Lipoma
Lipoma is a benign tumor of mature adipose tissue. As a group, lipomas are the most common soft-tissue tumors in adults. There are a few variants: angiolipoma is composed of fat mixed with vascular tissue; fibrolipoma is composed of fat mixed with bland fibrous connective tissue; spindle cell lipoma is composed of fat mixed with benign spindle cells; chondrolipoma is composed of fat mixed with benign cartilaginous tissue; and hibernoma is a tumor of brown fat with variable numbers of foamy adipocytes. Each subtype has slightly different clinical features and treatment considerations. Atypical lipomatous tumor is a lipoma-like tumor that has malignant transformation potential and must be routinely monitored or otherwise treated with marginal excision and surveillance (and because of its malignant potential, it is described in a separate chapter.)
Epidemiology
Lipomas tend to arise in middle-aged to older adults. They affect men more often than women and exist as either solitary or multiple lesions. Although most lipomas develop spontaneously, lipomatosis is a syndrome of multiple, infiltrating fatty masses and is hereditary in about 30% of cases.
Clinical Features
A benign lipoma within the subcutaneous tissue presents as a soft, nontender, compressible, mobile mass. Although many lipomas remain stable in size, some will exhibit further growth. Even if otherwise asymptomatic, growing tumors are usually treated for cosmetic reasons before they become too large. Deep lipoma, however, can “hide” within the muscle and stay relatively asymptomatic until they become very large. At this point, the tumor can start to compress subfascial structures and cause pain. Angiolipoma typically develops in subcutaneous locations. Unlike the other subtypes, it is very tender to palpation, as are many benign vascular tumors.
Of note, it is impossible to definitively diagnose the histology of a superficial soft tissue mass by palpation and inspection. As such, great care should be taken in the clinical approach to superficial lesions.
Radiologic Features
Lipoma is often not detected on plain radiographs, though a deep lipoma may be seen as a relatively radiolucent soft-tissue mass within the muscle compartment (Figure 1). Metaplastic ossification or calcification can be seen in certain subtypes of lipoma, such as chondrolipoma and angiolipoma. Notably, such calcification does not indicate malignant transformation.
Lipoma is one of the few soft-tissue tumors that can be definitively diagnosed on magnetic resonance imaging (MRI). A plain benign lipoma “looks like fat” on all sequences: it is isointense to subcutaneous fat on T1 sequences, hypointense on T2 fat-suppressed or short tau inversion recovery (STIR) sequences, and does not enhance with contrast (Figure 2). It also lacks the stranding found in atypical lipomatous tumors though these non-fatty components are also seen in the lipoma variants noted above. A subcutaneous lipoma is often “camouflaged” by the normal surrounding fat and can be difficult to distinguish from routine adipose tissue on MRI and also clinically at the time of surgical excision.
Pathology
Grossly, lipomas are usually round or egg-shaped and resemble normal adipose tissue enclosed in a well-circumscribed, thin capsule (Figure 3). Lipomas are soft and yellow but can have areas of hemorrhage.
Microscopically, the mature adipocytes of a lipoma look similar to those in subcutaneous fat, with small, eccentric nuclei and large amounts of lipid within the cytoplasm. There are virtually no spindle cells mixed with the adipocytes (Figure 4).
Although all subtypes contain mature adipocytes, each subtype appears slightly different on histopathologic examination due to its defining characteristic features. Angiolipoma is characterized by a mixed population of mature adipocytes, spindle cells, and blood vessels (lined by endothelial cells and containing red blood cells). Chondrolipoma is characterized by a chondromyxoid matrix and chondrocyte-like spindle cells. Fibrolipoma is characterized by variable amounts of collagen and fibrous tissue interwoven between mature adipocytes. Finally, spindle cell lipoma is characterized by a benign spindle cell stroma interwoven between mature adipocytes, with no cellular atypia.
Compared to the other subtypes, hibernoma exhibits greater vascularity and contains both mature adipocytes and lipoblasts (Figure 5). A lipoblast is an immature adipocyte with intracytoplasmic fat vacuoles and an indented nucleus.
Differential Diagnosis
Lipoma can be definitively diagnosed on MRI because it “follows the fat” on all sequences. Whereas most soft-tissue tumors are hypointense on T1 sequences and hyperintense on fluid-sensitive sequences, lipomas are homogeneously hyperintense on T1 sequences and hypointense on fat-suppressed sequences, always looking like normal fat. Some atypical lipomas demonstrate very little stranding and so MDM2 testing on biopsy material may be needed for correct diagnosis. The differential diagnosis for lipoma includes all of the lipoma subtypes. Of note, frank liposarcoma contains very little fat despite its name and appears similar to other soft-tissue sarcomas on MRI (i.e., hypointense on T1 sequences and hyperintense on fluid-sensitive sequences).
Disease Course: Treatment and Prognosis
Excluding atypical lipomatous tumor, lipomas do not undergo malignant transformation. Therefore, if they are not symptomatic (as noted, angiolipoma can be painful or tender to palpation) or cosmetically displeasing, lipomas can be “treated” with benign neglect. If a lipoma does need to be removed, marginal excision is adequate. Furthermore, because the risk of recurrence is very low and the risk of malignant transformation is essentially zero, routine surveillance imaging is unnecessary after surgery. Patients however should be counseled to return should a mass recur.
Key Test Topics
All lipomas can be treated with marginal excision.
MDM2 gene amplification can distinguish plain benign lipoma from atypical lipomatous tumor. Unlike lipoma, atypical lipomatous tumor has a higher rate of local recurrence and a 10% risk of malignant transformation. (See Liposarcoma chapter)