Implementation of the Clarity®Autoscan (Elekta) Transperineal Ultrasound (TPUS) system in Bristol is the first of its kind in the UK and we have already shown its utility in interfractional Image Guided Radiotherapy (IGRT).14This study establishes the extent of intrafraction prostate motion as measured by Clarity and explores the potential benefits of TPUS for intrafraction monitoring.