Genetic Mutations
Lung cancer does not start off with the ability to spread to other organs. For it to spread (known as metastasis) the tumor cells must develop genetic mutations that allow them to break off from the primary tumor and travel to other parts of the body. Tumor cells already have certain genetic abnormalities that allow them to grow so rapidly and often have developed mutations that make it easier for additional mutations to occur. For example, many cancer cells have mutations in genes that are important for the repair of DNA. Problems with DNA repair lead to mutations occurring at random with some ultimately giving the cells the ability to metastasize.
Escape from the Lungs
In order for lung cancer to spread it first has to leave the lungs. This is normally prevented by a structure called the extracellular matrix, which is created by the body to hold cells in place and give them support. To escape from the lungs the cancer cells need to break down the matrix, which can be done using proteins called matrix metalloproteinases. If the genes for these proteins mutate, the cancer cells can use them to break free. This allows the tumor to spread through the body using the bloodstream or the lymphatic system.
Spread
Once the cancer cells make their way to the bloodstream or lymphatic system, they are able to spread to other tissues. According to the National Cancer Institute, lung cancer typically spreads to the bones or to the brain, although why it does is unknown. Once the metastatic cancer has found a suitable spot, it will begin to grow a new tumor (known as a secondary tumor). Because this new tumor will need blood to supply it with nutrients and oxygen, it will either make use of existing blood vessels or generate chemical signals to cause new blood vessels to form. The cells in the secondary tumor usually closely resemble the original one. As a result, even if lung cancer spreads to other organs the new cancer cells will still appear like lung tissue.
