Fig. 1: Precursors of invasive adenocarcinoma.

A Atypical adenomatous hyperplasia can be identified at low magnification by the increase in lining cell size and sharp demarcation with adjacent lung. B Alveolar walls can be thickened, and the lining with both type 2 pneumocytes of variable shape and type 1 pneumocytes. C Adenocarcinoma-in-situ, non-mucinous contain areas of alveolar spaces more uniformly replaced by type 2 pneumocytes, often low grade, without the gaps of residual type 1 cells. D While adenocarcinoma-in-situ can consist of high cuboidal cells or columnal cells with greater atypia, they are usually single layers with occasional folds or tufts of 2 layers (arrow). E Macroscopic image of a minimally invasive adenocarcinoma, grossly indistinct but with area of central distortion. F At low magnification, the alveolar growth pattern predominates with an area of greater density and distortion. G At high magnification, proliferation includes multi-layered epithelium along with angulated glandular profiles.