Two canine malignant vascular tumours with features of human retiform haemangioendothelioma

Within the human medical literature, retiform haemangioendothelioma (RHE) is an established and well-recognized histopathological variant of endothelial tumours, but to date RHE has not been reported in animals. These tumours are characterized by the presence of elongate, arborizing vascular channels lined by neoplastic endothelium with prominent, often bulging ('hobnail') nuclei supported by a dense collagenous matrix and accompanied by abundant lymphoplasmacytic inflammation. Immunohistochemically, the neoplastic cells typically express endothelial markers such as von Willebrand factor and CD31. Human RHEs are categorized as low-grade malignancies. This report describes two canine vascular tumours with features consistent with RHE. In both cases there was suspected or known widespread tumour metastasis.