A non-papillary neoplasm of the urinary bladder, whose classification categories include low-grade intraurothelial neoplasia and urothelial carcinoma in situ. The disease is bladder flat intraepithelial lesion (MONDO_0006111). Also known as: flat intraepithelial lesion of the bladder, flat intraepithelial lesion of the urinary bladder, urinary bladder flat intraepithelial lesion.