A 26-year-old man presented with a 1-month history of chest pain, a palpable and painful right inguinal mass, and edema in the right lower extremity. One month earlier, he started to experience left chest pain with no cough. Pulmonary CT angiography (CTA) revealed a left lower lobe segmental pulmonary embolus. The local hospital made a diagnosis of pulmonary embolism (PE). He received anticoagulants, and his chest pain was gradually relieved. At the time of current presentation, the patient was experiencing right lower extremity swelling and pain. Physical examination revealed a 4 × 3 cm palpable right inguinal mass with no redness. His medical history and family history were negative. The results of laboratory work-up were normal, with a D-dimer level of 0.16 mg/L fibrinogen equivalent units (reference range, <0.46 mg/L) and an international normalized ratio of 2.45 (therapeutic range, 2.0–3.0 for a patient taking warfarin), except the prothrombin time was 28.2 seconds (reference range, 9.6–12.8 seconds) and the activated partial thromboplastin time was 52.2 seconds (reference range, 24.8–33.8 seconds). Echocardiography, chest radiography, chest CT , and contrast-enhanced CT revealed no abnormalities. The patient underwent right lower extremity vascular conventional US (Philips IU22; Philips) with an L9–3 probe (3–9 MHz, venous condition) and contrast-enhanced US (1.5–2.0 mL, SonoVue; Bracco) with an intravenous bolus injection at the initial evaluation. T wo days later, noncontrast and contrast-enhanced CT images of the lower abdomen (1.5 mL per kilogram of body weight, 300 mg/mL iomeprol, Iomeron; Bracco) were acquired for further evaluation (Figs 1–3).The initial conventional US images revealed a 3.0  2.9 cm heterogeneous multilocular mass with well-circumscribed walls and hyperechoic internal septa arising from the right external iliac vein and the right common femoral vein (Fig 1a). Color Doppler flow imaging showed abundant blood flow signals in the mass, and spectral Doppler imaging depicted arterial blood flow signals with a 35.3 cm/sec peak systolic velocity and a resistance index of 0.81 in the mass (Fig 1b). CE US showed that the mass was enhanced inhomogeneously and simultaneously with the femoral artery at 18 seconds (Fig 2a[ID] FIG2[/ID]) and that the mass was multilocular with several unenhanced areas (Fig 2a, 2b). The CE US images indicated an intravascular tumor with neovascularization in the right external iliac vein and right common femoral vein. According to the contrasting features on CE US images, a tumor with neovascularization was more likely than a bland thrombus in the right external iliac vein and right common femoral vein. Thus, the patient was sent for plain CT and CE CT scans of the lower abdomen. Further evaluation with a plain CT scan of the lower abdomen showed a 3.9 × 2.9 cm multilocular soft-tissue mass with uneven density arising from the right external iliac vein and right common femoral vein, and some punctate calcifications were present at the periphery of the mass (arrow in Fig 3a). CE CT enabled confirmation of the presence of enhanced neovessels in the mass and heterogeneous enhancement of the mass with enhanced internal septa and unenhanced cystic areas. There were no abnormal signs of the other vessels, the bladder or prostate, or pelvic and femur bones on the plain CT and CE CT images of the lower abdomen (Fig 3). Given the imaging findings of the tumor on the US and CT scans, an intravascular heterogeneous multilocular soft-tissue tumor of the right external iliac vein and right common femoral vein with neovascularization was diagnosed. In combination with the unenhanced CT findings, characteristic peripheral punctate calcifications of the tumor (1), it was thought to most likely represent an intravascular cystic synovial sarcoma. The patient underwent tumor resection surgery, and the gross specimen was a tumor with an appearance like that of fish meat, mixed with hemorrhage and liquefactive necrosis (Fig 4a). Histology revealed a typical biphasic synovial sarcoma (Fig 4b), which was confirmed with SYT-SSX fusion gene detection of fluorescence in situ hybridization (Fig 4c). What is your diagnosis?