Intravascular bronchiolo-alveolar tumor (IVBAT): A low-grade sclerosing epithelioid angiosarcoma of lung.

Intravascular bronchiolo-alveolar tumor (IVBAT) is a rare and highly distinctive pulmonary tumor of disputed cellular nature. Both epithelial and endothelial differentiation of this neoplasm have been suggested. We have studied multiple nodules of IVBATs from three patients by light and electron microscopy and by immunohistochemical methods for Factor VIII-related antigen (FVIII RAG). Our light and ultrastructural studies are in essential agreement with the previous suggestion of the endothelial nature of the neoplasm and our demonstration of the presence of FVIII RAG in many of the tumor cells offers new evidence strongly supportive of their endothelial differentiation. We believe that IVBAT and a group of extrapulmonary tumors described as epithelioid hemangioendothelioma and endovascular papillary angioendothelioma are similar biologically indolent neoplasms of epithelioid and dendritic endothelial cells characterized by stromal sclerosis, intravascular spread, a low incidence of metastases and slow clinical evolution. Thus, we regard IVBAT as a low-grade sclerosing angiosarcoma of the lung.