Other types of carcinoma

As a vast majority of bladder cancers (over 90% in Western countries) are urothelial carcinoma in histology, non-urothelial carcinomas, such as squamous cell carcinoma (SCC) and adenocarcinoma, often present diagnostic and therapeutic challenges. SCC can occur in both non-bilharzial- and bilharzial-related bladders; the two subtypes differ in epidemiology, pathogenesis, and clinicopathological features. In addition, SCC of the bladder can present as several histological variants including verrucous, basaloid, and sarcomatoid SCCs. Primary adenocarcinoma of the urinary bladder is also an uncommon neoplasm, histologically subdivided into enteric, mucinous, not otherwise specified, and mixed types. These primary adenocarcinomas frequently become a source of diagnostic confusion with adenocarcinomas secondarily involving the bladder from the adjacent organs, such as the colon, prostate, and female genital tract. Urachal adenocarcinoma is usually discussed together with bladder adenocarcinoma, because they share many clinical and histological features. The 2016 World Health Organization classification delineates urachal glandular tumors into two broad categories: mucinous cystic tumors and non-cystic adenocarcinomas. Clear cell carcinoma (tumor of the Müllerian type) primarily arising from the urinary bladder, a distinct form of non-urothelial carcinoma, is also discussed in this chapter.