ABSTRACT
Renal interstitial cell tumors are benign tumors of renomedullary origin; however, malignant features have not been reported in dogs, to our knowledge. A 17-y-old spayed female Maltese dog was presented to a local animal hospital with a mass in the right abdomen. Clinicopathologic findings prior to surgery revealed renal insufficiency and anemia. Imaging revealed that the right kidney was enlarged by an amorphous mass with opaque areas, indicative of mineralization. Upon histologic examination, the mass was comprised of malignant mesenchymal cells that produced mucinous matrix. The tumor cells were positive for vimentin and COX-2, but negative for pancytokeratin; the matrix stained positively with alcian blue. Therefore, the mass was diagnosed as a renal interstitial cell tumor, with malignant features. COX-2 may be useful in the diagnosis of canine renal interstitial cell tumors, similar to its diagnostic role in humans.