Lipomas are benign tumors of slow-growing, mature fat cells. They are the most common soft tissue tumor.

Clinically, lipomas present as soft, rubbery, freely mobile subcutaneous masses without overlying skin change. They are most often solitary but can be multiple. They can occur anywhere on the body where fat is found, with the highest predilection for the trunk, extremities, buttocks, and abdominal wall; they rarely occur on the head and neck. They can be quite small (1-2 mm) to quite large (greater than 10 cm) and occasionally are multilobular, weighing as much as 1 kg. They are usually asymptomatic; however, large tumors that compress nerves or limit normal tissue movement can cause lymphedema with discomfort and pain. Tumors with a vascular component (angiolipomas) also tend to be painful; pain may be precipitated by cold. They grow slowly to a stable size and do not spontaneously regress.

Solitary idiopathic lipomas are slightly more common in women, whereas multiple lipomas are more often seen in men. These lesions commonly present between the third and seventh decade, rarely before puberty. They may grow with weight gain but do not shrink with weight loss. Since the fat in lipomas is not available for metabolism, excessive weight loss may make the lesion more prominent. Deeper lesions may become more visible on muscle contraction. Some lipomas are familial and have autosomal dominant inheritance. Malignant transformation is rare.

The etiology of lipomas continues to be investigated. An increased incidence is associated with diabetes, obesity, and hypercholesterolemia. Fat cells of lipoma reflect an increased adipocyte turnover rate along with transcriptional changes distinct from the adipose tissue hypertrophy seen in obesity. There also appears to be less adipocyte apoptosis, owing to the tumor-like growth. In one case report, growth of multiple subcutaneous tumors was associated with protease inhibitors (PI). The homology between the PI catalytic site and the proteins involved in lipid metabolism may be significant in this finding. Moreover, PIs are known to induce several effects, including hyperlipidemia, diabetes, and lipomatosis. Another hypothesis suggests that lipomas occur as a result of trauma, which may precipitate the conversion of preadipocytes to mature adipocytes secondary to inflammatory changes.

Multiple lipomas are associated with several rare syndromes:

• Diffuse lipomatosis – Characterized by the infiltration of nonencapsulated fat into multiple tissues including muscle, skin, fascia, and bone. This entity can be seen in association with tuberous sclerosis.
• Familial multiple lipomatosis – Characterized by multiple encapsulated lipomas in several family members.
• Proteus syndrome – Characterized by multiple hamartomas (including lipomas) and disproportionate overgrowth of multiple tissues.
• Hemihyperplasia / multiple lipomatosis syndrome – Characterized by multiple lipomas, asymmetric tissue overgrowth, capillary malformations, and accentuation of plantar skin creases.
• Benign symmetric lipomatosis (Madelung disease) – Characterized by symmetric fat deposits around and above the shoulders associated with alcohol use disorder.
• Adiposis dolorosa (Dercum disease) – Characterized by multiple painful lipomas in postmenopausal women associated with weakness and depression.
• Gardner syndrome – Characterized by multiple lipomas, colon polyposis, odontomas, epidermoid cysts, osteomas, leiomyomas, desmoid fibromatosis, and hypertrophy of the retinal pigment epithelium.
• Bannayan-Riley-Ruvalcaba syndrome – Characterized by multiple lipomas, macrocephaly, intestinal polyposis, lentigines of the penis, and hemangiomas.