MALIGNANT RHABDOID TUMOR IN THE GASTRIC WALL OF AN AGED ORANGUTAN (PONGO-PYGMAEUS)

A 34-year-old female orangutan (Pongo pygmaeus) developed renal failur e and became uremic. At necropsy, large gastric masses were present ar ound the cardia and in the corpus. Abdominal metastases occurred in th e liver, pancreas, and right ovary. Light microscopic examination of t he tumor revealed polygonal cells with vesicular nuclei and prominent nucleoli. The growth pattern was predominantly solid. Focal areas cont ained excentric cytoplasmic intermediate filament inclusions, as ident ified by immunohistochemistry and electron microscopy. Immunohistochem ical procedures demonstrated mainly the vimentin type of intermediate filaments. Except for occasional cytokeratin, other intermediate filam ent markers and neural, lymphocytic, and histiocytic markers stained n egative. The morphologic and ultrastructural characteristics are typic al for a malignant rhabdoid tumor, a term used in human pathology to d escribe a rare and extremely aggressive malignancy of uncertain histog enesis. Although usually located in the infant kidney, a few reports h ave documented the occurrence of similar lesions in extrarenal sites o f adults. In human tumors, vimentin is often combined with the express ion of cytokeratins. The sparsity of the cytokeratin filaments in this case might be due to species-specific variations and/or may reflect t he hypothesis of a phenotypic concept encompassing a spectrum of histo genetic diversity.