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case_images: (a) CT findings: Sagital reconstruction showing diaphyseal lytic lesion with cortical erosion, causing bone remodeling. (b) MRI findings: Post-contrast transverse T1 sequence showing surrounding soft tissue mass with homogeneous and intense contrast uptake. In both images, the lesion is delineated between the yellow asterisks.
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case_images: Additionally, there was edema and contrast uptake in the adjacent musculature, which could be secondary to inflammatory changes due to the fracture, although infiltration could not be ruled out (Figure 1). HMB45. H3K27me3 was not valuable, and the Ki67 index was 7% (Figure 4 and Table 1).
case_images: Macroscopic findings. A poorly defined yellowish lesion measuring 4.7 x 2.3 cm was observed within the medullary cavity, expanding into the surrounding soft tissues.
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case_images: The lesion was located 2.6 cm from the proximal margin, 1.7 cm from the distal margin, and 1 cm from the circumferential margin, all of which were macroscopically free of the lesion (Figure 2).
case_images: Microscopy findings. (a) The lesion occupied the medullary cavity, breaking through the cortex and extending into the surrounding soft tissues (HE; 4x). (b) Diffuse 'herringbone' pattern (HE; 10x). (c) Elongated and interwoven fascicles composed of atypical cells with moderate pleomorphism and medium size, with hyperchromatic nuclei and poorly defined cytoplasm. (HE; 20x). (d) Isolated mitoses were observed, with a notable lymphoid inflammatory component. No osteoid formation and areas of necrosis were present within the lesion. (HE, 40x).
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case_images: These cells were arranged in long, highly intertwined fascicles, breaking through the cortical bone and extending into the soft tissues (Figure 3a,b). The nuclei were oval to elongated, hyperchromatic, and granular with nuclear pseudo-inclusions, and the cytoplasm was elongated and eosinophilic (Figure 3c). There were 5 mitoses in 10 high-power fields (Figure 3d).
case_images: Immunohistochemical findings. (a) Diffuse positivity for SATB2 (20x). (b) Weak and focal positivity for osteonectin (20x). (c) Absence of expression for Smooth Muscle Actin (20x). (d) Ki67 index was established in 7% (20x).
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case_images: HMB45. H3K27me3 was not valuable, and the Ki67 index was 7% (Figure 4 and Table 1).
case_images: FISH and NGS results. (a) FISH shows BRAF fusion. (b) NGS shows the fusion point between EVI5 and BRAF. Made with the SeqOne  in vitro diagnostic medical device.
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case_images: This rearrangement was later studied by Next-Generation Sequencing (Figure 5), confirming the fusion.