RESUMEN
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.