It is not always possible to say that an operation will definitely cure a cancer. The surgeon removes the tumour and a surrounding area (margin) of normal tissue, and although scans may look clear, cells could have broken away from the main tumour and spread to another part of the body before the surgery. These early secondary cancers, called micro-metastases, may be too small to pick up on a scan. It is because of the potential risk of micro-metastases that other treatments, such as chemotherapy and radiotherapy, are sometimes given before or after surgery.

Sometimes a surgeon will find during an operation that a cancer cannot be completely removed. Scans do not always give the true picture and surgeons will not always know the extent of the cancer until they operate.