Polyethylene glycol (PEG) has promoted the prospective cancer treatment applications of gold nanoparticles (GNPs). PEG is widely used in providing GNPs with stealth properties, hence prolonging blood circulation times. GNPs coated with PEG (PEG-GNPs) take advantage of the enhanced permeability and retention effect in tumor environments, making them suitable for targeted treatment. The cellular uptake of PEG-GNPs is significantly lower than uncoated GNPs in vitro. PEG minimizes PEG-GNP interaction with ligands that mediate cancer cell uptake, causing reduced GNP uptake in comparison to uncoated GNP. As intracellular localization of GNPs maximizes its therapeutic enhancement, there is a need to improve the uptake of PEG-GNPs. To improve cell entry, receptor mediated endocytosis peptides were conjugated with PEG-GNPs of varying core sizes. Spherical GNPs of diameters 14, 50 and 70 nm with a PEG chain length of 2 kDa were used to determine a preferred core size for uptake in vitro in HeLa and MDA-MB-231 cells. A preliminary study using surface-modified GNPs as a radiosensitizer to a megavoltage clinical photon beam was done to assess its therapeutic application.