Lung cancer happens when cells in the lungs become abnormal and grow out of control. Lung cancer can start anywhere in the lungs, but most often it starts in the breathing (bronchial) tubes. The cancer cells can form tumors, invade nearby tissues, and spread to other parts of the body.

The main types of lung cancer are small cell lung cancer (SCLC) and non-small cell lung cancer (NSCLC). NSCLC is by far the most common type of lung cancer. SCLC usually grows and spreads more quickly than NSCLC.