More than 2.3 million new cases of breast cancer are diagnosed each year. During surgery, it has become standard practice to remove the primary tumour as well as the so-called sentinel lymph nodes (SLN), the lymph nodes that have the highest probability of being invaded by cancer cells. Pre-operative detection of the tissues of interest using X-ray computed tomography (CT) combined with fluorescence detection during surgery could provide complementary information to detect tumours and SLNs, but current CT contrast agents and fluorescence probes suffer from downsides such as low lymph node retention, undesirable fast migration to higher-tier lymph nodes and inability to be used in a targeted manner. In this fundamental project, we start from the clinical need for improved methods to perform tumour and SLN detection and removal in breast cancer patients, to enable these improved methods we present dual-modality hafnium oxide nanocrystals as a powerful new imaging tool in the battle against breast cancer. In this project we aim to gain a deeper understanding on how the surface chemistry of the nanocrystals influences their lymphatic and intravenous interactions, safety, and final in vivo faith. We also aim to develop nanocrystal probes that can be used in a targeted manner. Finally, a large animal in vivo study will provide fundamental insights in how the material behaves in a more "human-like" model.