The lung contains a very rich network of both blood vessels and lymphatic channels through which cancer cells can metastasize. The cancerous cells can spread to nearby lymph nodes or to other organs; the tumor in the lung is known as the primary tumor, and the tumors in the other parts of the body are called secondary or metastatic tumors.
The lung is also a common site for cancers that start in other organs to spread to, i.e. breast, colon, pancreas, kidney, bladder or skin. When this occurs it is not considered lung cancer, it is instead cancer of the primary site where it started that has metastasized to the lung.