The majority of cancers present at a relatively advanced stage in which invasion within the primary organ is well established and metastases to lymph and distant organs are either clinically apparent or present at the microscopic level. However, it is increasingly recognized that the natural history of cancer formation is a long and complex path taking many years to develop to a clinically apparent stage in most cases. Furthermore, for most solid tumours there is a pre-invasive or intraepithelial stage of disease. This affords the opportunity for early detection and prevention of invasive...