Lipoma and Other Benign Lipomatous Tumors

Sean V. McGarry, MD, FAAOS

Dr. McGarry or an immediate family member serves as a paid consultant to or is an employee of Musculoskeletal Transplant Foundation; has received research or institutional support from Musculoskeletal Transplant Foundation; and serves as a board member, owner, officer, or committee member of National Comprehensive Cancer Center Bone Committee and the Soft Tissue Committee.

ABSTRACT

Lipomas and lipomatous tumors are the most common tumors observed by orthopaedic surgeons. There are several histologic subtypes. The principal importance of recognizing lipoma variants is not to mistake them for malignancies. Atypical lipomatous tumor is a nonmetastatic premalignant lesion that can be identified histologically or with molecular markers.

INTRODUCTION

Lipomas are benign tumors consisting of mature adipose tissue. They can be classified as superficial or deep in relation to the deep fascia. There are several histologic subtypes showing subtle pathologic and prognostic differences. Atypical lipomatous tumor (ALT), sometimes referred to as atypical lipoma or well-differentiated liposarcoma (WDL), is a borderline premalignant lesion. Considering only reported tumors, lipomas are the most common mesenchymal tumors (benign or malignant) found in humans. Still, there is a relative paucity of literature on their treatment and outcomes. Beyond case reports of rare or interesting subtypes, most of the recent literature on lipoma focuses on the use of molecular and cytogenetic markers to better classify and diagnose the various histologic variants of lipoma. Little has changed in the actual treatment of lipoma.

INCIDENCE AND ANATOMIC LOCALIZATION

Considering the indolent behavior of most lipomas and the fact that many are never treated surgically, the incidence of lipomas is likely significantly higher than reported. In addition, many surgically excised lipomas are never confirmed by pathology. Therefore, the true incidence of lipoma is difficult to assess. Most studies describe a slight male-to-female predominance in lipoma. These tumors are extremely rare in children and uncommon in young adults. They most commonly present in middle-aged adults. They are more common in individuals who are sedentary and those with obesity. There is no racial discrepancy in the incidence.

Lipomas can occur either superficial or deep to the fascia. Superficial lipomas occur significantly more frequently than deep lipomas and are considered the most common soft-tissue tumors in humans. Common anatomic locations include the back, shoulders, posterior neck, and abdomen (coincident with the highest densities of subcutaneous fat in the body). Truncal locations are more common than the extremities. The proximal extremity is a more common location than the distal extremity.

CLINICAL PRESENTATION

Superficial and deep lipomas have a bimodal presentation. In the subcutaneous tissue, lipoma presents as a small (<5 cm), slowly enlarging, nonpainful mass just below the skin. The tumor has a rubbery consistency and is freely mobile. With a deep lipoma, the patient presents with either an incidental finding on imaging performed for another reason or fullness in the involved area. The tumor is usually a much larger mass (≥10 cm) and may not be palpable.

Patients may describe an increase in the size of the lesion in proportion to weight gain. The lesions are typically not painful unless they are encroaching on or impeding on an
adjacent anatomic structure (eg, a nerve). Frequently, the presenting symptom is cosmetic asymmetry.

RADIOGRAPHIC FINDINGS

Plain radiographs are not very specific but may show a soft-tissue shadow suggesting the presence of a mass. CT shows a homogenous soft-tissue mass with the same radiodensity as subcutaneous fat. MRI can establish the presence of a lipomatous tumor, showing a homogenous mass with signal intensity that is isointense to that of subcutaneous fat in all sequences. Specifically, a lipoma has high signal intensity on T1-weighted imaging and low signal intensity on T2-weighted imaging (Figure 1). Occasionally, a lipoma will have a more heterogeneous signal intensity as a result of necrosis or calcification within the lesion, or secondary to prominent vascular septa. This makes a radiographic diagnosis more difficult. MRI does not reliably distinguish lipoma from atypical lipoma.¹

FIGURE 1 A and B, Axial T1-weighted and axial T2-weighted magnetic resonance images show the tumor (black arrow) is isointense to subcutaneous fat (white arrow) in both sequences.

PATHOLOGIC FINDINGS

Gross Pathologic Findings

Subcutaneous lipomas are roughly spherical masses, beige to yellow in color, ranging in size from 1 to 2 cm to 6 to 8 cm in diameter. They are usually well circumscribed by a thin capsule that, when opened, demonstrates a more lobular irregularly shaped lesion. Deep lesions are usually larger and can be as large as 15 to 20 cm or more when found in the thigh. They are yellow to orange on gross inspection. They are more irregularly shaped, with the shape often determined by the space in which they are growing. They show a centripetal growth pattern in the axial plane, and when they meet resistance, they grow in a more longitudinal plane filling the available space in the compartment in which they reside (Figure 2).

FIGURE 2 A and B, Axial and coronal T1-weighted magnetic resonance images show growth in a centripetal and then longitudinal plane.

Histopathologic Findings

Lipoma cells are similar in appearance to mature adipocytes. The cells are oval with a central lipid droplet comprising most of the cell; the remainder of the cytoplasm and bland basophilic nuclei are pushed to a more peripheral location, resembling a signet ring. In a large microscopic field, the cells form an interlocking pattern similar to chicken wire (Figure 3). Lobules of these cells are separated by fibrous septa through which the blood vessels course. A thin capsule usually surrounds the tumor. Occasionally, muscle fibers are present at the periphery of deep lipomas where the lipoma is infiltrating into the adjacent muscle. An admixture of other cell types or matrix (eg, spindle cells, blood vessels, or chondroid matrix) distinguishes the various subtypes of lipomas.

FIGURE 3 A histogram at 20× magnification shows a lipoma with chicken-wire appearance of cells.

Cytogenetic Findings

Most lipomas show chromosomal abnormalities. Frequently, these are balanced translocations involving the chromosomal segments 12q13-15 and 3q27-28. The genes involved are HMGA2 and LPP, which form a fusion gene. The expression of HGMA2-LPP or LPP-HGMA2 is found in almost 25% of all lipomas.²

Most other cytogenetic abnormalities in lipoma include the HGMA2 gene and other genes including NFIB, CXCR7, EBF1, and LHFP. The translocation partner and respective chromosome segments for those genes are as follows: t(9;12)(p22;q13-15), t(2;12)(q37;q13-15), t(5;12)(q32-33;q13-15), and t(12;13)(q13-15;q12).³

TREATMENT AND PROGNOSIS

Lipomas show an indolent growth pattern. After the initial growth, most tumors stabilize and do not show significant further growth. When located in a subcutaneous location and smaller than 5 cm, many patients can be treated with observation. Many patients find these tumors cosmetically unacceptable and request excision. Other surgical indications include larger-size lesions or lesions that continue to grow. If any questions remain as to the diagnosis after appropriate imaging is performed, the patient should be referred to an orthopaedic oncologist. Deeper lesions, as noted previously, generally present as much larger lesions. They also tend to be more of a diagnostic challenge and may warrant referral to an orthopaedic oncologist.

Treatment for patients with lipomas is marginal resection. Recurrences are likely the product of incomplete resection. Poorly encapsulated or infiltrative tumors are more difficult to completely excise and hence more likely to recur. Malignant degeneration of a lipoma into a liposarcoma is extremely rare. The more likely scenario is transformation of an ALT into a liposarcoma. Postoperative radiologic surveillance following lipoma resection is unnecessary.

HISTOLOGIC VARIANTS OF LIPOMA

Lipomatosis

Lipomatosis is a diffuse overgrowth of normal adipose tissue that affects the trunk or extremities, which can be concerning based on the rapid growth and infiltrative nature of this condition. It is essentially impossible to completely excise all lipomas in these patients; therefore, treatment consists of debulking when clinically appropriate or excision of symptomatic lesions.

Lipoma Arborescens

Lipoma arborescens is a fatty infiltration of the subsynovial tissue of a joint, most commonly the knee. The most common location within the knee is the suprapatellar pouch. There is some question as to whether this is a reactive process because it almost always occurs in association with other intra-articular pathology. Treatment of patients with this condition consists of synovectomy of the involved joint when symptomatic.

Lipoblastoma

Lipomas are rare in patients younger than 20 years. However, the equivalent tumor in infancy and young childhood is a lipoblastoma. A 2018 case series showed
17 of 23 patients were age 5 years or younger and 21 of 23 patients were age 12 years or younger at the time of diagnosis.⁴ Lipoblastoma occurs more commonly in males than in females. Similar to lipoma, lipoblastoma can occur in a diffuse form known as lipoblastomatosis. Radiographic appearance and clinical presentation are similar to those of adult lipomas and show abundant fat. Lipoblastomas present most commonly in the subcutaneous tissues. MRI findings include nonenhancing cystic change (representing necrosis and myxoid stroma) and enhancing areas containing soft-tissue nodules (representing the more vascular and cellular nodules of lipoblasts).⁵ Histologically, lipoblastoma consists of lobules of lipoblasts (immature adipocytes) in various stages of differentiation separated by vascular fibrous stroma with an underlying myxoid stroma. Immunohistochemical staining, fluorescence in situ hybridization (FISH), and cytogenetic studies for the PLAG1 gene rearrangement (8q11-13) can help confirm the diagnosis of lipoblastoma.⁶^,⁷^,⁸ Patients with lipoblastoma are treated with marginal resection.

Only gold members can continue reading. Log In or Register to continue