Diagnostic Pitfall in Cardiac Angiosarcoma: Initial Misdiagnosis as Masson Tumor Due to Sampling of Necrotic Tissue

Figure 1. Axial contrast-enhanced CT of the chest demonstrating pericardial effusion. Axial CT image shows a circumferential low-attenuation fluid collection (arrow) surrounding the cardiac silhouette, consistent with pericardial effusion.

Figure 2. CTA chest demonstrating large mediastinal mass with mass effect. CTA demonstrates a large heterogeneous mass (>10 × 11 cm) in the right mediastinum/right lung base (Red arrow) causing significant mass effect on the right heart with leftward displacement. An expansile lytic lesion of the left posterior ninth rib (6–8 cm) is also noted (Yellow arrow).

Figure 3. Axial FDG PET/CT demonstrating predominantly photopenic mediastinal and left posterior ninth rib masses with peripheral metabolic activity. Axial fused FDG PET/CT images show a large mediastinal mass and associated left posterior ninth rib lesion that are largely photopenic (stars) with mild heterogeneous peripheral FDG uptake (arrows), suggesting central necrosis with viable tumor at the periphery.

Figure 4. Cardiac MRI demonstrating heterogeneous pericardial mass with mass effect. (A): Cardiac MRI shows a mass arising from the right anterior epicardial/pericardial region with central non-enhancement suggestive of necrosis (stars) and peripheral nodular enhancement (arrows), compressing the right atrium and right ventricle. (B): Left posterior ninth rib lesion with central non enhancement suggestive of necrosis (star) and peripheral enhancement (arrow).

Figure 5. H&E image of representative tumor. An infiltrative mass of endothelial cells forming irregularly shaped, abortive vessels with abundant background hemorrhage, characteristic of angiosarcoma. Scale bar: 300 µm.

Figure 6. Histopathologic and immunohistochemical characterization of angiosarcoma. (A) Hypercellular region with epithelioid morphology and multiple mitotic figures (arrowheads; H&E, 40×). (B–D) Immunohistochemical stains demonstrating diffuse positivity for vascular markers ERG, CD31, and CD34, confirming endothelial differentiation. Scale bar: 300 µm.