Reply to Comment on ‘Implications on clinical scenario of gold nanoparticle radiosensitization in regards to photon energy, nanoparticle size, concentration and location’

Abstract

In a comment on a recent paper by Lechtman et al (2011 Phys. Med. Biol. 56 4631–47), McMahon critiques one of Lechtman's conclusion that gold nanoparticle radiosensitization may not be applicable to megavoltage radiotherapy. He refers to recently published experimental studies showing radiosensitization with 6 MV x-rays and low gold concentrations. However those published studies show conflicting results, presenting survival curves with a small cell death increased with gold and some with no difference. In regards to gold nanoparticle radiosensitization physical, chemical, pharmacological and biological constraints all interplay. There are plenty of experimental and theoretical data to confirm the strong dependence to the primary photon energy and gold concentration. The manuscript of Lechtman added the dependence to microscopic localization, analysing the spatial distribution and the quality of secondary electrons, as a major player in the feasibility of the technique. We agree that radiobiological dose modification factor should be considered, but it is unlikely that accounting for a maximum RBE of 2 can compensate for the drastic decrease of photoelectric events shifting from kV to MV. Lechtman calculated that to achieve similar radiosensitization for low energy beams and intra-cellular gold concentration of 0.5%, concentrations 300 times higher are required for 6 MV beams. To date it seems unlikely that concentration higher than 1% could be achieved such that it is unlikely that megavoltage would yield a measurable clinical effect.