Urothelial carcinoma with tonsillar metastasis in a dog.

Given its unusual lymphatic drainage system, the tonsil is a rare site of metastasis, with few reports in the human and veterinary literature. Prognosis in cases of tonsillar metastasis is reportedly poor. We describe here a unique case of urinary bladder urothelial carcinoma (UC) with metastasis to the tonsil in an 11-y-old, spayed female, mixed-breed dog. At presentation, the patient had a history of a growing neck mass and increasing lethargy, hyporexia, weight loss, drooling, and diarrhea for 2 wk. A carcinoma was diagnosed by cytology. Given the poor prognosis, the patient was euthanized. Postmortem examination revealed masses in the inguinal region, cranioventral neck region including tonsil, and urinary bladder. Histologically, the masses were composed of large polyhedral cells arranged in dense sheets and nests with occasional large, clear, intracytoplasmic vacuoles. Neoplastic cells were multifocally positive for uroplakin III and cytokeratin 8/18 by immunohistochemistry. UC with metastasis to tonsil and lymph nodes was diagnosed.