Lung cancers normally start to form in the cells that line the bronchi and in parts of the lung such as the bronchioles or alveoli where they are presumed to begin as areas of pre-cancerous changes. These initial changes are thought to occur in the genes of the cells, and may cause them to grow at a faster than normal rate. At this early stage, the cells may look slightly abnormal under a microscope, however, they have not as yet formed a mass or tumor, cannot be seen on an x-ray and do not cause any symptoms.
Over time, the pre-cancerous changes in the cells may progress to an actual cancer, which as it develops, makes chemicals that cause new blood vessels to form nearby. The new blood vessels then nourish the cancer cells, allowing them to continue to grow and ultimately to form a tumor large enough to be seen on imaging tests. Eventually, cells from the cancer may break away from the original tumor and spread to other parts of the body.
There are three primary ways in which cancer can spread:
The most common way for cancer to spread is through the lymphatic system. This process is known as “embolization”.
Lymph nodes are small, bean-shaped collections of immune system cells that are connected by lymphatic vessels. Lymphatic vessels are similar to small veins, except rather than carrying blood they carry a clear fluid called lymph 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 the lymphatic vessels and start to grow in the lymph nodes around the bronchi and in the mediastinum (the area under the breastbone between the two lungs). Once lung cancer cells have reached the lymph nodes, they are more likely to have spread to other areas of the body as well. The stage of the cancer (how far it has progressed) and decisions on appropriate treatment options are based on whether or not the cancer has spread to nearby lymph nodes.
Cancer can also spread through the bloodstream. In order to spread, cancer cells break off from the primary tumor, and then invade the wall of a blood vessel to gain access to the bloodstream. Tumors almost always spread through the veins rather than the arteries since the walls of the veins are thinner and can be invaded more easily. The cancer cells then follow the pattern of the venous flows, until they get stuck somewhere, usually in a very small blood vessel called a capillary. They then invade the wall of the capillary and migrate into nearby tissue where they begin formation of a secondary tumor.
Cancer can spread by local invasion, meaning that it intrudes on healthy tissue surrounding the tumor. Although it is not fully understood how tumors spread by direct extension, there are three likely possibilities, but which way is most frequent may be dependent on the type of tumor and where in the body it is located.
The first possibility is that as the tumor grows and begins to take up more space, it forces itself through nearby normal tissue. As it continues to grow, it begins to block small blood vessels in the area, causing low blood and oxygen levels which in turn, causes normal tissue to die. This tissue death makes it easier for the tumor to continue to push its way through since tumors often follow the path of least resistance.
The second possibility is that many normal blood cells produce chemicals known as enzymes that break down cells and tissue. These normal cells use their enzymes for good, by attacking bacteria and viruses and by clearing damaged cells so that new cells can replace them. It has been found that many tumors contain larger quantities of these enzymes than would be found in normal tissues, therefore it is possible that cancer cells manufacture the enzymes and then use them to break down tissue making it easier for the tumor to cut a pathway through healthy tissue.
Thirdly, one thing that differentiates normal cells from cancer cells is that cancer cells appear to have the ability to move more easily, therefore it seems reasonable that cancer can spread through nearby tissue simply by directly moving. Given this, scientists have discovered a substance made by cancer cells that stimulates them to move. Although research is not conclusive at this time, it seems that this substance could play a part in how cancer spreads. Some cancer cells seem to “home in” on certain places, perhaps because of substances on their surfaces that stick to cells in these organs. In other cases, the organ itself may release substances that cause the cancer cells to grow faster.