Renal cell carcinoma, especially with extensive involvement of peritoneal cavity, is rarely described in horses. This study described a case of renal cell carcinoma in an 8-year-old mare, which presented with an enlargement in the left flank, decreased appetite and straining to defecate. Clinically, a tumor in the left retroperitoneal space was detected by rectal examination and ultrasonography. The animal died after one month of clinical evolution. At necropsy, the left kidney was completely effaced by a 50cm in diameter, irregular, round, lobulated cystic mass with necro-hemorrhagic areas. Within the peritoneal cavity, there was severe hemoperitoneum and numerous sessile and pedunculated masses ranging from 2.0 to 15.0cm diameter that were attached to the visceral and parietal peritoneum. Histologically, the primary neoplasia and its metastasis were composed by a proliferation of epithelial cells, which were arranged in a tubulopapillary pattern. Diagnosis of renal cell carcinoma with peritoneal carcinomatosis and hemoperitoneum was based on macroscopic aspects, and mainly on the histological features.
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