The overarching purpose of this study is to improve precision medicine through more refined therapy selection for breast cancer patients who are candidates for ICI therapy (monoclonal antibodies targeting the programmed death ligand 1 (PD-L1) or programmed cell death protein 1 (PD-1)). The reference standard biomarker for ICI therapy selection is PD-L1 protein expression measured by immunohistochemistry (IHC). Several disadvantages exist with this method, the most important ones being inter- and intralesional as well as spatial heterogeneity in PD-L1 expression, as well as the need for invasive procedures to obtain material for analysis. The study hypothesis is that Positron Emission Tomography combined with Computed Tomography (PET/CT) imaging with a contemporary radiotracer (89Zr-atezolizumab), visualizing PD-L1 expression in the whole body, could be a better predictive biomarker to select which patients benefit from ICI. The use of PET/CT imaging with new radiotracers enables a non-invasive assessment of the presence of the target of treatment in the whole body and provides the possibility to combine functional information with anatomical details.