Lipomas 

Lipomas are the most common soft-tissue tumor. These slow-growing, benign fatty tumors form soft, lobulated masses enclosed by a thin, fibrous capsule. Although it has been hypothesized that lipomas may rarely undergo sarcomatous change, this event has never been convincingly documented. It is more probable that lipomas are at the benign end of the spectrum of tumors, which, at the malignant end, include liposarcomas (see Pathophysiology). Because more than half of lipomas encountered by clinicians are subcutaneous in location, most of this article will be devoted to that subgroup. Additional information about other locations (eg, intramuscular, retroperitoneal, gastrointestinal) will be included as appropriate.

Recent studies

Pang et al compared outcomes in 238 patients who underwent total or near-total (T/NT) resection for dorsal, transitional, or chaotic spinal cord lipomas (with 16-y follow-up), along with complete reconstruction of the neural placode, with results from 116 patients who underwent partial resection for spinal cord lipomas (with 11-y follow-up). Although in the T/NT and partial resection groups the rate of immediate symptom stabilization or improvement was similar (more than 95%), the combined cerebrospinal fluid leakage and wound complication rate was only 2.5% for T/NT resections, compared with 6.9% for partial resections. Moreover, the overall progression-free survival probability (Kaplan-Meier analysis) was 82.8% for T/NT resection patients at 16 years postoperative, compared with 34.6% for partial resection patients at 10.5 years postoperative.

Evidence indicated that the superior results in the T/NT resection patients were associated with the fact that lower cord-sac ratios were achieved in these patients than in the partial-resection group.^{[1, 2]}

Lipomas must be differentiated from other masses or tumors.

In the subcutaneous location, the primary differential diagnosis is a sebaceous cyst or an abscess. Sebaceous cysts are also rounded and subcutaneous. They can be differentiated from lipomas by their characteristic central punctum and the surrounding induration. Treatment requires removal of a small ellipse of overlying skin to avoid entering the cyst. Abscesses typically have overlying induration and erythema. Incision and drainage is the appropriate management.

Hibernomas are uncommon tumors that arise from brown fat. They are also benign but with a slightly greater tendency to bleed during excision and to recur if intralesional excision is performed.

Atypical lipomatous tumors are considered to be well-differentiated liposarcomas. They have a predilection for local recurrence but do not generally metastasize. This diagnosis should be suspected when a fatty tumor is encountered in an intramuscular or retroperitoneal location.

Liposarcomas are malignant tumors that arise from adipocytes. They may recur locally and may metastasize. Fatty tumors of the retroperitoneum or in intramuscular locations should be considered to be potential liposarcomas until proven otherwise.

In the breast, a lipoma will be mammographically radiolucent. It must be differentiated from a similar benign tumor, a mammary hamartoma, and a pseudolipoma (a soft-tissue mass that may surround a small, scirrhous cancer).

Conversely, lipomatous lesions in the adrenal gland that have calcifications on radiologic examinations have been confused with teratoma. Many of these are angiomyolipomas.

In the spermatic cord, a finger of retroperitoneal fat termed a "lipoma of the cord" is frequently encountered during hernia repair. Removal is advocated to allow the internal inguinal ring to be tightened around the cord and to minimize the risk of recurrence of the hernia. During laparoscopic exploration for a palpable inguinal mass, no identifiable peritoneal orifice may be found if the inguinal mass purely consists of a lipoma of the cord.

Frequency

Lipomas occur in 1% of the population. Most of these are small subcutaneous tumors that are removed for cosmetic reasons. These subcutaneous lipomas will be considered separately from lipomas in other locations in the discussion below. In the intestine, lipomas constitute 16% of benign, small neoplasms; this percentage is less than that of leiomyomas (18%) and more than that of adenomas (14%).

Speculation exists regarding a potential link between trauma and subsequent lipoma formation.^[3] One theory suggests that trauma-related fat herniation through tissue planes creates so-called pseudolipomas. It has also been suggested that trauma-induced cytokine release triggers pre-adipocyte differentiation and maturation. To date, no definitive link between trauma and lipoma formation has been prospectively demonstrated.

While the exact etiology of lipomas remains uncertain, an association with gene rearrangements of chromosome 12 has been established in cases of solitary lipomas, as has an abnormality in the HMGA2-LPP fusion gene.^[4]

Lipomas are common benign mesenchymal tumors. They may develop in virtually all organs throughout the body.

In the gastrointestinal tract, lipomas present as submucosal fatty tumors. The most common locations include the esophagus, stomach, and small intestine. Symptoms occur from luminal obstruction or bleeding.

Duodenal lipomas are mostly small but may become pedunculated with obstruction of the lumen. They may cause pain, obstructive jaundice, or intussusception in younger patients.^[5] Mucosal erosion over the lipoma may lead to severe bleeding, as demonstrated in the image below. Small intestinal lipomas occur mainly in elderly patients. They tend to be pedunculated submucosal lesions. They are more common in the ileum than in the duodenum or jejunum. As with duodenal lipomas, severe hemorrhage or intussusception may occur. Colonic lipomas are usually discovered on endoscopy. Gentle palpation with a biopsy forceps reveals the soft nature of the submucosal mass. A biopsy specimen of the mucosa may reveal underlying fat, the so-called naked fat sign. As with lipomas in other locations, colonic lipomas may cause pain with obstruction or intussusception.

As noted above, a fatty protrusion of preperitoneal fat termed a "lipoma of the spermatic cord" is a common finding on groin exploration for hernia repair.

Numerous case reports document the presence of lipomas in other, rare locations, with these tumors having been found virtually everywhere in the body.^[6] Lipomatous involvement of endocrine organs, including the thyroid, adrenal glands, pancreas, and parathyroid glands, has been described. Maxillofacial lipomas, including intralingual, parotid, orbitonasal, maxillary sinusoidal, and parapharyngeal space masses, have also been documented. In rare instances, intraosseous and intra-articular involvement occurs. Involvement of the structural components of the mediastinum, including the airways and pleura, has also been reported. Gynecolic lipomas may occur in the uterus, ovaries, and broad ligament. Critical organ involvement of the heart (causing ventricular tachycardia), superior vena cava, brain, and spinal cord may pose a significant clinical challenge.^{[7, 8]}

Mixed histologies, such as angiolipomas and fibrolipomas, are often encountered and are usually benign. Differentiation from liposarcoma may be difficult. Other fatty tumors include lipoblastomas, hibernomas, atypical lipomatous tumors, and liposarcomas. Lipoblastomas occur almost exclusively in infants and children. They have a benign clinical course and a low recurrence rate after surgical excision. Hibernomas, also rare, derive their name from the morphologic resemblance to the brown fat of hibernating animals. They presumably arise from fat that may occur in the back, hips, or neck in adults and infants. Atypical lipomatous tumors are generally considered to be low-grade sarcomas, with a strong propensity to recurrence but little metastatic potential. Liposarcomas are true mesenchymal malignancies.

Lipomas are most often asymptomatic. When they arise from fatty tissue between the skin and deep fascia, typical features include a soft, fluctuant feel; lobulation; and free mobility of overlying skin. A characteristic "slippage sign" may be elicited by gently sliding the fingers off the edge of the tumor. The tumor will be felt to slip out from under, as opposed to a sebaceous cyst or an abscess that is tethered by surrounding induration. The overlying skin is typically normal.

Symptoms in other sites depend on the location and can include the following:

• Lipomas in the major airways can cause respiratory distress related to bronchial obstruction. Patients may present with either endobronchial or parenchymal lesions.
• Previously undiagnosed lipomas of the oropharynx may also lead to airway difficulty at the time of intubation.
• Patients with esophageal lipomas can present with obstruction, dysphagia, regurgitation, vomiting, and reflux; esophageal lipomas can be associated with aspiration and consecutive respiratory infections.
• Cardiac lipomas are located mainly subendocardially, are rarely found intramurally, and are normally unencapsulated. They appear as a yellow mass projecting into the cardiac chamber.
• Intramediastinal lipomas may impinge on the superior vena cava, thereby leading to superior vena cava syndrome.
• Intestinal lipomas may manifest as classic obstruction, intussusception, volvulization, or hemorrhage.
• Lipomas arising from fat in the intramuscular septa cause a diffuse, palpable swelling, which is more prominent when the related muscle is contracted.
• Lipomas in intra-articular joint spaces or intraosseous sites, such as the calcaneus, may lead to joint dysfunction and pain that preclude normal ambulation.
• Lipomas may also arise in the dural or medullary components of the spinal cord, thereby leading to cord compression and attendant sequelae.^[7]
• Lipomas occur frequently in the breast but not as frequently as expected considering the extent of fat that is present.
• Lipomas may arise from the subcutaneous tissues of the vulva. They usually become pedunculated and dependent.

Lipomas are removed for the following reasons:

• Cosmetic reasons
• To evaluate their histology, particularly when liposarcomas must be ruled out
• When they cause symptoms
• When they grow and become larger than 5 cm

Obtain biopsies of large lipomas or of those tethered to fascia to rule out a liposarcoma.

The anatomy depends on the tumor site. Subcutaneous lipomas are usually not fixed to the underlying fascia. The fibrous capsule must be removed to prevent recurrence.

No contraindications to removing a lipoma exist, unless the patient is unfit for surgery or anatomic location makes removal unfeasible (as in the case, for example, of an intraspinal lipoma). Benign lipomas are simply "shelled out," with complete removal of the capsule in an extracapsular plane. This is an inadequate operation for a liposarcoma, and hence, performing an initial biopsy to exclude this lesion may be considered for large fatty tumors or for those in the retroperitoneum or the intramuscular spaces.