Intraosseous Lipoma of the Tibia

Clinical History

A 40-year-old man with no past medical history presents with four months of mild right leg pain that had progressed in the last three weeks.

Imaging Findings

Plain X-ray films show a lytic lesion with a thick sclerotic rim within the proximal tibia. The cortex was intact. The lesion showed a lucent area with trabecular thickening and coarse septa but no calcified matrix was seen(Fig.1)

Computed tomography (CT) demonstrated a centric bone lesion that causes mild bone expansion without disrupting the cortex. Within the lesion, CT images show a predominant low homogeneous fluid attenuation content with a rim of sclerotic bone; no calcified matrix was seen. Extensive areas with a low CT attenuation coefficient resulting from fatty content were enclosing the lesion.(Fig.2)

At magnetic resonance (MR) imaging, the lesion showed predominantly an isointense signal on T1-weighted images surrounded by a thin hyperintense rim. PD SPIR weighted fat suppressed MR showed a hyperintense lesion with thin hypointense margins. No associated extraosseous soft-tissue mass nor bone-marrow oedema were seen on MR images.(Fig.3a and 3b)

Discussion

Lipomas associated with bone can be classified according to their intraosseous, cortical, or parosteal location. Intraosseous lipomas are rare [2].

The common sites for this lesion include the intertrochanteric and subtrochanteric regions of the femur and calcaneus, but they may arise from marrow fat in any skeletal bone.
They commonly present in the middle-aged population, with a slight male predominance [1].

Intraosseous lipomas are frequently incidental findings but they can cause pain or swelling as in our case [1].

In most cases, the imaging characteristics of this entity when they are considered together with the clinical history, could be pathognomonic making biopsy unnecessary.
To recognize the imaging features of intraosseous lipomas it is important to understand the histopathology [1].

Milgram classified intraosseous lipoma in three categories [3].
Stage 1:solid lesion of viable lipocytes.
Stage 2:areas of partial fat necrosis and focal calcification with persistent viable lipocytes.
Stage 3:advanced cell necrosis of the lesion with variable amounts of calcification, cyst formation and reactive new bone formation.

The radiographic appearance may show lucent areas with a well-defined benign-appearing osteolytic bone lesion, often with radiodense areas of dystrophic calcification. Expansile remodeling of bone might be seen. The tumour may be associated with thick sclerotic borders[1].

CT and MR imaging demonstrate the fatty component of the lesion which is diagnostic for intraosseous lipoma.
In an early stage, a CT may show resorption of the bone trabeculae with areas of lucency corresponding to fat attenuation. Some lesions may appear with patchy areas of calcification and fat necrosis. Stage 3 intraosseous lipomas exhibits resorption of trabecular bone and predominantly central calcification with reactive peripheral ossification and cyst formation caused by necrosis of the fat component, as in our case [1].

MR Imaging findings include fat signal intensity similar to that of subcutaneous fat[1]. With involution, fibrous proliferation and cystic degeneration may develop and might be the predominant finding showing variable fluid signal intensity on T1- and T2- weighted sequences, as shown in our case [1].

The finding of an intraosseus lytic lesion that demonstrates mild expansile remodeling with a sclerotic rim and peripheral fat tissue margins was consistent with a stage 3 intraosseus lipoma with advanced cell necrosis and cystic degeneration.

Malignant transformation is very rare but it has been described [4].

Surgical treatment may be required for symptomatic lesions.

Differential Diagnosis List

Final Diagnosis

Intraosseous lipoma of the proximal tibia

URL:	https://eurorad.org/case/14090
DOI:	10.1594/EURORAD/CASE.14090
ISSN:	1563-4086