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Infiltrative areas are seen in approximately one-third of cases of urothelial carcinoma. The invasive component usually consists of nests, cords, clusters, or individual tumor cells arising from the base of the lesion. Invasion is more often seen with high-grade tumors, although occasional cases may have low-grade cytology (as seen here). The invasion in this case was limited to lamina propria (pathologic Stage T1). Muscularis propria was uninvolved.