Iodinated contrast agents, which are routinely used to improve contrast in x-ray diagnostic radiography, have been successfully proven to enhance radiation effects in kilovoltage x-ray radiation therapy beams. The studies determined the influence of iodine on the level of radiation biotoxicity to cells as an indicator of the radiation dose enhancement. The use of other high-atomic-number materials such as gold nanoparticles (AuNPs) may also provide advantages in terms of radiation dose enhancement. In this work AuNPs have been used for the enhancement of radiation effects on bovine aortic endothelial cells of superficial x-ray radiation therapy and megavoltage electron radiation therapy beams. Results reveal an increase of cell damage with increasing concentration of AuNPs. At 1 mM concentration of AuNPs, enhancement of radiation peaked at 25 times for a kilovoltage x-ray beam. AuNPs showed similar effects on electron beams but to a lesser extent. This study showed that AuNPs can be used to enhance the effect of radiation doses from kilovoltage x-ray radiation therapy and megavoltage electron radiation therapy beams. In the prevailing clinical circumstances, wherein radiation therapy dose is constrained by normal tissue tolerance, this enhancement could in the future be used to improve local control in superficial x-ray treatments, megavoltage electron beam radiation therapy, microbeam radiation therapy, and intraoperative irradiation using kilovoltage x-rays or megavoltage electron beams. Moreover, the value of this work also stems from the fact that the damage to the endothelial cells lining the highly vasculature structure of tumors deprives tumors of their oxygen and nutrients supply and enhances the efficiency of radiation therapy treatment, where it has been proven that more of the AuNPs injected into animals ends up into the blood than in the tumor.