Lung cancer develops if the cells of your lungs grow abnormally and out of control. This creates a lump (tumour) which can either be malignant (cancerous) or benign (not cancerous). You can get cancer in the lining of your bronchi or in the lung tissue itself. Cancers that start in other organs can spread through the bloodstream or the lymphatic system (part of your immune system) to your lungs where they may grow and form 'secondary' tumours. This is called metastasis. It's important to know where the cancer started as this affects the choice of your treatment.
Attribution: BUPA