The Change in the Linear Energy Transfer of a Clinical Proton Beam in the Presence of Gold Nanoparticles

Gold nanoparticles are promising radiosensitizers for proton radiotherapy. However, the physical mechanisms of gold nanoparticles radiosensitization remain unclear. In the present study, the Geant4 toolkit was used to estimate by the Monte-Carlo simulation the changes (1) in the contribution of primary and secondary particles to the absorbed dose, (2) in the dose-averaged linear energy transfer, and (3) in the relative biological effectiveness of a 150 MeV proton beam caused by the addition of 50 mg/mL of gold nanoparticles to the irradiated water phantom. In the presence of gold nanoparticles no significant changes in the absorbed dose and the Bragg peak position were found, at the same time a redistribution of the contribution of secondary particles to the absorbed dose was recorded. An increase in the contributions from protons (~16%), recoil nuclei (~58%), α-particles (~400%), deuterons (~900%), tritons (~3000%), and photons (~7000%) was observed ~10 mm beyond the Bragg peak. The contribution of the secondary electrons decreased by ~35%. This redistribution led to ~5-fold increase in the dose-averaged linear energy transfer at the distal edge of the Bragg curve; this, in turn, may cause the ~1.4−2.2-fold increase in the relative biological effectiveness within this region. Thus, it is critically important to take into account the presence of gold nanoparticles when dosimetric planning proton radiotherapy in order to avoid unwanted damage to the normal tissues around the tumor.