Malignant rhabdoid tumor in the gastric wall of an aged orangutan (Pongo pygmaeus).

A 34-year-old female orangutan (Pongo pygmaeus) developed renal failure and became uremic. At necropsy, large gastric masses were present around the cardia and in the corpus. Abdominal metastases occurred in the liver, pancreas, and right ovary. Light microscopic examination of the tumor revealed polygonal cells with vesicular nuclei and prominent nucleoli. The growth pattern was predominantly solid. Focal areas contained excentric cytoplasmic intermediate filament inclusions, as identified by immunohistochemistry and electron microscopy. Immunohistochemical procedures demonstrated mainly the vimentin type of intermediate filaments. Except for occasional cytokeratin, other intermediate filament markers and neural, lymphocytic, and histiocytic markers stained negative. The morphologic and ultrastructural characteristics are typical for a malignant rhabdoid tumor, a term used in human pathology to describe a rare and extremely aggressive malignancy of uncertain histogenesis. Although usually located in the infant kidney, a few reports have documented the occurrence of similar lesions in extrarenal sites of adults. In human tumors, vimentin is often combined with the expression of cytokeratins. The sparsity of the cytokeratin filaments in this case might be due to species-specific variations and/or may reflect the hypothesis of a phenotypic concept encompassing a spectrum of histogenetic diversity.