The lungs are the two spongy organs in the chest. They are responsible for inhaling oxygen and exhaling carbon dioxide. When cancer cells form in the lungs, a patient is said to have lung cancer. Cancer cells may spread to the lymph nodes. They may even reach the brain. When lung cancer has spread, it has metastasized. Lung cancer is usually grouped into two categories. Small cell lung cancer is when malignant cells form in the lung tissues. Non-small cell lung cancer is the more common type, accounting for about 85% of cases. This type starts in the substances that secrete mucus.