Unusual biphasic renal tumour in a cat: gross and microscopic features and differential diagnosis

Renal tumors with biphasic pattern are a subset of neoplasms rarely occurring in humans. Mixed epithelial and stromal tumor of the kidney (MESTK) is the term given to uncommon biphasic renal tumours, characterized by a mixture of epithelial and stromal components [1]. To the best of our knowledge, biphasic renal tumours are not described in veterinary medicine. The aim of this report is to describe the unusual gross and histological features of a renal tumour occurring in a 10-year-old, male castrated, European shorthair cat, presented with abdominal enlargement. Ultrasound examination revealed a heterogeneous mass originating from right renal parenchyma, occupying most of the caudal abdomen. CBC, serum chemistry with SDMA and urinalysis were within normal limits. Total right nephrectomy was performed. On gross examination, the affected kidney showed markedly increased volume (15x9x9 cm), with renal parenchyma substituted by a solid and cystic mass, characterized by multiple cysts of varying size, containing a clear to haemorrhagic fluid, admixed with white to greysh, solid areas, with multifocal mixoid appearance. No infiltration in the adjacent tissues or metastasis were detectable at the time of surgery. Samples of the mass were routinely processed for histology and immunohistochemistry for pan-cytokeratin (CK), vimentin, CAM5.2, α-smooth muscle actin (α-SMA). Histopathological examination revealed a neoplasm with biphasic features comprising epithelial and mesenchymal elements. The epithelial component was constituted by tubules and cysts, lined by cuboidal, well-differentiated epithelium, with frequent clear cell change. Urothelial-like epithelium also lined some of the larger cysts. The epithelial elements were variably interspersed throughout the mesenchymal component, characterized by bundles of spindle cells, multifocally surrounded by abundant mixoid matrix. Spindle cells showed slightly pleomorphic, ovoid nuclei, with occasional mitotic figures. The epithelial component showed intense and diffuse immunoreactivity for CK, CAM5.2 and vimentin, whereas spindle cells revealed intense, diffuse immunostaining for vimentin, and intense, variably distributed positivity for α-SMA. Histological and immunohistochemical findings suggested a biphasic renal tumour with features compatible with MESTK, which is tipically composed of tubules and cysts, interspersed between spindle cells showing smooth muscle, fibroblastic, or myofibroblastic differentiation. Although an aggressive behaviour was rarely reported, MESTK usually behaves in a benign fashion following surgical excision [1]. The cat is healthy after 2 months of follow-up. Differential diagnosis included leiomyomatous renal cell carcinoma, a rare human tumour composed of nests and cords of clear epithelial cells forming solid areas, tubules or papillary structures, immersed in a stroma composed of mature smooth muscle [3]. Renal leiomyosarcoma, rarely described in cat [4], was also excluded due to the biphasic pattern, the bland cytological features and variable α-SMA expression of spindle cells.