Multimodal imaging, i.e. combining (simultaneous) positron emission tomography (PET)-magnetic resonance imaging (MRI) and bioluminescence imaging (BLI) allows in-depth assessment of changes in tumor microenvironment over time. In this study, the development of human ovarian xenografts in mice was longitudinally monitored using BLI and PET-MRI to identify the most suitable time window to test nanomaterial-based therapies. Under these conditions, xenografts are viable, vascularized and metabolically active tumor masses with leaky blood vessels after four weeks, indicating a suitable time point for nanomaterial administration. This suggests that a combination of PET, MRI and BLI allows the identification of potential therapeutic windows.