Histologic and immunohistochemical characterization and frequency of rare subtypes of canine invasive urothelial carcinoma

While most cases of human muscle-invasive bladder cancer (MIBC) are diagnosed as conventional urothelial carcinoma (UC), there is growing recognition of histologic subtypes and divergent differentiation within conventional UC. In this study, 31 dogs with UC (33 slides) were evaluated for histologic subtypes and divergent differentiation, as specified by the recent World Health Organization update on the classification of urinary bladder cancers. Slides were reviewed by a medical uropathologist and 3 board-certified veterinary pathologists and assessed for expression of uroplakin III, vimentin, and E-cadherin. All tumors were classified as UC. Fifteen cases were identified as conventional UC. Among the remainder, 8 displayed glandular differentiation, 4 were classified as sarcomatoid UC, 2 showed squamous differentiation, 1 case was classified as a large nested subtype, and 1 case was classified as a tubular/microcystic subtype. This study found a higher frequency of certain histologic subtypes and divergent differentiation in canine UC, particularly sarcomatoid UC and UC with glandular differentiation, compared to previous reports of both canine UC and human MIBC. The relatively high prevalence of the sarcomatoid UC and UC with glandular differentiation in dogs observed in this study suggests that canine UC may serve as a valuable translational model for evaluating novel therapeutic agents, particularly for these rare and aggressive subtypes in humans.