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Proton computed tomography (pCT) has the potential to improve the accuracy of proton treatment planning, which is currently based on X-ray computed tomography (XCT). However, at this time no method exists for pCT to selectively increase the contrast of tumor tissues with respect to normal tissues. One possibility to achieve this goal is to bind gold-nanoparticles, i.e., microscopic structures in the size range of 1 nm to several hundred nanometers, to the surface of tumor cells with cell-seeking antibodies conjugated to the surface of the nanoparticles. In order to test the suitability of this approach, we used the Monte Carlo transport code GEANT4, to provide simulated histories of 200 MeV protons going through a cylindrical water phantom with embedded inhomogeneities enhanced with traces of gold. The energy loss of several million protons traversing the cylinder from 180 different angles formed the basis for reconstructing the single proton CT image representing the relative volume electron density map of the phantom using an approximate filtered back projection algorithm. Our results demonstrate that with hundred nanoparticles of 100 nm diameters per tumor cell, it may be possible to enhance the visibility of tumors in pCT.