Lung adenocarcinomas may spread through lymphatic channels to involve ipsilateral and contralateral lymph nodes at the hilum and in mediastinum. Blood-borne metastases can cause spread to the liver, bone, brain, adrenal glands, and contralateral lung. Spread along the pleural surface can give rise to an appearance that mimics mesothelioma. Yet another mechanism is spread through air spaces (STAS). It consists of micropapillary clusters, solid nests, or single cells present in the air-spaces in the lung parenchyma at the periphery of the tumor. STAS is considered a sign of invasion and is used to separate lepidic adenocarcinomas from minimally-invasive adenocarcinomas and adenocarcinoma-in-situ. The center of this image shows a few groups of tumor cells with papillary features - a sign of invasion. See higher magnification in the next image.