On gross examination, PPAs are hemorrhagic tumors involving the lung and/or the pleura, with hemorrhage into the surrounding tissues.24 Microscopic examination usually reveals atypical endothelial cells forming variably sized vascular channels and occasional solid spindles or epithelioid cell nodules. Although intraarterial and periarteriolar involvement as well as extensive hemorrhage are more characteristic for lung-originating tumors, there are no histologic criteria allowing for the determination of the primary origin, either pulmonary or extrapulmonary, of the vascular tumor.1,24 Therefore, an extensive clinical and radiologic evaluation to exclude primary extrapulmonary tumor sites is mandatory to confirm PPA. Positive results for immunohistochemical stains for endothelial markers (CD31, CD34, and von Willebrand factor) play an important role in distinguishing PPA from other sarcomas, lung carcinomas, and pleural mesotheliomas. High grades of tumor cells and mitotic activity are used to distinguish angiosarcomas from benign hemangiomas and low- to intermediate-grade hemangioendotheliomas of the lung.24,25 Large amounts of RBCs filling the neoplastic vessels, hemorrhages, and hemosiderin-laden macrophages are used to differentiate between angiosarcomas and tumors deriving from lymphatic vessels. An epithelioid appearance of tumor cells, lack of inflammatory components, and lack of slit-like vascular spaces are used to distinguish angiosarcoma from Kaposi sarcoma.