In some cases cancer can come back after treatment. It may come back in the same area in which it first started, this is known as a recurrence. This can happen because tiny cancer cells, that may have been left behind when the tumour was removed, or that weren't destroyed by treatments such as radiotherapy and chemotherapy, have begun to divide again and form a tumour. 

Or it may develop in a different part of the body, often referred to as a metastasis or secondary cancer. This can happen because 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.