Benign tumors and reactive conditions
Lipoma
Definition
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A benign dermal or subcutaneous mesenchymal tumor composed of mature adult-type adipose tissue
Clinical features
Epidemiology
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The most commonly encountered mesenchymal tumor
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Majority of patients are adults
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No sex predilection
Presentation
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Most cases are asymptomatic
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Occasionally painful when compressing nerves
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Slowly growing, but size is variable
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Usually solitary
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Multiple lipomas may be associated with neurofibromatosis, multiple endocrine neoplasia syndromes, or Bannayan syndrome (hemangiomas and multiple lipomas)
Prognosis and treatment
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Treated mostly for aesthetic reasons
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Cured by conservative surgical excision
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Recur only if incompletely excised and then only rarely
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Not associated with progression to liposarcoma
Pathology
Histology
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Resemble normal adult-type adipose tissue
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Lobules of adipocytes bounded by thin fibrous septa
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Adipocytes have large single lipid vacuole in their cytoplasm and eccentrically located small nuclei
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May show areas of fat necrosis and increased fibrosis after trauma
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No atypia, pleomorphism, mitoses, or hyperchromasia
Immunohistochemistry/special stains
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Adipocytes express S100 protein
Genetic profile
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True neoplasms
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Translocation at 12q14 involving HMGA2 gene is the most common aberration, often with LPP (3q27)
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6p21-23 rearrangements involving HMG1B gene are also seen
Main differential diagnoses
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Normal adipose tissue
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Pseudolipomatosis cutis
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Atypical lipomatous tumor
Lipomatosis
Definition
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Diffuse overgrowth of mature adipose tissue
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The disease presents in two forms: multiple symmetric lipomatosis (Launois-Bensaude syndrome) and asymmetric lipomatosis
Clinical features
Epidemiology
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Rare disorder, commonly associated with obesity
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Some familial cases show evidence of autosomal-dominant mode of inheritance
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Common in children less than 2 years of age
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Adults can occasionally be affected
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Can present in trunk, head and neck, and extremities
Presentation
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Painless, diffuse enlargement of affected area due to fat accumulation
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Rapid growth is characteristic
Prognosis and treatment
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Recurrence after palliative surgical removal is common
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Usually cured by radical surgical excision
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Although benign, can be fatal due to laryngeal obstruction
Pathology
Histology
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Sheets and lobules of mature adipocytes
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No encapsulation
Immunohistochemistry/special stains
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The mature adipocytes strongly express S100 protein
Main differential diagnoses
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Intramuscular lipoma
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Atypical lipomatous tumor
Multiple symmetric lipomatosis (Launois-Bensaude syndrome)
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The most common form of lipomatosis characterized by symmetrical deposition of adipose tissue in the trunk and head and neck area
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Two variants exist: diffuse and localized
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The diffuse variant usually affects the trunk of male children
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The localized variant characteristically affects the cervical area of adult males and can present with laryngeal obstruction
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Patients can present with neuropathy
Asymmetric lipomatosis
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Asymmetric deposition of adipose tissue affecting any part of the body
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Less common than the symmetric form of lipomatosis
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Can present with macrodactyly or gigantism of affected body area
Lipomatosis of Nerve
Definition
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A rare hamartomatous condition characterized by fatty infiltration of epineurium of a major nerve
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Also known as fibrolipoma of nerve, fibrolipomatous hamartoma of nerve, macrodystrophia lipomatosa, perineural lipoma, and intraneural lipoma
Clinical features
Epidemiology
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Affected individuals are mostly newborns and young children
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Female predilection
Presentation
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The median nerve is the most common location, followed by the ulnar nerve
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Slowly growing masses or macrodactyly
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Can be asymptomatic or associated with pain, paresthesia, or motor deficit
Prognosis and treatment
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Benign condition, but management can be challenging
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Surgical excision may result in permanent sensory and/or motor deficit
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Incomplete removal is associated with high rate of recurrence
Pathology
Histology
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Infiltration of epineurium and perineurium by mature adipose tissue admixed with fibrous tissue
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Metaplastic bone formation is rarely seen
Main differential diagnoses
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Intraneural lipoma
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Diffuse lipomatosis
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Traumatic neuroma
Adiposis Dolorosa
Definition
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A rare condition characterized by multiple painful plaques of accumulated subcutaneous fat
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Also known as Dercum disease
Clinical features
Epidemiology
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Rare disorder, commonly associated with obesity
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Affected individuals are mainly postmenopausal women
Presentation
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Painful and tender plaques
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Usually multiple
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Most cases are located in the lower extremities and pelvic area
Prognosis and treatment
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Benign condition cured by surgical excision
Pathology
Histology
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Subcutaneous lobules of mature adipocytes
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Fat necrosis can be seen
Immunohistochemistry/special stains
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The mature adipocytes strongly express S100 protein
Main differential diagnoses
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Lipoma
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Other forms of lipomatosis
Piezogenic Pedal Papules
Definition
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Multiple papules located at the internal aspects of the heels
Clinical features
Epidemiology
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Commonly affects athletes, particularly marathon runners
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Associated with Ehlers-Danlos and Prader-Willi syndromes as well as rheumatoid arthritis
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Rarely familial
Presentation
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Multiple asymptomatic skin-colored papules
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Lesions become more pronounced after prolonged standing
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Pressure may elicit pain
Prognosis and treatment
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Conservative management includes intralesional steroid and anesthetic injection or compression treatment
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Therapy outcome might not be satisfactory
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Surgical intervention is usually more effective
Pathology
Histology
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Mature adipose tissue herniating into the dermis
Immunohistochemistry/special stains
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The mature adipocytes strongly express S100 protein
Main differential diagnoses
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Intradermal lipoma
Fat Necrosis of the Morbidly Obese
Definition
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Necrosis of subcutaneous adipose tissue as a result of obesity
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It is thought to result from medial calcification of subcutaneous arterioles with subsequent ischemic necrosis of fat
Clinical features
Epidemiology
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Usually affects distal extremities
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Can affect thighs, abdomen, and other areas
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Associated with chronic renal failure and hyperphosphatemia
Presentation
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Painful induration and purplish discoloration of skin
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Associated skin necrosis can lead to ulcer formation
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Skin ulceration progresses rapidly as level of phosphate peaks
Prognosis and treatment
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High mortality rate due to development of secondary infection
Pathology
Histology
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Massive necrosis of subcutaneous adipose tissue with minimal inflammatory reaction
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Calcification of subcutaneous arterioles with luminal narrowing
Nevus Lipomatosus Superficialis
Definition
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A rare cutaneous hamartomatous lesion characterized by multiple small collections of adipocytes within the dermis
Clinical features
Epidemiology
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Affected individuals are children or adolescents
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Adults present with solitary variant
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No sex predilection
Presentation
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Multiple painless papules
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Size ranges from few millimeters up to 2 cm
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Most commonly affect the gluteal area, upper thighs, and lower back
Prognosis and treatment
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Cured by simple excision
Pathology
Histology
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Nonencapsulated lesion composed of variable number of fatty lobules deposited in the upper portion of the dermis
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The lobules condense around blood vessels and are interspersed by loose fibrous tissue
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Secondary changes include perifollicular fibrosis, follicular hyperkeratosis, and epidermal atrophy
Immunohistochemistry/special stains
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The mature adipocytes strongly express S100 protein
Main differential diagnoses
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Lipofibroma
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Fibroepithelial polyp
Fibrohistiocytic Lipoma
Definition
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A rare variant of lipoma characterized by the presence of fibrohistiocytic proliferation in addition to the conventional lipomatous component
Clinical features
Epidemiology
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Strong predilection for young adult males
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Anterior aspect of trunk is most commonly affected
Presentation
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Small, painless, subcutaneous lesions
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Usually solitary
Prognosis and treatment
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Cured by local excision
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No reports of recurrence or metastasis
Pathology
Histology
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Well-circumscribed subcutaneous lipomas with fibrohistiocytic foci
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Fibrohistiocytic foci depict slightly plump, bland-looking spindle cells arranged in fascicles in a collagenous stroma
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May exhibit minimal inflammation and/or hemosiderin deposition
Immunohistochemistry/special stains
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Tumor cells express CD34 and calponin
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Fibrohistiocytic component lacks S100 protein expression
Main differential diagnoses
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Benign fibrous histiocytoma
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Spindle cell lipoma
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Atypical lipomatous tumor
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Dermatofibrosarcoma protuberans
Lipofibromatosis
Definition
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A benign fibrofatty neoplasm presenting in childhood
Clinical features
Epidemiology
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More common in males
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Develops in infancy and childhood (mean age 1 year)
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Already present at birth in a substantial number of cases (about 15%)
Presentation
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Slowly growing and ill-defined painless mass
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Size of the lesion usually between 2 and 5 cm
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Wide anatomical distribution with predilection for hands and feet
Prognosis and treatment
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Benign proliferation, associated with high rate of nondestructive recurrence(s) (about 70%)
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Diffuse involvement of the limbs can result in deformities of the underlying bone
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Risk factors for local recurrence(s) include congenital onset, male gender, incomplete excision, and increased mitotic rate
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Complete surgical excision generally curative, but may be difficult to achieve
Pathology
Histology
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Poorly delineated proliferation in the subcutis and/or deep soft tissues
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Two main components are mature fat and fibroblastic spindle-shaped cells
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Adipose tissue component
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Integral part of the tumor
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Usually the predominant component
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Composed of mature adipocytes
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Variably sized and poorly demarcated lobules
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Fibroblastic spindle cell component
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Formation of fascicles, typically growing along the fat septa
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Mild cytological atypia
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Mitoses absent or rare in the majority of cases
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Increased mitotic rate associated with increased likelihood of local recurrence
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Small to moderate amounts of collagen
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Focal myxoid change occasionally present
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Univacuolated cells
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Usually encountered at the interface between the fibroblastic and adipocytic component
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Characterized by a single vacuole in the cytoplasm
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Likely represent degenerating adipocytes, lipid-rich fibroblasts, or transitional cells between fibroblast and adipocyte
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Seen in the majority of lesions
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Entrapment of normal structures, including vessels, nerves, skin adnexa, and skeletal muscle within the tumor
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Pigmented melanocytic spindled or dendritic cells (similar to the ones found in Bednar tumor) exceptionally seen among the lesional cells
Immunohistochemistry/special stains
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Not contributory
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Focal S100 protein positivity in the spindle cell component occasionally detected
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β-catenin negative
Main differential diagnoses
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Juvenile hyaline fibromatosis
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Fibrous hamartoma of infancy
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Calcifying aponeurotic fibroma
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Lipoblastoma
Lipoblastoma/Lipoblastomatosis
Definition
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Lipoblastoma and lipoblastomatosis are lesions characterized by the presence of lobules of fetal-type adipose tissue
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Lipoblastoma is localized while lipoblastomatosis is the diffuse form of the disease
Clinical features
Epidemiology
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Lipoblastoma/lipoblastomatosis are considered the fetal counterpart of the adult lipoma and lipomatosis, respectively
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Most patients are infants or young children less than 3 years of age
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Boys are much more commonly affected than girls
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The extremities are the most common location followed by the mediastinum, the trunk, the retroperitoneum, and the head and neck
Presentation
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Discrete or diffuse superficial, painless masses of fatty tissue
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Small lesions, usually measure 2 to 5 cm
Prognosis and treatment
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Benign condition
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Localized form of disease is readily cured by total resection
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Lipoblastomatosis may recur on some occasions mainly due to inability to completely excise the lesion
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Untreated cases may undergo maturation into an adult-type lipoma/lipomatosis
Pathology
Histology
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Lobules of mature adipocytes admixed with lipoblasts in varying stages of development
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Number of lipoblasts present in the tumor tends to decrease as the patient age increases
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Lipoblastomatosis is less lobulated and may infiltrate skeletal muscle tissue
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May have prominent fibrous septa and myxoid matrix
Genetic profile
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8q11~13 rearrangements resulting in HAS2 – PLAG1 or COL1A2 – PLAG1 fusion genes is common
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Gain of chromosome 8 seen in some cases
Main differential diagnoses
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Atypical lipomatous tumor
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Myxoid liposarcoma
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Lipofibromatosis
Lipoblastoma-Like Tumor of the Vulva
Definition
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An extremely rare mesenchymal neoplasm characterized by adipocytic differentiation
Clinical features
Epidemiology
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Adolescent and young adult females
Presentation
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Superficial, painless vulvar masses
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Can be cystic
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Can have mucoid consistency
Prognosis and treatment
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Cured with surgical resection
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Recurrences and metastases have not been reported
Pathology
Histology
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Well-circumscribed and lobulated
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Lobules are composed of uniform spindle cells showing slightly eosinophilic cytoplasm and elongated nuclei and variable number of lipoblasts separated by thin fibrous septa
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Lacks nuclear atypia and hyperchromasia
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Plexiform, “chicken-wire” vascular networks are commonly seen
Immunohistochemistry/special stains
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Fat cells express S100 protein
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No CD34 expression
Main differential diagnoses
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Lipoblastoma
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Myxoid liposarcoma
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Spindle cell lipoma
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Aggressive angiomyxoma
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Angiomyofibroblastoma
Angiolipoma
Definition
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A benign dermal or subcutaneous encapsulated mesenchymal tumor composed of mature adult-type adipose tissue and thin-walled small blood vessels
Clinical features
Epidemiology
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Mainly affects adolescents and young adults
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Predilection for females
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Familial in 5% of cases and shows autosomal-dominant mode of transmission
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Most commonly encountered in the upper limbs, trunk, and distal extremities
Presentation
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Many are painful
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Commonly multiple
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Size ranges from a few millimeters to 2 cm
Prognosis and treatment
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Cured by conservative, complete surgical excision
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No recurrence or metastasis reported
Pathology
Histology
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Encapsulated tumors composed of adipocytes and variable number of thin-walled small blood vessels
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Blood capillary vessels commonly show microthrombi
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No atypia, pleomorphism, mitoses, or hyperchromasia
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A rare cellular variant is recognized
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Older lesions can show increased fibrosis
Immunohistochemistry/special stains
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The adipocytes express S100 protein
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The endothelial cells are highlighted by CD34, CD31, and ERG
Genetic profile
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Low-level mutations of protein kinase D 2 have been demonstrated in 80% of cases
Main differential diagnoses
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Lipoma
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Hemangioma
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Kaposiform hemangioendothelioma
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Kaposi sarcoma
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Angiosarcoma
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