Lung cancer describes the growth of abnormal cells inside the lung. These cells divide and grow more quickly than normal cells and form a tumour, which is a cluster of cells. If cancer starts in the lungs, it is called primary lung cancer. It can also spread (metastasize) to other parts of the body. Lung cancers are divided into non-small cell lung cancer and small cell lung cancer based on where cancer started. Other types of lung cancer are mesothelioma and a carcinoid tumour.