Pleomorphic Variant of Leydig Cell Tumor in a Dog

Leydig cell tumor is relatively common in dogs, accounting for approximately a quarter to half of all canine testicular tumors. While most Leydig cell tumors are benign, malignant forms have been reported. A 13-year-old Pomeranian visited a local hospital with a chief complaint of edema in the left testicle. Radiography and ultrasonography revealed a mass in the left testicle; therefore, orchidectomy was performed. Microscopic analyses revealed the growth of a neoplastic mass compressing adjacent seminiferous tubules. Immunohistochemistry confirmed that the neoplasm was derived from Leydig cells. Neoplastic cells showed marked cellular and nuclear pleomorphisms, which is one of criteria for malignant Leydig cell tumor. However, the immediate postsurgical outcome was good, and seven months after surgery, the patient recovered well without any complications or recurrence. Therefore, the present study indicates that cellular and nuclear polymorphisms cannot be alone inform the differentiation between benign and malignant Leydig cell tumor in dogs.