Lung cancer is a disease of uncontrolled cell growth in tissues of the lung. This growth may lead to metastasis, which is the invasion of adjacent tissue and infiltration beyond the lungs. The vast majority of primary lung cancers are carcinomas of the lung, derived from epithelial cells.
Malignant tumors, on the other hand, grow aggressively and invade other tissues of the body, allowing entry of tumor cells into the bloodstream or lymphatic system and then to other sites in the body. This process of spread is termed metastasis; the areas of tumor growth at these distant sites are called metastases. Since lung cancer tends to spread or metastasize very early after it forms, it is a very life-threatening cancer and one of the most difficult cancers to treat.
The lung also is a very common site for metastasis from tumors in other parts of the body. Usually lung cancer, develops within the wall or epithelium of the bronchial tree. But it can start anywhere in the lungs and affect any part of the respiratory system. Lung cancer mostly affect people between the ages of 55 and 65 and often takes many years to develop. Once it occurs, cancer cells can break away and may spread to the lymph nodes or other tissues in the chest, including the lung opposite to where it originated. It may also spread to other organs of the body, such as the bones, brain or liver.
The principal function of the lungs is to exchange gases between the air we breathe and the blood. The major airways entering the lungs are the bronchi, which arise from the trachea. Lung cancers can arise in any part of the lung, but 90%-95% of cancers of the lung are thought to arise from the epithelial cells, the cells lining the larger and smaller airways (bronchi and bronchioles); for this reason, lung cancers are sometimes called bronchogenic cancers or bronchogenic carcinomas.
