Bilateral renal histiocytic sarcoma with disseminated lung involvement in a Siberian Forest Cat

Case summary A 5-year-old female neutered Siberian Forest Cat presented with a 7-day history of lethargy, hyporexia and weight loss. Abdominal ultrasonography revealed bilateral renal changes suggestive of neoplasia. Thoracic radiography documented diffuse pulmonary nodules. The cat was euthanased during diagnostic investigations. Histopathological assessment and immunohistochemical staining of post-mortem renal biopsies were consistent with a histiocytic lesion, most likely histiocytic sarcoma (HS). The lung lesions were suspected of representing disseminated disease. Relevance and novel information HS is considered a rare neoplastic process in cats. This report describes a case of feline bilateral renal HS with suspected concomitant pulmonary involvement. A primary renal origin was suspected, with the lung lesions being a result of disseminated disease. Renal HS should be included as a differential diagnosis when renal ultrasonography reveals changes suggestive of neoplasia.