
• pelvis
• abdominal wall
• rarely subcutaneous
• intramuscular / intermuscular lipoma:
• large muscles of the extremities (thigh, shoulder, upper arm)
• paraspinal muscles
• larynx
• deep location often
• chondroid lipoma:
• subcutaneous - limbs and limb girdles
• deeper soft tissues
• spindle cell lipoma and pleomorphic lipoma:
• posterior neck
• shoulder
• back

Gross features:

• angiolipoma
• multiple usually (2/3)
• painful
• intramuscular and intermuscular lipomas:
• firm spherical mass on muscle contraction
• minute to 20cm or more
• cut section:

• gradual replacement of muscle tissue by fat
• may extend beyond the muscle fascia into the intermuscular connective tissue spaces
• striated appearance often (muscle fibres with fat in between)

• spindle cell lipoma:
• similar to lipoma but with gray-white gelatinous areas

Histologic features:

• conventional:
• well-encapsulated
• mature adipocytes
• no pleomorphism
• no significant variation in cell size
• angiolipoma:
• mature fat separated by a branching network of small vessels
• perivascular and interstitial fibrosis maybe
• fibrin thrombi within the vascular channels characteristically
• mast cell often conspicuous
• myolipoma:
• variable admixture of mature fat and bundles or sheets of well-differentiated smooth muscle
• sieve-like appearance at low-power
• no nuclear atypia, no floret-like giant cells, no lipoblasts
• absence of medium-caliber arteries with thick musclular walls (characteristic of angiomyolipoma)
• intramuscular lipoma / intermuscular lipoma (infiltrating lipoma):
• diffuse infiltration of muscle by lipocytes
• bland mature lipocytes
• no lipoblasts or cells with atypical nuclei (consider WD liposarcoma)
• atrophy of muscle fibres to varying degrees (often few changes though)
• chondroid lipoma:
• may be mistaken for a myxoid liposarcoma or chondrosarcoma
• lobular pattern
• strands and nests of round cells
• some with eosinophilic granular cytoplasm
• others with lipid vacuoles resembling lipoblasts
• not pleomorphic
• myxochondroid or hyalinized fibrous background
• no significant mitotic activity
• thick-walled blood vessels and cavernous thin-walled vascular spaces
• spindle cell lipoma:
•  characterized by replacement of mature fat by collagen-forming spindle cells
• varying amounts of mature fat and spindle cells arranged in short parallel bundles
• often striking nuclear palisading reminiscent of a neural tumour
• spindle cells:
• uniform
• single elongated nucleus
• inconspicuous nucleoli
• narrow, bipolar cytoplasmic processes
• floret-like multinucleated giant cells in some cases (similar to those in pleomorphic lipoma)
• histologic spectrum between spindle cell lipoma and pleomorphic lipoma
• mast cells conspicuous in almost all cases
• mucoid matrix composed of hyaluronic acid and mixed with varying number of birefringent “ropey” collagen fibres
• vascular pattern:
• inconspicuous
• few small or intermediate-size thick-walled vessels
• some have a prominent plexiform pattern reminiscent of myxoid liposarcoma
• some have a hemangiopericytoma-like pattern
• inconspicuous mitoses
• pleomorphic lipoma:
• variable amounts of mature fat
• scattered, bizarre giant cells
• frequently with a concentric floret-like arrangement of hyperchromatic nuclei
• deeply eosinophilic cytoplasm at the centre
• ropey collagen bundles characteristic
• extensive myxoid change may be seen
• mast cells maybe
• other histologic types
• fibrolipoma
• myelolipoma

Immunophenotype:

Marker:	Sensitivity:	Specificity:
Vimentin
S100 (variable staining) (negative in spindle cells)
CD34 (spindle cell and pleomorphic lipoma) (strong)	Almost all spindle cell lipomas

Molecular features:

•  conventional lipoma:
• translocations involving 12q13-15
• interstitial deletions of 13q
• rearrangements involving 6p21-23
• angiolipoma:
• normal karyotype
• myolipoma:
• chondroid lipoma:
• t(11:16) involving 11q13
• spindle cell and pleomorphic:
• loss of 16q13
• unbalanced 13q alterations (less frequent)
• absence of giant marker and ring chromosomes usually
• lipoblastoma / lipoblastomatosis:
• translocations involving 8q11-13
• hibernoma:
• translocations involving 11q13
• translocations involving 10q22

Other features:

• all benign
• spindle cell lipoma:
• EM – abundant rough ER
• Well-formed golgi
• Non-membrane-bound lipid vacuoles
• Interrupted basal lamina in some

References:

• Robbins 2005
• Enzinger and Weiss’ Soft Tissue Tumours (2001)