International multidisciplinary classification of lung adenocarcinoma. (a) Adenocarcinoma in situ. Histologic specimen showing a tumor with atypical pneumocytes proliferating along slightly thickened but preserved alveolar walls; (b) Minimally invasive adenocarcinoma. Histologic specimen of a tumor exhibiting a bronchioloalveolar growth pattern with minimal invasion. The tumor is invading in the fibrous stroma.