Nearly 1 in 4 breast cancer patients in the UK will see their cancer return after surgery. The consequences include repeat operations, delayed adjuvant treatment, increased likelihood of distant recurrence, poorer cosmetic outcomes, emotional distress, and enormous financial cost to the NHS. Breast cancer recurs after surgery primarily due to incomplete excision of the tumour or inadequate clearance of the surgical margins. Surgeons are unable to completely remove the cancerous tissue because the only tools they have to detect cancer during surgery are visual and tactile assessment. Consequently, there is a tremendous medical need for improved tools to detect cancer during surgery. Lightpoint Medical is developing a molecular imaging fiberscope to detect cancer in real time during surgery, and thereby tailoring the surgery to the individual patient's disease. The technology is based on Cerenkov Luminescence Imaging (CLI), a ground-breaking imaging modality that can perform optical imaging of Positron Emission Tomography (PET) imaging agents. Initial market research indicates a high demand for the technology among breast cancer surgeons in the UK and US. In this project Lightpoint and Guy’s Hospital will undertake a pilot clinical trial of the CLI fiberscope in breast-conserving surgery in order to assess the device's diagnostic performance. The results will be used to de-risk the technology before embarking on large-scale, pivotal clinical trials and to collect surgeon-user feedback on the device usability and performance.