A portable intra-operative scanning terahertz (THz) probe has been developed for medical use by TeraView Ltd to aid surgeons in the removal of breast tumors by providing positive identification of diseased tissue in real-time. Initial ex vivo tests have been carried out using this system at Guypsilas Hospital, London, its ability to perform in a clinical environment has been tested by comparing THz data with histological diagnosis of localized areas of breast tissue from 27 patients as well as examination of sentinel node biopsies (SNB) of 37 patients. In this paper the system is described and findings of this initial study are discussed.