Variant histology in bladder cancer: how it should change the management in non-muscle invasive and muscle invasive disease?

Bladder cancer (BC) is a frequent type of carcinoma with an estimated incidence of approximately 100,000 men and women each year in the European Union (EU) with an associated mortality of 30,000 of these patients. In more than 70% the disease is diagnosed in a non-muscle invasive stage with the chance of minimally invasive, local treatment only, which might be required repetitively due to high rate of recurrence. In contrast, muscle invasive or metastatic stages need multimodal treatment strategies including surgical treatment and chemotherapy (CTX) in neoadjuvant (NAC), adjuvant, or palliative settings. Therapy recommendations and guidelines mainly refer to the most common histological type of BC, pure urothelial carcinoma (UC). However, BC can be classified as urothelial and non-UC. Non-urothelial BC and variants of UC account for up to 25% of all BCs. Further discrimination can be made into epithelial and non-epithelial non-UC. Most of the non-UCs are of epithelial origin (approximately 90%) including squamous-cell carcinoma, adenocarcinoma and small-cell carcinoma. Non-epithelial tumors are rare and include variants as sarcoma, carcinosarcoma, paraganglioma, melanoma and lymphoma. Even though it is unclear whether the prognosis of non-urothelial cancer truly differs from that of UC, there is evidence that additional variant histology might prognosticate an impaired prognosis. Accordingly, aggressive behavior and often advanced stages at primary presentation are frequently observed in non-UC arguing for radical and sometimes different treatment strategies as compared to pure UC. This review aims to summarize the available data for the most common histological variants of non-urothelial BC.