Liapis E, Klemm U, Karlas A, et al. - In view of the significance of understanding temporal and spatial hemodynamic heterogeneity as a function of tumor growth or therapy for the development of novel therapeutic strategies, researchers here used eigenspectra multispectral optoacoustic tomography (eMSOT) as a next-generation optoacoustic technique to convey high accuracy in resolving tumor hemodynamics during bevacizumab treatment in two types of breast cancer xenografts (KPL-4, MDA-MB-468). Control and bevacizumab-treated tumors were assessed for patterns of tumor total hemoglobin concentration (THb) and oxygen saturation (sO2) via imaging over the course of 58d (KPL-4) and 16d (MDA-MB-468), and the evolution of functional vasculature "normalization" was resolved macroscopically. The initiation of bevacizumab treatment was shortly followed by an initial sharp drop in tumor sO2 and THb content; this was followed by a recovery in oxygenation levels. Rim-core subregion analysis indicated steep spatial oxygenation gradients in growing tumors that were decreased after bevacizumab treatment. Critically, histopathologic assessment of hypoxia (pimonidazole) and vascularity (CD31) were used to directly validate eMSOT imaging findings. Data thereby demonstrate how eMSOT allows precise observation of entire tumor responses to challenges at spatial and temporal dimensions not available by other methods today.