Mixed-Grade Urothelial Carcinoma: Insights Into Clinical Behavior and Prognostic Implications Compared to Pure Low-Grade and High-Grade Urothelial Carcinomas.

Context.—: Low-grade urothelial carcinoma (LGUC) and high-grade urothelial carcinoma (HGUC) are distinguished based on architectural and cytological features, with the anticipation that HGUC exhibits more aggressive behavior and a worse prognosis compared to LGUC. The current World Health Organization classification recognizes mixed-grade urothelial carcinoma (MGUC, for the purposes of this study) as a separate category that behaves like LGUC if the high-grade component is <5% and states that any tumor with ≥5% high-grade component should be graded as HGUC.

Objective.—: To evaluate the risk of tumor recurrence, grade, and stage progression of MGUC compared to LGUC and HGUC.

Design.—: A total of 150 de novo noninvasive polypoid urothelial carcinomas (41 cases of MGUC, 59 of LGUC, and 50 of HGUC) were included. Tumor recurrence, grade, and stage progression were compared among the MGUC, LGUC, and HGUC cases.

Results.—: Tumor recurrence was observed in 14 of 41 (34.2%) of MGUC, 33 of 59 (55.9%) of LGUC, and 28 of 50 (56%) of HGUC. Grade progression occurred in 5 of 41 (12.2%) of MGUC cases and 5 of 59 (8.5%) of LGUC cases. No stage progression was observed in LGUC or MGUC cases, while 7 of 50 (14%) of HGUC cases showed stage progression. MGUC was associated with lower odds and hazard of recurrence compared to LGUC. The rate of grade progression was higher in MGUC and occurred after a shorter interval compared to LGUC.

Conclusions.—: MGUC showed a prognosis closer to LGUC. Our study supports the current recommendation to classify tumors with <5% high-grade component as MGUC, as these tumors display clinical characteristics and outcomes close to that of pure LGUC.