The receptor activator of the nuclear factor κ B ligand (RANKL) is an important component in carcinogenesis, specifically in the maintenance of self-renewal of cancer stem cells and up-regulation of anti-apoptotic pathways. In the tumor microenvironment, RANKL expression by tumor cells is associated with poor prognosis and more aggressive disease, of amongst others head-and-neck and breast cancer; two malignancies with poor outcome and in urgent need of better prognostic biomarkers and treatment options. However, current research on RANKL is hampered by the lack of a non-invasive biomarker of RANKL expression and dynamics in the tumor microenvironment. We propose a novel use of immuno-positron emission tomography (PET) by radiolabeling the anti-RANKL monoclonal antibody denosumab as longitudinal non-invasive imaging biomarker. The current proposal of this innovative approach includes developing and validating the labeling procedure, establishing the preclinical mouse models, evaluating the biodistribution, and biomarker validation in xenograft and metastatic mice models of oral squamous cell cancer (OSCC) and triple-negative breast cancer (TNBC). To this end, tumor models will be created with high and low RANKL expression, as well as modulation of tumor-derived RANKL using pharmacological intervention. Both a long (zirconium-89) and a short (gallium-68) half-life PET emitter will be studied to facilitate translation to human applications. Novel techniques will have to be developed to optimize antibody labeling with specific application to RANKL imaging, to derive unique immuno-PET imaging signatures of RANKL expression, and to establish the predictive value of this new biomarker. This challenging project will contribute to the understanding of the heterogeneity of RANKL expression, the dynamics of RANKL binding, and impact of RANKL-directed treatment on the tumor microenvironment. This can ultimately impact and improve the selection of patients in trials of RANKL-directed cancer treatments in these two frequent and aggressive diseases (i.e. OSCC and TNBC).