BilIN with varying degrees of cytologic atypia, ranging in severity from BilIN1 to BilIN3

(A) BilIN1: Flat or micropapillary architecture with basally located and relatively uniform nuclei (arrows).
(B) BilIN2: Pseudopapillary or micropapillary architecture with nuclear pseudostratification reaching the luminal surface (arrows). The nuclei demonstrate dysplastic nuclear changes, including enlargement, hyperchromasia, and irregular nuclear membrane. Some variations in nuclear sizes and shapes are seen (arrowheads).
(C) BilIN3: Pseudopapillary lesion, cytologically resembling carcinoma, but invasion through the basement membrane is absent. Loss of nuclear polarity resulting in cellular piling at luminal surface (arrows) and "budding off" of small clusters of cells in the lumen (arrowhead). The cells have cytologically malignant features with hyperchromasia, nuclear membrane irregularities, and large nuclei (dashed arrows).