Mature teratoma arising from an undescended testis in a horse: comparison between ultrasonographic and morphological features.

Scant information is available on the ultrasonographic appearance of different testicular tumours in the stallion. Preoperative ultrasound imaging and gross and microscopic features of a testicular teratoma in a horse is described. An asymptomatic 4 years old cryptorchid Arabian horse was admitted for orchiectomy. Combined transabdominal and inguinal ultrasound examination revealed a large complex ovoid mass, containing both solid and cystic elements with internal echoes, located dorsally to the superficial inguinal ring. Two main hypoechoic cavities divided by a linear hyperechoic septum were evident, with a hyperechoic circular structure inside the lumen of the largest one. A tumour of the undescended testis was suspected. Ultrasound findings guided a provisional diagnosis of teratoma that was confirmed by histology following surgical removal of the tumour. Due to the correspondence between ultrasonographic and morphological features, trans-abdominal ultrasonography was conclusive in the preoperative planning.