Lung cancer. Scanning electron micrograph (SEM) of a section through a mesothelioma. This malignant (cancerous) lung tumour is usually associated with asbestos exposure. Sheets of elongated cells form the structure seen. Mesotheliomas develop on the pleural membrane outside the lungs, eventually growing to form a white mass that covers both lungs and obliterates the pleural cavity (space around lungs). This form of cancer is difficult to treat and is often fatal. Cancers are malignant growths that result from abnormal and uncontrolled cell division.