Histological description: The tumor has both epithelial and mesenchymal differentiation. The epithelial component consists of predominantly low grade non-invasive papillary urothelial carcinoma. The mesenchymal component is composed of both nonspecific malignant spindle cells and osteogenic sarcoma.

Discussion. This lesion is a classic sarcomatoid carcinoma with biphasic epithelial and mesenchymal components. It is unusual in that typically the epithelial component, when present, is high grade. Nonetheless, primary osteogenic sarcomas of the bladder for the most part do not exist such that to propose a collision tumor with a low grade urothelial carcinoma would be incorrect. In the past, some experts would have used the term “carcinosarcoma” for this case and “sarcomatoid carcinoma” when the mesenchymal component was nonspecific malignant spindle cells. It is now accepted that both these terms should be used synonymously with the latter term preferred. As some urologists may not be familiar with both terms, I diagnose these cases as “sarcomatoid carcinoma (carcinosarcoma)”. I also list both the epithelial and mesenchymal components, although there is no prognostic significance to the different elements. If the patient develops metastases and the original material was not available for review, the pathology report would have detailed information on the various histological patterns present in the original tumor to compare to the more recent material. In the absence of an overt epithelial component, sarcomatoid carcinoma can be diagnosed when there is a nonspecific malignant spindle cell component that expresses epithelial markers immunohistochemically. The most sensitive and specific marker is high molecular weight cytokeratin.