A 19-year-old man presented with soreness in right leg for over half a year, which was tolerable without limb numbness. The symptom was not severe or aggravated by movement. He had no history of recent trauma or injuries. Then, he was admitted to the Department of Osteology of the Second Affiliated Hospital of Zhejiang University School of Medicine due to a soft tissue mass in the right fibula found by plain X-ray. Coagulation blood tests revealed that prothrombin time was 14.8 s (normal range: 12–14 s). Routine blood laboratory tests, including full blood count, erythrocyte sedimentation rate (ESR), electrolyte panels, renal and liver function test were normal, except 49.8% neutrophils (normal range: 50–70%) and 43.5% lymphocytes (normal range: 20–40%). And routine fecal and urine laboratory tests including a fecal occult blood test indicated normal findings. Routine serum tumor biomarkers were all within normal ranges, including carcinoembryonic antigen (2.7 ng/mL), carbohydrate antigen 125 (CA125, 4.3U/ml), carbohydrate antigen 199 (CA199, 2.0 U/ml), alpha fetoprotein serum (2.0 ng/mL), and prostate-specific antigen (0.245 ng/mL). Plain X-ray revealed an irregular, low-density and well-demarcated region of bone destruction in the right distal fibula with scattered patches of slightly high density inside and slight periosteal reaction. Slight swelling change of the adjacent soft tissue was also observed. Computed tomography (CT) images of bony window showed centrally expansile osteolytic changes with scattered punctate bony sclerosis inside (). Axial CT images demonstrated a soft tissue mass with heterogeneous density () and mild enhancement in the arterial phase () and moderate enhancement in the venous phase (). There were no signs of fibular artery, venous, or small saphenous venous invasion. Magnetic resonance imaging (MRI) revealed a focal, 2.5cm × 3.0 cm × 3.2 cm, hypo- and isointense mixed mass on T1-weighted images (T1WI) (), and hypo- and hyperintense mixed mass on T2-weighted images (T2WI) (). There were no signs of necrosis, hemorrhage, or cyst formation within the mass. However, adjacent soft tissue edema and swelling was seen. Gadolinium-enhanced T1WI revealed obvious perilesional enhancement, particularly in the region adjacent to the normal tissue (). The MRI report suggested an osseous malignant tumor in the right distal fibula. Based on the radiological diagnosis, on March 5th, 2018, tumor resection of the right distal fibula and external fixation with autogenous fibular graft and reconstruction were performed. The tumor was well-demarcated, and located in the right distal fibula 6 cm from the right ankle without infiltrating the surrounding blood vessels and nerves. No visible metastatic nodules were found surrounding the tumor. Histological staining showed that fibroblastic epithelioid cells were ovoid and arranged in nests, cords, or sheets within a collagen-rich extracellular matrix (). Immunohistochemical staining showed the tumor cells were positive for CD31 (weak positive), ERG, Fli-1, and Myoglobin, and negative for CD34, S-100, smooth muscle actin (SMA), Desmin, CK (AE1/AE3), CAM5.2, MyoD1, and Myogenin. The Ki-67 proliferation index was low (5%) in tumor cells. A diagnosis of primary SEF in the fibula was finally determined. After 24 months of follow-up, the patient had no signs of recurrence or metastasis. The timeline of diagnosis and treatment was shown in.