Clinical History
A 50-year-old man presented with increase in the volume of his right thigh since 4 years. He also complained of recurrent pain and heaviness in his right thigh. He was referred for imaging to our department and subsequently underwent surgical resection of a mass.
Imaging Findings
Radiograph of thigh (Figure 1) showed a well defined low density mass with associated bony excrescence originating from the anterior femoral cortex.
Ultrasonography (Figure 2) showed a well defined elongated hypoechoic mass located deep to the vastus intermedius muscle. A broad based bony projection was seen posterior to mass with irregular outline merging with the external cortex of femur.
CT (Figure 3) revealed a well defined fat density mass in anterior thigh with adjacent bony excrescence along the anterior femoral cortex. There was no continuity of the bony excrescence with medulla of the femur, differentiating it from an osteochondroma.
MRI (Figures 4 and 5) confirmed a lobulated lesion measuring 5.2 x 2.4 x 8.6cm(ML x AP x SI). The lesion was hyperintense on T1- and T2-Weighted images (WI) and suppressed on STIR and fat saturated T2-WI.Thin intralesional septations within showed subtle enhancement. The bony excrescence attached to anterior femoral cortex appeared hypointense on all pulse sequences.
Discussion
Lipomas are benign tumours of mature adipose tissue without cellular atypia. Parosteal lipoma is benign tumour of adipose tissue intimately associated with periosteum of bone. Parosteal lipomas share histopathologic features with commonly occurring soft-tissue lipomas, and cytogenetic evidence suggests common histopathogenesis [1].
The parosteal lipoma is rare tumour accounting for 0.3% of all lipomas [2].It affects adults with age 40 -60 years and presents with palpable mass or mild-intensity pain.
Paraosteal lipomas exhibit distinctive radiographic features that helps in diagnosis. These are fat containing lesions adjacent to the cortical bone often associated with reactive changes.These changes include bone deformity, cortical erosion, and overproduction of the cortical bone (hyperostosis).More than half are associated with hyperostosis.The most frequently affected sites are the diaphysis and metaphyseal regions of long bones [3].
MRI is the imaging modality of choice. It mainly reveals fat signal intensity on all sequences. However low-signal-intensity strands on T1W which appear as high signal intensity on STIR/T2 fat saturated images correspond to fibrovascular strands that are commonly found in lipomatous lesions [4]. Cartilaginous components reveal intermediate signal on T1-weighted and high signal on T2-weighted images. Bony protruberance appears as hypointense signal on all sequences.
At gross examination, these have mature fat identical to soft-tissue lipomas. Cartilage, osteoid metaplasia, and areas of osseous excrescences or cortical thickening extend from bone surface to the lesion. These osseous excrescences do not show cortical or medullary continuity with the adjacent bone. The cartilage and osteoid metaplasia typically occur adjacent to the osseous excrescences.
Bone destruction is typically absent in paraosteal lipomas. Osseous excrescences may mimic osteochondromas, although osteochondromas characteristically are continuous with medullary marrow of the underlying bone. This tumour should also be differentiated from liposarcoma. Parosteal lipomas show thin fibrous septa which show either no or minmal enhancement as compared to liposarcomas which have thickened or nodular fibrous septae showing moderate to intense enhancement. "Associated nonadipose soft tissue masses, prominent foci of high T2 signal, and prominent areas of enhancement also favour liposarcoma.[5]"
Parosteal lipomas commonly cause nerve compression with motor and/or sensory function deficit. Nerve compression is most common with parosteal lipomas of the proximal forearm which can ultimately progress to muscle atrophy. MRI initially shows muscle edema with hyperintense signal on T2W images which can progress to muscle atrophy (decrease in volume) and/or fatty infiltration which has high-intensity linear streaks of fat within muscle , best seen on T1W images.
Treatment of parosteal lipoma is complete surgical resection.
Differential Diagnosis List
Parosteal Lipoma
Osteochondroma
Liposarcoma
