Figure 1.

Histological features of AAH (H&E staining). A: The lesion has well-defined boundaries and no central scar formation or collapse. B: Low-grade AAH: the cell density is low to moderate, with the cells arranged in a single layer, intermittently or focally and continuously, on the alveolar septa. C: High-grade AAH: the cells are continuously and densely arranged in a single layer, but do not exhibit the piled-up structure often observed in adenocarcinoma. Original magnifications: ×2 (A), ×150 (B), ×100 (C).