 Radiotherapy RT is an important treatment for prostate cancer with over 10,000 men receiving RT per year in the UK. Prostate RT aims to deliver radiation dose to the prostate to kill cancer cells whilst minimizing the dose given to surrounding normal tissues, such as the bladder or the rectum. Radiation is delivered in a number of daily treatments which are called fractions. The position of the prostate varies each day and during the radiation delivery. To accurately aim the radiation the prostate needs to be located before daily treatment. Clarity is an ultrasound image guidance system that locates the prostate and monitors its position during treatment. Ultrasound does not give a radiation dose, is non-invasive, provides fast imaging and can easily visualise soft tissues. Before Clarity is implemented the investigators wish to ensure that it can accurately locate and monitor the prostate by comparing Clarity with marker-based techniques in 24 patients. Patients receive markers as part of other trials which are currently recruiting at The Royal Marsden for example, patients in the DELINEATE trial and the PACE trials receive implanted gold markers and patients in the PROSPARE trial have received electromagnetic markers. Patients who have received or will be receiving markers as part of other studies will be asked to also to take part in this study. This means that there will be no additional intervention for patients as a result of this study. Clarity will be considered a safe and effective imaging system if the mean and standard deviation of the differences between Clarity measurements and marker measurements, of prostate position, are less than 1mm. It is possible that markers will influence the Clarity measurements. To find out if this happens, the mean and standard deviation of Clarity measurements in a group of 24 patients without markers to the group with markers will be compared. 