Figure 1. Cytohistological correlation of four cases originally diagnosed as ‘atypical urothelial cells’. Such cases are reclassified as negative for high-grade urothelial carcinoma (NHGUC), HGUC, or low-grade urothelial neoplasm (LGUN) under the Paris System (TPS). NHGUC: A: Cytologically, a few scattered urothelial cells exhibit slightly enlarged nuclear-to-cytoplasmic ratio (<0.5). The presence of neutrophils in the background suggests active inflammation. B: The nuclei of urothelial cells display smooth nuclear membrane and fine chromatin pattern, compatible with NHGUC under TPS. C: Histologically, the urothelium shows reactive nuclear atypia associated with mixed inflammatory infiltrates, without evidence of HGUC. HGUC: D: Cytologically, atypical urothelial cells show hyperchromatic nuclei with increased nuclear-to-cytoplasmic ratio (>0.7). E: The atypical cells form a small cluster displaying severe nuclear pleomorphism and hyperchromasia, indicating HGUC under TPS. F: Histologically, HGUC consists of neoplastic urothelial cells showing high-grade nuclear atypia. The papillae are frequently fused and anastomosed, forming cords, trabeculae, or small nests. G: Cytologically, a small, hyperchromatic cellular cluster (green arrow) of neoplastic urothelial cells showing increased nuclear-to-cytoplasmic ratio (>0.7), nuclear membrane irregularity, clumped chromatin, and hyperchromasia are visible, diagnostic of HGUC under TPS. Some urothelial cells show degenerative-appearing chromatin pattern (yellow arrows), including irregular clumping, clearing with prominent nuclear membrane (ground-glass appearance), pyknosis, and karyorrhexis. These features might interfere with a cytopathologist's judgment in deciding malignancy. H: Histologically, HGUC consists of haphazardly arranged neoplastic urothelial cells and a fibrovascular core covered with a thickened urothelial layer, forming solid, sheet-like architecture. LGUN: I: A three-dimensional papillary structure with a fibrovascular core consists of monotonous cells, which are mildly crowded and overlapped each other but maintain polarity. These urothelial cells exhibit minimal nuclear atypia, compatible with LGUN under TPS. J: At low-power magnification, variable-sized nests and trabeculae of urothelial cells invaginate into the underlying lamina propria, producing endophytic mass covered by normal urothelium. K: A high-power view of inverted urothelial papilloma reveals inversion of papillary fronds surrounded by fibrovascular stroma. Microcysts containing eosinophilic material are evident. A, B, D, E, G, and I: Papanicolaou stain; C, F, H, J, and K: hematoxylin and eosin stain. Original magnification, A, B, C, F, H, and I: ×200; D, E, and G: ×400; J, ×40; K, ×100.