[Why do tumors metastasize and why to the lung or liver?]

Biological alterations of tumor cells which lead to metastasizing properties can now be identified in some cases in specific molecules (adhesion molecules, proteases, cytoskeleton, angiogenesis factors). To explain organ-specific metastasis, organ-specific mechanisms of adhesion to endothelial and parenchymal cells have been in the forefront for two decades. Based primarily, but not exclusively, on animal models, we must include more and more paracrine growth factors produced by the target organ, including the lung.