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A collision urothelial carcinoma and leiomyosarcoma in the urinary bladder of a dog

Collision tumors occur when two independently coexisting, histologically distinct neoplasms are closely associated, but microscopically separate within a single macroscopic mass. These tumors are rarely reported, and their frequency in dogs is unknown. In the dog, they are mostly reported in the skin (four cases) and oral cavity (two cases), while there are single reports of these tumors in the spleen, testicle and in the iliac lymph node. A 2 cm x 1 cm mass removed from the mucosa of the urinary bladder of a mixed-breed bitch of unknown age, with reported hematuria, was submitted for histopathology. Grossly, the mass was brown with multifocal yellow areas, and a rough surface. On the cut section the tissue was white and firm. Histology revealed a high-grade urothelial carcinoma in the mucosa, which reached to the margins. Beneath the basal lamina, adjacent to the urothelial carcinoma, was an infiltrative mesenchymal neoplasia composed of interlacing bundles of spindle cells. All neoplastic mesenchymal cells were strongly positive for vimentin, most cells stained with smooth muscle actin, and few cells stained with desmin. PTAH stining was negative in neoplastic mesenchymal cells. Based on results of histology and immunohistochemistry, the neoplasia was diagnosed as collision urothelial carcinoma and leiomyosarcoma. Urothelial carcinoma is the most commonly diagnosed, while leiomyosarcoma is an uncommon tumor in the urinary bladder of the dog. These two tumors have so far not been diagnosed concurrently, hence there is no data on prognosis and biological behavior.

Collision tumor, urothelial carcinoma, leiomyosarcoma, urinary bladder, dog

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