(A) For a typical MNP, administered nanoparticles are quickly removed from the circulation by the RES, substantially limiting the fraction of MNPs in each pass that are returned to the tumor for additional magnetic targeting, as indicated by the dashed line (). Such phenomena results in limited tumor exposure of the administered dose and, thus, limited MNP delivery. (B) Conversely, long-circulating MNPs, that avoid RES sequestration, would pass through tumor vasculature many times over, improving the probability of interaction with the magnetic field and, thus, achieving enhanced tumor delivery.