Studies have suggested that more efficient killing of cancer cells, and therefore, better cure rates, can be achieved by increasing the radiotherapy dose. However, in the past, this was not possible due to side effects. Intensity Modulated Radiotherapy (IMRT) is a new radiotherapy delivery technique that allows better shaping of the dose to the areas that need irradiating with the potential for fewer side effects. If the investigators use IMRT to deliver an intentionally higher dose of radiation (called a boost) to small selected areas whilst, at the same time giving standard treatment doses to the remaining areas, this approach is called IMRT dose-painting. These selected areas can be identified by a scan called 18F-FDG-PET (18F-fluorodeoxyglucose-positron emission tomography, also known as a 'PET' scan) which is a type of scan that can give information about the activity of a cancer.