Urothelial carcinoma in situ with "overriding" features can evade detection by mimicking umbrella cells

Urothelial carcinoma in situ (uCIS) is typically recognized by overtly malignant cells with characteristic nuclear features; multiple histologic patterns have been described. A rare "overriding" pattern, in which uCIS tumor cells extend on top of normal urothelium, has previously been mentioned in the literature, but not well described. Herein, we report 3 cases of uCIS with "overriding" features. Detailed morphologic evaluation revealed somewhat subtle cytologic atypia: variably enlarged hyperchromatic nuclei and scattered mitotic figures but with abundant cytoplasm and limited to superficial urothelium. Immunohistochemical (IHC) analysis showed a distinctive diffuse positive aberrant p53 pattern, limited to the atypical surface urothelial cells; these cells also showed CK20+, CD44-, and increased Ki-67. In 2 cases, there was a history of urothelial carcinoma and adjacent conventional uCIS. In the third case, the "overriding" pattern was the first presentation of urothelial carcinoma; therefore, next-generation sequencing molecular testing was also performed, revealing pathogenic mutations in TERTp, TP53, and CDKN1a to further support neoplasia. Notably, the "overriding" pattern mimicked umbrella cells, which normally line surface urothelium, can have abundant cytoplasm and more variation in nuclear and cell size and shape, and show CK20+ IHC. We therefore also evaluated umbrella cell IHC patterns in adjacent benign/reactive urothelium, which showed CK20+, CD44-, p53 wild-type, and very low Ki-67 (3/3). We also reviewed 32 cases of normal/reactive urothelium: all showed p53 wild-type IHC in the umbrella cell layer (32/32). In conclusion, caution is warranted to avoid overdiagnosis of usual umbrella cells as CIS; however, "overriding" uCIS should be recognized, may have morphologic features that fall short of the diagnostic threshold of conventional CIS, and requires further study.