How to distinguish between reactive and neoplastic flat urothelial lesions

Flat urothelial lesions are common and recognition is important for patient management. Over- and undertreatment can be a consequence of misdiagnosis. Reporting the right diagnosis is also important for the follow-up. We describe the most frequent entities with a focus on clinical meaning. Several of these described lesions do not figure in the WHO 2022 classification; therefore knowledge of them is important. We also discuss benign, precursor and malignant lesions and suggest the latest nomenclatures given by international societies. Molecular data, as far as currently known, are included where possible. The aim is to give a practical and precise overview of this complicated topic.