The use of nanoparticle carriers for the sustained release of cytotoxic drugs in cancer therapy can result in fewer adverse effects and can thus be of great benefit for the patient. Recently, a novel nanocomposite, prepared by the encapsulation of THCPSi nanoparticles within solid lipids (SLN), was developed and characterized as a promising drug delivery carrier in vitro. The present study describes the in vivo evaluation of unmodified THCPSi nanoparticles and THCPSi-solid lipid nanocomposites (THCPSi-SLNCs) as potential drug delivery carriers for cancer therapy by using (18)F radiolabeling for the detection of the particle biodistribution in mice. Passive tumor targeting of (18)F-THCPSis and (18)F-THCPSi-SLNCs by the enhanced permeation and retention effect was investigated in a murine breast cancer model. Encapsulation of THCPSi nanoparticles with solid lipids improved their accumulation in tumors at a 7 week time point (tumor-to-liver ratio 0.10 ± 0.08 and 0.24 ± 0.09% for (18)F-THCPSis and (18)F-THCPSi-SLNCs, respectively).