Lung cancers can start in the cells lining the bronchi and parts of the lung such as the bronchioles or alveoli.
Lung cancers are thought to start as areas of pre-cancerous changes in the lung. The first changes in the genes (DNA) inside the lung cells may cause the cells to grow faster. These cells may look a bit abnormal if seen under a microscope, but at this point they do not form a mass or tumor. They cannot be seen on an x-ray and they do not cause symptoms.
Over time, the abnormal cells may acquire other gene changes, which cause them to progress to true cancer. As a cancer develops, the cancer cells may make chemicals that cause new blood vessels to form nearby. These blood vessels nourish the cancer cells, which can continue to grow and form a tumor large enough to be seen on imaging tests such as x-rays.
At some point, cells from the cancer may break away from the original tumor and spread (metastasize) to other parts of the body. Lung cancer is often a life-threatening disease because it tends to spread in this way even before it can be detected on an imaging test such as a chest x-ray.
The lymph (lymphatic) system
The lymph system is one of the ways in which lung cancers can spread. This system has several parts:
Lymph nodes are small, bean-shaped collections of immune system cells (cells that fight infections) that are connected by lymphatic vessels.
Lymphatic vessels are like small veins, except that they carry a clear fluid called lymph (instead of blood) away from the lungs.
Lymph contains excess fluid and waste products from body tissues, as well as immune system cells.
Lung cancer cells can enter lymphatic vessels and begin to grow in lymph nodes around the bronchi and in the mediastinum (the area between the 2 lungs). Once lung cancer cells have reached the lymph nodes, they are more likely to have spread to other organs of the body as well. The stage (extent) of the cancer and decisions about treatment are based in part on whether or not the cancer has spread to the nearby lymph nodes in the mediastinum. These topics are discussed later in the section “How is non-small cell lung cancer staged?”
