the pharmacokinetics of nanoparticle-borne drugs targeting tumors depends critically on nanoparticle design. Empirical approaches to evaluate such designs in order to maximize treatment efficacy are timeand cost-intensive. We have recently proposed the use of computational modeling of nanoparticlemediated drug delivery targeting tumor vasculature coupled with numerical optimization to pursue optimal nanoparticle targeting and tumor uptake. Here, we build upon these studies to evaluate the effect of tumor size on optimal nanoparticle design by considering a cohort of heterogeneously-sized tumor lesions, as would be clinically expected. the results indicate that smaller nanoparticles yield higher tumor targeting and lesion regression for larger-sized tumors. We then augment the nanoparticle design optimization problem by considering drug diffusivity, which yields a twofold tumor size decrease compared to optimizing nanoparticles without this consideration. We quantify the tradeoff between tumor targeting and size decrease using bi-objective optimization, and generate five Paretooptimal nanoparticle designs. the results provide a spectrum of treatment outcomes-considering tumor targeting vs. antitumor effect-with the goal to enable therapy customization based on clinical need. this approach could be extended to other nanoparticle-based cancer therapies, and support the development of personalized nanomedicine in the longer term. Chemotherapy is the treatment of choice to control metastatic cancer-a stage often reported in patients at the time of clinical presentation. Unfortunately, patients undergoing chemotherapy may have low median survival, especially for pancreatic, lung, and liver cancer 1. Negative response to treatment is attributed to a number of factors, including tumor microenvironmental barriers, evolution of resistance to drug, and chemotherapeutic toxicity. It has been shown that nanoparticle-mediated drug delivery may substantially enhance the pharmacokinetics of anticancer drugs while addressing some of these factors 2. However, while many nano-based formulations have undergone pre-clinical and clinical evaluation, few have been translated to the clinic 3. The targeting potential of nanotherapy is strongly associated with nanoparticle biophysical and biochemical properties 4-6. These properties include size 7 , shape (e.g., sphere or ellipsoid) 5 , stiffness 4,8 , and binding affinity of nanoparticle surface ligands to receptors upregulated in cells in tumors 5. Computational studies have investigated the effect of these properties on treatment efficacy in an attempt to find nanoparticles optimized for maximal anti-tumor activity. Nanoparticle margination 6 and adhesion to tumor vasculature 5 have been modeled as a function of nanoparticle properties (size, aspect ratio, ligand surface density, and ligand-receptor binding affinity). Uncertainties in ligand surface density and ligand-receptor affinity have been quantified and incorporated in a nanoparticle-tumor adhesion model using a Bayesian hierarc...