Characteristics and clinical significance of histological variants of bladder cancer

In the past 10 years evidence for the clinical relevance of variant histology in urinary bladder cancer has been increasing. This increase has resulted in new classifications of urothelial cancers by the WHO in 2016, highlighting the importance of an accurate morphological description of pathological specimens for the therapeutic management of patients with bladder cancer. The rising awareness of the importance of an accurate pathological report manifests itself in the increasing prevalence of reporting of variant histology in daily practice. Histological variants can generally be divided into urothelial and nonurothelial. Urothelial variants often have similar features that also have specific morphological phenotypes, whereas nonurothelial variants have independent features. Overall, histological variants follow a more aggressive clinical course than conventional urothelial carcinoma, but conclusive data on their effect on survival are currently lacking. The clinical relevance of variant histology can manifest at three different levels: diagnostic, as identification is challenging and misinterpretation is not uncommon; prognostic, for patient risk stratification and outcome estimation; and therapeutic, as particular variants could be responsive to specific treatment strategies. An accurate morphological description of histological variants is necessary for patient consultation and therapy planning. Moreover, the association of variant histology with specific mutation patterns promises to be helpful in discovering targeted therapeutic approaches based on specific molecular pathways.