Keywords

1 Definition

  • A rare benign intraosseous neoplasm constituted of adipose cells, which can also arise on the surface of bone.

2 Etiology

  • Unknown

3 Epidemiology

  • Less than 0.1% of primary bone neoplasms; 15% of them are surface tumors.

  • Can be seen at any age, but its main incidence is in the fifth decade of life.

  • There is a slight male predominance (4:3).

4 Sites of Involvement

  • Most cases occur in the calcaneus or in the metaphysis of long bones, especially the proximal femur (Figs. 44.1, 44.2, 44.3, and 44.4).

  • Flat bones are occasionally affected.

  • Surface tumors may be seen over the diaphysis of long tubular bones of the extremities (Fig. 44.5).

Fig. 44.1
figure 1

Radiograph and CT of a case of multiple lipomas, in a characteristic calcaneal location (a, b) and in the lower tibia (b, c)

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Fig. 44.2
figure 2

(a) Radiograph of a lipoma in the proximal end of the fibula, with some central areas of calcification. (b) Multiplanar reconstruction (MPR) CT of the same case (coronal view). (c) MRI T1-weighted image, coronal view. Lesion signal intensity is similar to normal adipose bone marrow. (d) MRI proton-density (PD) fat-saturated image, coronal view. The lesion presents the same signal as normal adipose bone marrow

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Fig. 44.3
figure 3

Bilateral calcaneal lipomas with central mineral calcifications, a common finding

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Fig. 44.4
figure 4

T1-weighted MRI (a) and macrophotography (b) of a parosteal lipoma

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Fig. 44.5
figure 5

Specimen cut surface with characteristic yellowish color (a). Microphotography of a surface lipoma. The histology is the same as for classic lipomas of soft parts (b)

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5 Clinical Symptoms and Signs

  • Lipomas are mostly asymptomatic but may present local aching pain and/or the presence of a mass.

  • Pathological fracture is rare.

6 Imaging Features

6.1 Radiographic Features (Figs. 44.1, 44.2, 44.3, and 44.4)

  • Radiographs show a lucent, usually well-defined intramedullary lesion, with or without some trabeculation and a thin marginal sclerosis.

  • Expansion of the bone is slight and infrequent, except in small or thin bones.

  • Parosteal lesions may produce a solid periosteal reaction.

  • Dense foci of calcification may be seen in a third of the cases and more frequently in the os calcis, where it is characteristically centrally located (Fig. 44.3).

6.2 CT and MRI Features

  • The tumor shows features similar to subcutaneous fat and may demonstrate the presence of intralesional cysts.

  • MRI presents high-intensity signal on T1- and T2-weighted images and low signal on STIR or fat-suppressed T2-weighted sequences.

7 Imaging Differential Diagnosis

7.1 Chondromyxoid Fibroma

  • The lesion may expand to soft parts, where it shows a thin mineralized shell of reactive bone.

  • MRI shows heterogeneous, predominantly high-intensity signal on T2-weighted images.

7.2 Bone Infarct

  • Calcifications have a different pattern, the so-called chimney smoke pattern.

7.3 Enchondroma

  • Calcifications also have a different, more scattered, pattern.

  • MRI shows low- to intermediate-intensity signal on T1-weighted images.

7.4 Osteochondroma

  • May be difficult to distinguish from parosteal lipoma on radiographs.

  • CT and MRI will show an absence of cortical and medullary continuity, besides evidence of the lesion’s fatty nature.

7.5 Simple Bone Cyst

  • It may mimic lipoma on radiographs, if calcification is not present.

7.6 Liposclerosing Myxofibrous Tumor

  • When in the inter-trochanteric area of the femur, intraosseous lipoma may participate in the genesis of this controversial lesion—at least in some cases.

7.7 Osteoporosis

  • It may present areas devoid of bone trabeculae that can be confused with a lipoma if no clear limits can be discerned.

8 Pathology

8.1 Gross Features (Fig. 44.5)

  • The cut surface shows a well-defined, yellow, and soft lesion, with a marginal sclerosis. It usually measures less than 5 cm in diameter.

  • Parosteal tumors may be large, up to 10 cm in diameter.

  • Dense, mineralized foci may be found inside the tumor in older lesions.

8.2 Histological Features

  • Lipomas consist of lobules of well-differentiated adipocytes that may surround small bone trabeculae.

  • Brown fat areas may be seen rarely.

  • Fat necrosis and/or cyst formation may occur, along with foamy macrophages and fibrosis.

  • Central areas of amorphous calcification may be present, more characteristically in the os calcis (Fig. 44.3).

  • Ossifying lipomas present a more abundant and diffuse bony trabeculation.

  • Parosteal lipomas consist of white fat with a periosteal cover (Fig. 44.4). Hyaline cartilage with endochondral ossification or reactive bone formation may be found at the tumor limit with the bone cortex.

9 Pathologic Differential Diagnosis

9.1 Normal Fat Marrow

  • Lipoma presents pushing borders (which may be easily overlooked) and only scarce or no bone trabeculae.

10 Genetics

  • The same translocation t(3;12)(q28;q14) and fusion transcript HMGA2-LPP seen in soft-tissue lipoma has been identified in parosteal lipoma.

11 Prognosis

  • Excellent; no metastatic potential.

  • Complete excision is curative.

  • Curettage or incomplete resection may rarely result in recurrence.

12 Treatment

  • Marginal resection or curettage is curative.

  • Non-evolutive or regressive tumors may not deserve treatment.