Lung cancer is an uncontrolled growth of cells that start off in the lungs. Usually, lung cancer starts in the cells that line the air passages. Instead of developing into healthy lung tissue, the cells divide rapidly and form tumours.

Lung cancer can grow and spread beyond the lung to reach other parts of the body through metastasis. Lung cancers can begin in any part of the lung, but 90 percent of lung cancers begin in the epithelial cells, which are the cells lining the larger and smaller airways also known as bronchi and bronchioles.

This is the reason why lung cancers are sometimes called bronchogenic cancers or bronchogenic carcinomas. Lung cancer is the most common cancer in the world, among both men and women. It is the leading cause of cancer deaths worldwide.