Multiple Sex Cord-stromal Tumors in a Standardbred Stallion Testis

A 12-year-old Standardbred stallion presented with a 5-month history of a growing mass in the left testis as well as an overall decrease in left testicular size. Palpation and ultrasonography of the left testis revealed a firm, hypoechoic, clearly delineated soft tissue mass in the craniolateral portion of the testis that measured 2.5 × 2.3 × 1.9 cm. Two smaller, hypoechoic regions also were visible ultrasonographically in the left testis, suggesting the presence of multifocal/multicentric neoplasia. The affected testis was very small (testicular volume of 40.3 cm³). The right testis was significantly larger (144.3 cm³), and the parenchyma was ultrasonographically normal. Due to the concern that these findings could indicate the presence of a more aggressive tumor type, unilateral orchiectomy was performed. Multiple soft tissue masses were identified grossly, and histopathologic evaluation identified the larger mass as a Sertoli cell tumor and the two smaller masses as mixed sex cord-stromal tumors with Sertoli cell and Leydig cell differentiation. To our knowledge, this the first report of concurrent Sertoli cell and mixed sex cord-stromal tumors in a single descended equine testis.