Ultrasonographic features and clinical outcome of an anaplastic carcinoma arising from a uterus masculinus in an intact, non-cryptorchid male dog.

A 6-year-old intact male dog was referred because of persistent hematuria despite multiple courses of antimicrobial therapy. Urogenital ultrasonography revealed a large, paraprostatic, uterus-like, fluid-filled structure containing echogenic fluid, associated with an adjacent large, heterogeneous mass located cranially and to the right of the lesion. These findings raised suspicion for malignant transformation of a uterus masculinus associated with persistent Müllerian duct syndrome. Exploratory laparotomy confirmed the presence of a uterus masculinus with blind-ended uterine horns, associated adhesions, and a large mass arising from the right cranial aspect. Surgical excision of the abnormal structure and orchiectomy were carried out. Histopathology confirmed an anaplastic carcinoma of Müllerian origin arising from persistent Müllerian duct remnants. The tumor exhibited highly aggressive biological behavior characterized by extensive local invasion. There was rapid clinical deterioration after surgery, including imaging findings consistent with pulmonary, hepatic, and peritoneal metastatic disease, ultimately leading to euthanasia 5 wk after diagnosis. Key clinical message: Persistent Müllerian duct remnants should be considered as a differential diagnosis for paraprostatic cavitary lesions in intact male dogs, even in the absence of cryptorchidism. Malignant transformation of a uterus masculinus, though rare, may exhibit highly aggressive biological behavior with rapid metastatic dissemination.