A tumour is an abnormal swelling or growth of tissue. Tumours can be either benign or malignant. Benign tumours are generally safe as they remain within a single organ or tissue and are not considered cancerous. Malignant tumours on the other hand are able to move from one site or organ in the human body (e.g. the lung) to another (e.g. breast) and therefore have the ability to invade numerous healthy organs in different parts of the body. The process of moving around the body is known as metastasis and tumours that are able to metastasise are cancerous.