Primary malignant histiocytosis of the brain in a dog

Malignant histiocytosis is a well-recognized canine tumour, occurring prima rily in Bernese mountain dogs and characterized by disseminated histiocytic infiltration of multiple visceral organs. This report describes the light microscopical and ultrastructural features of a, neoplasm composed of malig nant histiocytes and confined to the brain. A poorly demarcated mass in the right parieto-occipital lobe of a miniature schnauzer was composed of loos ely aggregated, pleomorphic cells with abundant eosinophilic cytoplasm, exp anding the meninges. Many binucleated and multinucleated giant cells and mi totic figures were seen. Immunohistochemically, the tumour cells reacted in tensely for lysozyme. Ultrastructurally, the neoplastic cells had features of histiocytic cells with abundant lysosomes. The findings in this case wer e strikingly similar to those of disseminated malignant histiocytosis descr ibed in other dog breeds. (C) 1999 W.B. Saunders Company Limited.