PECAM1 and neoplasm: Additionally, Prussian blue staining of tumor tissues showed large amount of 19F-RGD@USPIO distributed in the interior of the blood vessels (CD31-positive area) around the periphery of tumor, while subtle 19F-PDA@USPIO were mainly distributed in the interstitial space around the tumor and probably colocalizated with macrophages (Fig. 3D).