PMC5807841_01
a AP and lateral b plain radiographs demonstrate an ill-defined radiolucent lesion in the proximal tibial metadiaphysis (black arrows) with posteromedial cortical destruction and adjacent ossification (black arrowheads). A medial soft tissue mass with low density raises the possibility of a fatty matrix (white arrow). A pathological fracture is present (white arrowhead)
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Fig1
A pathological fracture was present (Fig. 1). 
Magnetic Resonance Imaging (MRI) of the proximal tibial lesion. a Coronal STIR, b coronal T1-, c axial T1-, d axial T2-weighted and, e axial proton density with fat saturation. The images demonstrate an ill-defined proximal tibial medullary based lesion extending to the subarticular surface. There is extensive cortical destruction with an almost circumferential soft tissue mass, which has a spiculated appearance (black arrows) displacing the posterior muscles and popliteal neurovascular bundle. Tumour breaches the deep fascia anteriorly to extend into the subcutaneous tissues and penetrates the interosseous membrane to extend around the anterolateral tibial cortex. Heterogeneous partly high T1 signal suppresses with fat saturation sequences supporting a partly fatty matrix (white arrowheads). The pathological fracture is clearly seen extending transversely across the proximal tibia (white arrows)
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Fig2
Magnetic resonance imaging (MRI) showed that the tumour filled the medullary cavity of the proximal tibial metaphysis; it had an ill-defined margin and crossed the physis extending to the subarticular surface but there was no intra-articular extension (Fig. 2).
Axial CT and PET/CT imaging of the proximal tibial lesion. a Axial CT imaging shows medial cortical destruction and a large soft tissue mass containing areas of fat attenuation (white arrowheads) and ossification (black arrows). b A coronal fused PET/CT image shows there is marked FDG uptake in the proximal tibial tumour as well as in a lateral distal femoral metastasis (white arrow). No other lipomatous lesion or tumour is present
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Fig3
Positron emission tomography with fluorodeoxyglucose (FDG) integrated with computed tomography (PET/CT) confirmed an ill-defined proximal tibial lesion with medial cortical destruction and a large soft tissue mass containing areas of fat attenuation and ossification (Fig. 3a). A further focus of markedly increased FDG uptake with an SUV of 13.5 was seen in a lateral distal femoral lesion which was presumed to represent a metastasis (Fig. 3b).
Histological analysis of the proximal tibial lesion shows primary pleomorphic liposarcoma of bone. a The tumor infiltrates cancellous bone and is composed of numerous lipoblasts and large pleomorphic cells, which have vacuolated cytoplasm and atypical nuclei. b Lipoblasts and brown fat-like cells are seen within the tumour. Immunohistochemistry shows the tumor cells express FABP4/aP2 (c) and UCP1 (d)
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Fig4
Histology of a biopsy of the tibial mass revealed a proliferation of malignant cells that had vacuolated cytoplasm and large atypical pleomorphic nuclei (Fig. 4). There were vacuolated tumour giant cells and numerous small lipoblast-like cells with a single cytoplasmic fat vacuole and hyperchromatic nuclei as well as brown fat-like cells with multiple small fat vacuoles (Fig. 4a, b).
Post-resection AP plain radiograph demonstrates a Stanmore prosthesis with integral distal femur and modular proximal tibial component
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Fig5
The patient underwent an uncomplicated surgical resection of the primary tumour in the proximal tibia and the lesion in the distal femur with reconstruction using a linked distal femur and proximal tibial endoprosthesis (Stanmore Implants, UK) (Fig. 5).