Nested variant of urothelial carcinoma.

Nested variant of urothelial carcinoma is a rare neoplasm that is histologically characterized by large numbers of small, closely packed, haphazardly arranged, poorly defined, confluent irregular nests of bland-appearing urothelial cells infiltrating the lamina propria and the muscularis propria. Due to the cells' deceptively bland appearance, the tumors are sometimes misdiagnosed as benign lesions, leading in some cases to a significant delay in establishing the correct diagnosis and thus contributing to this neoplasm's advanced stage. Nested variant of urothelial carcinoma must be differentiated from the benign proliferative lesions of urothelium, such as von Brunn nests, cystitis cystica, cystitis glandularis, nephrogenic adenoma, inverted papilloma, and paraganglioma.