Lipoma

A lipoma is a benign neoplasm of fat cells (adipocytes) and the most common soft tissue tumor in adults. The etiology is unknown, but obesity is a predisposing factor; genetics also play a role, with multiple lipomas occurring in various inherited disorders. Lipomas can arise in any site with adipose tissue (including the gastrointestinal tract, chest cavity, retroperitoneum, and glands), but are most common in subcutaneous tissues of the trunk or proximal extremities. The treatment is not necessary for small asymptomatic lipomas. Surgical excision is the treatment if there is a cosmetic, functional, or diagnostic concern.

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Overview

Definition

Lipoma is a benign neoplasm of fat cells (adipocytes).

Epidemiology

  • The most common benign soft tissue tumor
  • Age: mostly adults, age > 40 years; rare in children
  • Women > men
  • Associated with obesity
  • No gender or ethnic preference
  • Multiple lipomas account for 5% of cases: 
    • Often familial
    • Often associated with genetic disorders

Etiology

  • Unknown for most lipomas
  • Genetics in some cases: 
    • Solitary lipomas: HMGA2-LPP fusion gene defect in chromosome 12
    • Some are associated with genetic syndromes:
      • Familial multiple lipomatosis
      • Adiposis dolorosa
      • Neurofibromatosis
      • Multiple endocrine neoplasia
      • Gardner’s syndrome
      • Madelung’s disease
      • Bannayan-Riley-Ruvalcaba syndrome
  • Environmental factors: 
    • Obesity (definite)
    • Possible factors: 
      • Diabetes and other endocrine disorders
      • Trauma
      • Radiation
      • Corticosteroid therapy

Pathophysiology and Clinical Presentation

Pathophysiology

Anatomic sites:

  • Benign neoplasm composed of mature adipose cells
  • Can arise in any site where there is adipose tissue:
    • Most arise in subcutaneous tissue, on the trunk or upper extremities
    • Submucosal gastrointestinal sites, from the esophagus to the lower intestine
    • Less common sites: 
      • Retroperitoneal
      • Intraglandular
      • Intramuscular 
      • Parosteal lipoma

Morphology:

  • Size: 
    • Usually 2–3 cm
    • Range from < 1 cm to > 10 cm, especially if in deeper tissues
  • Single most common, but may be multiple
  • Soft, rubbery
  • Mobile (not fixed to surrounding tissues)
  • Round, oval, or multi-lobulated
  • Bright yellow homogeneous fat with a fine fibrous capsule (superficial lesions) and trabeculae
  • Greasy cut surface

Histology:

  • Mature adipose tissue, with adipocytes showing no atypia, but 2–5x variation in cell size
  • Lobular architecture with thin septae
  • No mitotic figures
  • Scant vascularity
  • Thin fibrous capsule
  • Fat necrosis: 
    • More likely in larger lipomas
    • Focal, with histiocytes +/- calcification
  • Some histologic variants:
    • Angiolipoma: 
      • Small, well-circumscribed, subcutaneous tumor
      • Mature adipose cells, capillary vessels with fibrin thrombi
      • Angiolipoma is 1 of 5 of the most common painful skin tumors. (The other 4 are: neuroma (traumatic), glomus tumor, eccrine spiradenoma, and leiomyoma (vascular), creating the mnemonic “ANGEL.”)
    • Fibrolipoma
    • Chondroid lipoma
    • Ossifying lipoma     

Clinical Presentation

  • Subcutaneous lipomas (most common):
    • 2–3 cm (can be > 10 cm) soft, mobile mass, usually on the trunk or upper extremities
    • Usually solitary 
    • Painless
    • No overlying skin changes
    • Can occur appear anywhere on the body
  • Gastrointestinal lipomas: 
    • Intestinal obstruction or intussusception (nausea/vomiting, abdominal pain)
    • Gastrointestinal hemorrhage (from mucosal ulceration)
  • Other sites, e.g. retroperitoneum, body cavities: compression effects on adjacent organs

Diagnosis and Management

Diagnosis

History:

  • A slowly growing lump (usually painless) for subcutaneous lipomas
  • For other types of lipomas reported, symptoms depend on the site.
  • Family history/genetic disorders

Physical exam:

  • Soft, rubbery, mobile nodule
  • “Slippage sign”: Tumor will slip out from under the fingers.
  • Deeper or intramuscular lipomas may present as swelling.

Imaging:

  • Not required for most subcutaneous lipomas
  • Ultrasound (US):
    • Can be used for subcutaneous lesions with atypical appearance
    • For deep soft tissue or retroperitoneal lesions
  • Computed tomography (CT):
    • Density < 50 Hounsfield units is indicative of a fatty tumor
    • However, cannot reliably distinguish lipoma from liposarcoma
  • Magnetic resonance imaging (MRI): also cannot reliably rule out malignancy

Endoscopy and endoscopic ultrasound (EUS):

  • From the esophagus to the colon
  • Appear as smooth, round submucosal nodules
  • Possible mucosal ulceration if large
  • EUS can help confirm the diagnosis of a lipoma.

Biopsy:

  • If diagnosis is uncertain and malignancy suspected
  • Core needle (for deep/retroperitoneal lesions)
  • Incisional biopsy is an option for large soft tissue lesions.
Large submucosal gastric lipoma

A large (12 x 8 x 6 cm) submucosal gastric lipoma (arrow)
Some contrast medium has leaked into the lipoma through focal ulcerated areas in the overlying mucosa, and can be seen tracking into the center from the surface.

Image: “Gastric lipoma presenting as a giant bulging mass in an oligosymptomatic patient: a case report” by Neto FA, Ferreira MC, Bertoncello LC, Neto AA, de Aveiro WC, Bento CA, Cecchino GN, Rocha MA. License: CC BY 2.0.

Management

Expectant management: appropriate for small (< 5 cm) subcutaneous asymptomatic lipomas

Surgical excision:

  • Subcutaneous lipomas:
    • Indications: 
      • Pain 
      • Cosmesis
      • Diagnosis in doubt
      • Increase in size
    • Excision needs to involve the fibrous capsule to reduce recurrence. 
  • Gastrointestinal lipomas:
    • Surgical resection if symptomatic/obstructive
    • Endoscopic removal may be feasible if small. 
  • Other sites (retroperitoneum, intraglandular): 
    • Resection usually needed if malignancy is in question
    • Always needed if symptomatic
    • Biopsy prior to resection, if feasible.

Complications of surgery:

  • Scarring
  • Seroma formation
  • Hematoma formation
  • Infection
Submucosal gastric lipoma

Submucosal gastric lipoma (surgical specimen)
Note the thickened wall of the proximal gastric wall on the right side, and the attenuated focally ulcerated overlying gastric mucosa.

Image: “Gastric lipoma presenting as a giant bulging mass in an oligosymptomatic patient: a case report” by Neto FA, Ferreira MC, Bertoncello LC, Neto AA, de Aveiro WC, Bento CA, Cecchino GN, Rocha MA. License: CC BY 2.0.

Differential Diagnosis

Subcutaneous lesions

  • Keratin-filled cysts: benign cysts lined by epidermal cells and filled with keratin. Also called by the misnomer “sebaceous cysts.” Two types: 
    • Epidermal inclusion cyst: formed by the invagination and cystic expansion of the epidermis. Commonly found on the head, face, or neck. Firm and has a central punctum; prone to rupture.
    • Trichilemmal cyst (pilar cyst): originates from the outer hair root sheath. Ninety percent occur on the scalp or scrotum. Often have an autosomal dominant pattern of inheritance. Usually firm with a thick cyst wall.
  • Abscess: pus-filled cyst with fibrotic wall; indurated, fixed, and erythematous; can be infectious (bacterial or fungal) or sterile (if an irritant drug or substance is injected into the skin, resulting in aseptic inflammation and abscess formation).
  • Well-differentiated liposarcoma (“atypical lipomatous tumor”): mostly in the deep thigh or retroperitoneum; multi-lobulated, with focal firm areas. Atypical spindle cells present; overexpresses MDM2 (which blocks p53 tumor suppressor function). Well-differentiated liposarcoma tend to recur if not completely resected. Do not metastasize if dedifferentiation into higher-grade elements does not occur. 
  • Pseudolipoma: tension on subcutaneous fat tissue by adjacent fibrous bands, which may be post-traumatic or related to scirrhous breast cancer

Deep lesions

  • Liposarcoma: a malignant neoplasm of adipocytes; usually arises de novo (not from a lipoma). Often intramuscular or retroperitoneal. Appears heterogeneous on imaging. Biopsy is needed for confirmation. 
  • Hibernoma: a benign tumor arising from brown fat; most commonly in inter-scapular area but also in the neck, axillae, thigh, or intrathoracic areas. Hibernomas have higher vascularity and density compared with lipomas.
  • Mammary hamartoma: a benign proliferation of fibrous, glandular, and fatty tissue surrounded by a thin, fibrous capsule.
  • Gastrointestinal submucosal tumors: include leiomyomas, granular cell tumors, gastrointestinal stromal tumors (GISTs), metastatic lesions, and others. Diagnosed by endoscopy, EUS, and biopsy.
  • Spermatic cord “lipoma”: not a true lipoma (i.e., a benign neoplasm of adipose tissue), but is only preperitoneal, mature, benign, fatty tissue that has gained access to the spermatic cord.

References

  1. Nickloes, T.A. (2020). Lipomas: Background, pathophysiology, etiology. https://emedicine.medscape.com/article/191233-overview#a4
  2. Kim, K.S., & Yang, H.S. (2014). Unusual locations of lipoma: Differential diagnosis of head and neck mass. Australian Family Physician, 43(12), 867–870.  https://www.racgp.org.au/afp/2014/december/unusual-locations-of-lipoma-differential-diagnosis-of-head-and-neck-mass/#5
  3. Lipoma (2020). In Kumar, V., Abbas, A. K., Aster, J.C., (Eds.). Robbins & Cotran Pathologic Basis of Disease. (10th ed., p. 1209). Elsevier, Inc.
  4. Rouse, R.V. (2017). Lipoma—Surgical pathology criteria—Stanford University School of Medicine. http://surgpathcriteria.stanford.edu/softfat/lipoma/
  5. Fairweather, M., & Raut, C.P. (2019). To Biopsy, or Not to Biopsy [a suspected retroperitoneal sarcoma]: Is There Really a Question?. Ann Surg Oncol 26, 4182–4184. https://doi.org/10.1245/s10434-019-07723-y
  6. Rotunda, A. M., Ablon, G., & Kolodney, M. S. (2005). Lipomas treated with subcutaneous deoxycholate injections. Journal of the American Academy of Dermatology, 53(6), 973–978. https://doi.org/10.1016/j.jaad.2005.07.068

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