Immunohistochemical staining patterns of Ki-67 and p53 in florid reactive urothelial atypia and urothelial carcinoma in situ demonstrate significant overlap.

Flat urothelial lesions with atypia may pose significant diagnostic challenges. Given frequent increased proliferation rates in florid reactive urothelial atypia and limited studies on the interpretation of p53 stains in the urothelium (following current standard guidelines for correlation with P53 mutation status), we sought to further study the discriminatory value of Ki-67 and p53 for florid reactive urothelial atypia versus urothelial carcinoma in situ (CIS). Bladder specimens diagnosed as reactive urothelial atypia (n = 40) and CIS (n = 40) were assessed by immunohistochemical staining with antibodies for Ki-67, p53, CD44, and CK20. Immunoreactivity was scored based on percent cells positive for Ki-67 and pattern of reactivity with p53 (aberrant: diffuse strong positive or negative; normal: patchy/wild type). CD44 and CK20 reactivity patterns served as adjunctive internal validation controls for reactive urothelial atypia and CIS, as previously described. In reactive urothelial atypia, Ki-67 ranged from 0% to 90% (mean, 34% ± 26) with 30 cases (75%) having >10%. In CIS, Ki-67 ranged from 5% to 95% (mean, 50% ± 25) with 17 cases (43%) having >50%. In all 40 cases (100%) of reactive urothelial atypia, p53 expression had a wild-type pattern. In CIS, aberrant p53 expression was identified in 15 cases (37%): 3 cases (7%) were p53 negative (i.e. null phenotype) and 12 cases (30%) showed strong and diffuse nuclear reactivity (in >85% of cells). The remaining 25 cases (63%) of CIS had a p53 wild-type pattern of expression. Cytoplasmic CK20 immunoreactivity in umbrella cells was seen in 34 cases (85%) of reactive urothelial atypia, and 6 cases (15%) were negative. In addition, 35 cases (88%) of reactive urothelial atypia demonstrated full-thickness CD44 expression, while 5 cases (12%) had expression confined to the basal/parabasal layers of the urothelium. Strong and diffuse CK20 positivity was present in 39 cases (98%) of CIS, and patchy positivity was detected in 1 case (2%). None of the CIS cases overexpressed CD44: 16 cases (40%) showed focal expression in the nonneoplastic basal cell layer; 24 cases (60%) demonstrated no staining. In summary, Ki-67 has poor discriminatory value for reactive urothelial atypia versus CIS and adds little to the classic CK20/CD44 immunophenotype. While p53 sensitivity for CIS is relatively low (30%) and interpretation as either wild type or negative may be challenging in a small subset of cases, strong and diffuse nuclear reactivity was 100% specific in the distinction from florid reactive urothelial atypia in this cohort.