Lung Cancer http://www.medicinenet.com What is lung cancer? Cancer of the lung, like all cancers, results from an abnormality in the body's basic unit of life, the cell. Normally, the body maintains a system of checks and balances on cell growth so that cells divide to produce new cells only when new cells are needed. Disruption of this system of checks and balances on cell growth results in an uncontrolled division and proliferation of cells that eventually forms a mass known as a tumor. Tumors can be benign or malignant ; when we speak of "cancer," we are referring to those tumors that are malignant. Benign tumors usually can be removed and do not spread to other parts of the body. 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 tum