Mixed-grade papillary urothelial carcinoma with a minor high-grade component shows a significantly worse clinical outcome than low-grade papillary urothelial carcinoma.

Grade heterogeneity is common in papillary urothelial carcinoma (PUC) of the urinary bladder. Mixed-grade PUC is generally treated as high-grade if the high-grade component accounts for ≥5 % of the tumor. However, the clinical behavior of predominantly low-grade PUC with a minor high-grade (<5 %) component remains uncertain. We retrospectively searched our pathology files and identified 52 cases of non-invasive mixed-grade PUC (predominantly low-grade PUC with a minor high-grade (<5 %) component). These patients' clinical outcomes were compared with those of 2 other cohorts, 52 cases of pure low-grade PUC and 50 high-grade PUC. During follow-up, 27 patients with mixed-grade PUC developed cancer recurrence in an average of 25 months, and 12 patients experienced progression to high-grade PUC, including invasive UC in 3 patients, in an average of 17 months. For comparison, 35 patients with low-grade PUC experienced cancer recurrence in an average of 17 months, but only 1 (2 %) experienced progression to high-grade non-invasive PUC; 34 patients with high-grade PUC experienced recurrence in an average of 13 months, and 10 (20 %) experienced progression to invasive UC. There was no significant difference in cancer recurrence in the 3 cohorts; cancer grade and stage progression rates in patients with mixed-grade PUC were significantly higher than those in those with low-grade PUC, but stage progression rate in patients with mixed-grade PUC was significantly lower than that in high-grade PUC. In conclusion, patients with mixed-grade PUC had a significantly worse clinical outcome than those with pure low-grade PUC but a significantly better outcome than those with high-grade PUC. Our results suggest that mixed-grade PUC should be treated as a distinct category from low-grade PUC.