4T1 tumor T1-enhancement corresponds to clusters of free iron oxides in central necrosis.

<p>(A) H&E stain of 4T1 tumor shows a necrotic core (square C) and highly cellular tumor periphery (square D, 100x magnification). (B) H&E stain of PymT tumor shows uniformly heterogeneous tumor tissue without central necrosis (100x magnifications). (C) Prussian blue stain (400x magnification) of the necrotic core of the 4T1 tumor shows diffuse distribution blue (iron) staining consistent with iron oxide nanoparticles in a non-compartmentalized, extracellular localization. (D) Prussian blue stain (400x magnifications) of the peripheral area of the 4T1 tumor shows blue iron staining in cells.