A 43-year-old woman with right peritrochanteric pain. She had a history of non-treated dyslipidemia and no previous surgical interventions. She did not report a recent trauma or fall. Hip radiographs were assessed in another institution and given the non-specific findings, CT and MRI were performed for further investigation.
First, multidetector computed tomography (fig.1) revealed a lytic lesion in the intertrochanteric region of the right femur with well defined, geographic sclerotic margins. The lesion neither insufflated the cortical nor produced a periosteal reaction. It measured 4x3x2 cm and had no-fat attenuation values. It was hyperattenuated compared to muscle.
MRI showed a well-defined lesion, which was hypointense on T1 weighted images (WI) (fig.2) and hyperintense on GRE T2*. The lesion was surrounded by a hypointense rim on both pulse sequences, which was interpreted as an area of sclerosis. The STIR sequence demonstrated lack of homogeneous suppression of fat signal within the lesion.
These findings would suggest the differential of: liposclerosing myxofibroid tumor, enchondroma, xantoma of the bone, fibrous dysplasia and non-ossifying fibroma.
Radiology report suggested liposclerosing myxofibroid tumour and fibrous dysplasia as the most probable diagnosis, based on semiology and location of the lesion.
Our patient underwent curettage and prophylactic internal fixation [1-3]
Xanthoma of the bone (XOB) is a rare bone lesion that has been associated with metabolic diseases like hyperlipidemia, however, XOB can occur in patients with normal lipid profiles [3-7]. XOB appears in almost all ages but is infrequent in pediatric patients . They occur in the appendicular or axial skeleton, including the pelvis and skull base .
Histological studies show cholesterol clefts, foamy histiocytes and inflammatory cells. Immunohistochemically, the xanthomatous cells are: CD 68 and vimentin-positive and negative for cytokeratine, AE1/AE3, S100, PAX 8 and CD1a.
Patients are usually asymptomatic but sometimes they can report pain.
Plain radiographs show subtle lucencies in the proximal metaphyseal to diaphyseal regions of the long bones. Perhaps the mechanism of the metaphyseal deposits is the slowing of regional blood flow in that region where the arteries form loops to drain into veins . Most cases show partially or completely sclerotic margins. Intralesional calcification is absent, excluding enchondroma from the differential. Sometimes XOB can exhibit an aggressive appearance, with an expansile border, lack of sclerotic rim, cortical disruption  and may mimic malignancy [3-5,10-12].
CT shows a loss of normal trabecular pattern in the medullary cavity. The lesion may have a mixed lytic and ground glass appearance  . The medullary cavity may have a density greater than normal bone marrow with or without cortical disruption[9,14] or endosteal reaction. The “soft tissue” density on the lesion may be due to a high percentage of cholesterol in hyperlipidemic patient’s xanthomas, containing relatively less triglyceride than the adipocytes in uninvolved bone marrow .
MRI is characterized by a circumscribed area of heterogeneous signal intensity on T1 and T2 WI (usually hypointense and hyperintense respectively), and a loss of signal when fat is suppressed, which is hypothesized to result from the cholesterol-laden histiocytes seen histopathologically. T2 WI can reveal the lesion to be surrounded by areas of low signal intensity, concordant with reactive bone sclerosis[9,12].
The nonspecific radiological appearance of the lesion and its low incidence makes it difficult to diagnose using only clinical and imaging techniques. The attenuation values let us exclude intraosseous lipoma from the differential diagnosis and the age of presentation make fibrous dysplasia and non-ossifying fibroma a less likely possibility. Histopathological and immunohistochemical staining is crucial to differentiate it from other entities involving histiocytes.
Written informed patient consent for publication has been obtained.
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