Metastases is the leading cause of death from all types of cancer and first appears in nearby lymph nodes. Due to their small size and low vascularisation, metastases are very difficult to detect, diagnose and treat. To address this, we have developed a immune-based nanoparticle platform for simultaneous Positron Emission Tomography (PET) and Magnetic Resonance Imaging (MRI) and targeted radionuclide therapy. This nanoplatform integrates high sensitivity detection (with PET) and high spatial resolution diagnosis (with MRI), as well as delivering radionuclide therapy that can target metastatic tumour cells in the lymph nodes by macrophage uptake and trafficking.