[Large nested variant of urothelial carcinoma of urinary bladder: a clinicopathological analysis of five cases].

To investigate the clinicopathologic features, diagnosis and differential diagnosis of large nested variant of urothelial carcinoma (LNUC) of urinary bladder. The clinical and pathologic data of five cases of LNUC of urinary bladder diagnosed between January 2014 and December 2018 at the Department of Pathology, Zhejiang Provincial People's Hospital were analyzed by immunohistochemistry (IHC) and Sanger sequencing. The relevant literature was reviewed. All five patients were male with a mean age of 63 years (range, 48 to 81 years). The mean tumor size was 3.4 cm (range, 1.7 to 4.7 cm). Histologically, the invasive LNUC tumor cells formed medium to large sized nests of varying shapes, from regular round, bulbous, oval to irregularly fused, branched, dumbbell shaped glands, with mild stromal reaction. In all five cases, focal central necrosis and microcystic changes in the tumor nests were identified. Cytologically, the tumor cells were low grade in four cases; the remaining case was overall low grade with focal high grade areas. Mitoses were scarce. All cases possessed surface urothelial tumors, including three low-grade papillary carcinomas, one high-grade papillary carcinoma and one carcinoma in situ. Three of the LNUC were accompanied by small nested variant of urothelial carcinoma and two by conventional high grade invasive urothelial carcinoma. Perineural involvement and angiolymphatic invasion were each noted in four tumors. Radical cystectomy was performed in four cases with TNM stages as followings: pT3aN0M0 in two cases, pT4aN0M0 and pT4aN1M0 in one case each. The remaining case had transurethral bladder resection and was of pT2 stage. By IHC, all five cases were positive for CK7 and p40; four were positive for GATA3; two were positive for CK20; and the mean Ki-67 proliferation index was 18%. TERT promoter mutation status were successfully performed in three cases, with one showing mutation (C228T) and two were wild type. All patients received postoperative chemotherapy. At a follow-up of 2 to 11 months, one patient died of unrelated causes, two patients developed metastases, and two were alive with no evidence of disease. LNUC is a histologic subtype of urothelial carcinoma with deceptively benign features but aggressive behavior, and appreciation of its unique infiltration patterns can aid in diagnosis and differential diagnosis. LNUC tends to coexist with small nested variant of urothelial carcinoma, suggesting these may represent different manifestations of the same urothelial carcinoma subtype.