Lung cancer cells, coloured scanning electron micrograph (SEM). Lung cancer occurs when abnormal cells divide in a chaotic and uncontrolled manner resulting in the formation of a tumour in the lungs. Lung cancer is the most common cause of cancer death in the UK. In 2016 it accounted for 21% of all cancer deaths in the UK. In terms of survival, approximately 5% will survive 10 years or more and 10% will survive 5 years or more. Magnification: x3000 when printed at 10cm wide.