A case of canine urothelial carcinoma of the urinary bladder with skull and skeletal metastases.

Canine urinary bladder neoplasia is uncommon, representing less than 1% of canine neoplasms. Amongst cases of urinary bladder neoplasia in dogs, primary urothelial carcinoma is the most frequent. Urothelial carcinomas are malignant invasive tumours which tend to be slow growing and metastasise late. However, the metastatic rate is high, with approximately 50% of cases developing metastases typically to the regional lymph nodes and lungs. Skeletal metastases are rare, with the most common reported sites being the femur and the vertebrae. Metastases to the skull are even rarer, with only a few previously documented cases in dogs. This report describes a case of primary urothelial carcinoma in the urinary bladder of a dog, with widespread metastases, including to the skull. The dog presented for chewing difficulties and limping of the left hindlimb, with large firm masses found on the skull and the left tibiotarsal joint. A necropsy revealed the masses on the head and left tibia were metastatic carcinomas, the latter site also having invaded into the tibiotarsal joint. Additional carcinomas of similar cellular morphology were found in the urinary bladder, ribs, skin and lungs. Neoplastic cells contained cytoplasmic Melamed-Wolinska bodies, a characteristic feature of urothelial carcinomas. Collectively, the findings are suggestive of a primary urinary bladder urothelial carcinoma with widespread metastases, including to rarely documented sites, such as the parietal bone of the skull, left tibia, left tibiotarsal joint and the ribs. This represents the third documented case of skull metastases from a urothelial carcinoma in a dog.