Why do cancers come back?

A cancerous (malignant) tumour consists of cancer cells which have the ability to spread beyond the original site. If left untreated they may invade and destroy surrounding tissues. Sometimes cells break away from the original (primary) cancer and spread to other organs in the body by travelling in the bloodstream or lymphatic system. When these cells reach a new area of the body they may go on dividing and form a new tumour, often referred to as a “secondary” or a “metastasis”.