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 [GI]) will be included as appropriate.

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

In the GI 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.[1] Mucosal erosion over the lipoma may lead to severe bleeding (see 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.

Upper gastrointestinal series shows duodenal lipoma with central ulceration where the overlying mucosa has thinned, ulcerated, and bled.

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.[2, 3, 4, 5, 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. Gynecologic 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.

Speculation exists regarding a potential link between trauma and subsequent lipoma formation.[9] 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.[10]

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 lower than that of leiomyomas (18%) and higher than that of adenomas (14%).

The outcome and prognosis are excellent for benign lipomas. Recurrence is uncommon but may develop if the excision was incomplete.

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-year 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-year follow-up).[11, 12]

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 post operation. 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.[11, 12]

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 (eg, 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 would be expected on the basis of the extent of fat that is present
• Lipomas may arise from the subcutaneous tissues of the vulva; they usually become pedunculated and dependent

Subcutaneous lipomas are primarily cosmetic issues. Lipomas in other locations may cause luminal obstruction or hemorrhage. The images below show a duodenal lipoma that caused gastrointestinal hemorrhage and required removal.

Upper gastrointestinal series shows duodenal lipoma with central ulceration where the overlying mucosa has thinned, ulcerated, and bled.

Duodenal lipoma resected through a duodenotomy. Overlying mucosa with central ulceration removed and lobulated fatty tumor shelled out intact with capsule. The mucosa was then sutured closed, and the duodenotomy closed. The stitch was placed to orient the specimen for pathologic examination.

A study by Yoshiyama et al suggested that the plasma D-dimer level could contribute to the differentiation between lipoma and well-differentiated liposarcoma, two lesions that are similar in terms of clinical, radiologic, and pathologic characteristics.[13]

For most subcutaneous lipomas, no imaging studies are required.

Lesions in the gastrointestinal (GI) tract may be visible on GI contrast studies (see the image below).

Upper gastrointestinal series shows duodenal lipoma with central ulceration where the overlying mucosa has thinned, ulcerated, and bled.

Imaging studies for lipomas in atypical locations (or those for which the differential diagnosis includes sarcoma) include ultrasonography (US), computed tomography (CT), and magnetic resonance imaging (MRI).[14]

In a retrospective review of the use of US by two musculoskeletal radiologists to evaluate 714 superficial soft-tissue tumors, Hung et al reported a sensitivity of 95.2% and a specificity of 94.3% for lipoma.[15]

When CT is employed, a radiodensity of less than 50 Hounsfield units is indicative of a soft-tissue tumor composed of fat, though no reliable distinction can be made between a benign lipoma and a malignant liposarcoma.[6]

MRI has been recommended as a reliable preoperative investigation.[16, 17, 18, 19, 6] It has been employed in intramuscular lipoma, pediatric lipoblastomas, and others. The findings of intramuscular lipomas, for example, range from small, homogeneous masses to large, inhomogeneous lesions with infiltrative margins. However, like CT, MRI does not allow an absolute, reliable distinction between a lipoma and a liposarcoma.[20]  A study by Thornhill suggested that the addition of computer-assisted diagnosis may improve the ability of MRI to make this distinction.[21]

A study by Yang et al found that imaging-based multimodality models using computer-assisted diagnostic models could be effective for discriminating between lipoma and well-differentiated liposarcoma.[22]

Because all lipomas are radiolucent, soft-tissue radiography can be diagnostic, but it is indicated only when the diagnosis is in doubt.

Biopsies are normally not indicated for small subcutaneous lesions, because the entire tumor is usually removed. All imaging techniques have been combined with fine-needle aspiration (FNA); this combination increases the accuracy of diagnosis. Obtaining tissue samples from different tumor components is important, because it provides samples for histopathologic analysis by means of various techniques, including fluorescence in situ hybridization (FISH).

Lipomas are benign mesenchymal tumors derived from adipocytes. Several variants have been described, including the following:

• Adenolipomas, a variation of lipomas that may occur in the breast, often have a marked fibrotic component; they are best regarded as a hamartoma
• Angiolipomas contain many small vessels
• Cardiac lipomas may calcify following fat necrosis; microscopically, they are composed of fatty tissue with interlacing muscle fibers

FNA biopsies of a lipoblastoma contain multivacuolated lipoblasts, myxoid areas, and a plexiform capillary network.

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.

No contraindications for 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.

Nonoperative therapy includes endoscopic excision of tumors in the upper gastrointestinal (GI) tract (ie, esophagus, stomach, or duodenum) or the colon. Colonoscopic snare removal has been described but may be associated with perforation if the base is broad. Japanese authors reported a safe technique in which a bipolar snare was used and the mucosa of the defective region was clipped.[23] Otherwise, surgical removal is indicated.

Complete surgical excision with the capsule is advocated to prevent local recurrence, whether the lipoma in question is subcutaneous or intracardiac in origin. These lesions may be lobulated, and it is essential that all lobules be removed.

Specific therapy depends on the location of the tumor.[24]

Subcutaneous lipomas are removed for cosmetic reasons, and hence, a cosmetically pleasing incision should be used.[25] The incision is usually placed directly over the mass and is oriented to lie in a line of skin tension.

Liposuction is an alternative that allows removal of the lipoma through a very small incision, the location of which may be remote from the actual tumor.[26, 27, 28, 29] The lesion may also be approached by means of advanced minimal-access tissue dissection methods, with the use of a dissecting balloon.[30] These latter two methods allow the incision to be placed in an inconspicuous location. For example, axillary incisions may be used to remove lipomas from the back.

Liposuction may be employed more often in small facial lipomas, because favorable aesthetic results have been obtained through strategically placed incisions. Liposuction is indicated for the treatment of medium-sized (4-10 cm) and large (>10 cm) lipomas; in small lipomas, no advantage has been reported, because these tumors can be extracted through small incisions.[26, 27, 28, 29]

Lipoma formation has been reported as an unusual complication of liposuction and has also been found to occur following trauma.[9, 31] The mechanism in these cases is unknown. Research on genetic markers of atypical lipomatous tumors and liposarcomas is ongoing. These tumors have been shown to express receptors for leptin.[32]

For more unusual locations, the method of removal must be tailored to the site and may require the expertise of a consultant, as follows:

• Local removal is indicated in intestinal lipomas causing obstruction or hemorrhage; uncertainty of diagnosis for an intramural intestinal mass also warrants resection, because liposarcomatous disease of the bowel has been described
• If esophageal lipomas cannot be endoscopically removed, surgical excision is indicated, whether by a transhiatal or a transthoracic approach
• Lipoma-related narrowing of the major airways warrants removal of the instigating mass; likewise, intraparenchymal lipomas of the lung may require thoracotomy and the expertise of a thoracic surgeon
• Breast lipomas are excised if their nature is in doubt, whether by means of wire or ultrasonographic localization or by means of direct palpation
• Vulvar lipomas may be locally excised
• Lipomas in critical locations, such as the heart, may require a more physiologically and technically demanding procedure for removal, including median sternotomy with bypass
• Intraosseous lipomas may be removed by means of endoscopy in combination with orthopedic expertise

Tumors can usually be enucleated. They may recur if not properly removed, which should include removal of the capsule. Hibernomas tend to be highly vascular. Lipomas in other locations may present unique difficulties during removal; for example, in a person presenting with a frontalis-associated subfascial lipoma as a protruding mass on the lateral forehead, the lipoma may be difficult to dissect because of the highly vascular muscle that invests it. Lipomas of the GI tract can frequently be shelled out of their submucosal location.