Lung cancer starts in the cells of the lung. A cancerous (malignant) tumour is a group of cancer cells that can grow into and destroy nearby tissue. It can also spread (metastasize) to other parts of the body. When cancer starts in lung cells, it is called primary lung cancer.
The lung is part of the respiratory system. You use your lungs when you breathe. The lungs are in the chest, one on each side of the heart. The right lung has 3 main parts, called lobes. The left lung is a bit smaller and has 2 lobes. The lungs are cushioned and protected by a thin covering called the pleura. Cells in the lung sometimes change and no longer grow or behave normally. These changes may lead to non-cancerous (benign) tumours such as hamartoma and papilloma. But in some cases, changes to lung cells can cause cancer.
Lung cancers are divided into non–small cell lung cancer and small cell lung cancer based on the type of cell in which the cancer started.