The nested variant of transitional cell carcinoma: a neoplasm resembling proliferation of Brunn's nests.

This report describes four cases in which a malignant transitional cell neoplasm initially manifested as small nests and abortive tubules infiltrating the lamina propria. Many of the tumor cells were only slightly atypical but careful examination revealed at least some significantly anaplastic cells in every case, the degree of cellular atypia tending to parallel the depth of invasion. Despite the initial impression of a benign lesion resembling a proliferation of Brunn's nests, these carcinomas tended to be persistent and aggressive.