Tonsillar lymphangiomatous polyp in an adult dog.

A female spayed cocker spaniel dog, aged 8 years, developed a large, smooth pedunculated mass arising from the right palatine tonsil. Histology revealed that the mass was composed of many, variably dilated, thin-walled lymphatic channels filled with pale eosinophilic fluid lacking red blood cells and embedded in a dense, fibrovascular stroma. The dilated lymphatic channels were lined by a single layer of flattened, discontinuous endothelium with scattered intraluminal valves. Periodic acid-Schiff staining highlighted the discontinuous basement membrane and immunohistochemistry revealed strong cell membrane and cytoplasmic immunoreactivity for CD31 and von Willebrand factor, respectively. The clinical and histological findings were consistent with a tonsillar lymphangiomatous polyp - an uncommon, benign tumour of the tonsil in man that has previously been unrecognized in veterinary medicine.