Lung cancer occurs when a malignant (cancerous) tumor grows inside the lungs, in structures such as the bronchi (small tubes that connect the windpipe to the inner surfaces of the lungs where gas transfer takes place). Like many other types of cancer, lung cancer is capable of spreading (metastasizing) to other parts of the body. In this case, cancer beginning in the lungs most commonly spreads to the brain, bones, adrenal glands and liver, via any of three mechanisms: direct extension, via the blood vessels, or via the lymph system. Direct extension occurs when a tumor grows rapidly in size such that it begins to touch an adjacent organ or structure, and then begins to penetrate itself into that adjacent organ or structure. Tumor cells are also able to get into the blood and lymph circulatory systems and travel, one by one, to distant structures.