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

Background and Clinical Significance: Cardiac and mediastinal angiosarcomas are rare, aggressive malignancies that often present with nonspecific symptoms and pose significant diagnostic challenges. Tumor heterogeneity and necrosis may lead to false-negative biopsy results; Case Presentation: We report a 64-year-old man who initially presented with cardiac tamponade of unclear etiology. Despite an extensive workup, the patient remained asymptomatic for five months before re-presenting with dyspnea and a large mediastinal mass compressing the right heart, along with a lytic rib lesion. Initial ultrasound-guided biopsy of the rib lesion demonstrated a benign vascular proliferation consistent with Masson tumor (intravascular papillary endothelial hyperplasia), which was discordant with aggressive imaging findings. Further evaluation with positron emission tomography-computed tomography (PET-CT) revealed peripheral metabolic activity, and cardiac magnetic resonance imaging (MRI) demonstrated a heterogeneous mass with central necrosis and peripheral enhancement. A repeat CT-guided biopsy targeting the metabolically active region confirmed angiosarcoma, with immunohistochemical staining demonstrating diffuse positivity for ERG, CD31, and CD34. The patient was treated with palliative radiation and paclitaxel-based chemotherapy but experienced rapid clinical decline and transitioned to comfort-focused care; Conclusions: This case highlights the importance of correlating imaging with pathology and emphasizes the risk of sampling error in necrotic tumors. PET-guided biopsy targeting viable tumor regions is essential in cases with discordant findings.