Lipoma is a benign neoplasm of fat cells (adipocytes).
- 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
- 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
- Multiple endocrine neoplasia
- Gardner’s syndrome
- Madelung’s disease
- Bannayan-Riley-Ruvalcaba syndrome
- Environmental factors:
- Obesity (definite)
- Possible factors:
- Diabetes and other endocrine disorders
- Corticosteroid therapy
Pathophysiology and Clinical Presentation
- 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:
- Parosteal lipoma
- 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
- 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:
- 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.”)
- Chondroid lipoma
- Ossifying lipoma
- Subcutaneous lipomas (most common):
- 2–3 cm (can be > 10 cm) soft, mobile mass, usually on the trunk or upper extremities
- Usually solitary
- 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
- A slowly growing lump (usually painless) for subcutaneous lipomas
- For other types of lipomas reported, symptoms depend on the site.
- Family history/genetic disorders
- Soft, rubbery, mobile nodule
- “Slippage sign”: Tumor will slip out from under the fingers.
- Deeper or intramuscular lipomas may present as swelling.
- 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.
- If diagnosis is uncertain and malignancy suspected
- Core needle (for deep/retroperitoneal lesions)
- Incisional biopsy is an option for large soft tissue lesions.
Expectant management: appropriate for small (< 5 cm) subcutaneous asymptomatic lipomas
- Subcutaneous lipomas:
- 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:
- Seroma formation
- Hematoma formation
- 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
- 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.
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