Abstract
A 7-year-old stallion with progressive left testicular enlargement was presented. Grossly, the excised testicle measured 25 × 15 × 12 cm and weighed 3.7 kg. It was multinodular with a gray-white surface; however, the right testis was normal. Histologically, the neoplastic cells were disseminated diffusely in the tumoral stroma with a minimal fibrovascular stroma. Neoplastic cells were round to polygonal with abundant eosinophilic cytoplasm and large round to oval vesicular or hyperchromatic nucleous with a single prominent nucleolus. Immunohistochemically, the neoplastic cells were positive nuclear immunostaining for C-KIT and negative for OCT3/4. According to gross, histopathological, and immunohistochemical characteristics, the diffuse type of seminoma was diagnosed. Nine months later, the follow-up observation of the case showed that the tumor had no recurrence and metastasis.