Purpose: Damage to individual molecular bonds within hydrated and dry DNA caused by proton beam irradiation at the beam entrance (low linear energy transfer (LET) region) and the Bragg peak (BP; high LET region) are studied using x-ray photoelectron spectroscopy (XPS).
Methods: Dry DNA samples and hydrated samples (dissolved in distilled water) were placed at the entrance (0 cm depth), in the dose build up region (10 cm depth), at the proton Bragg peak (BP) maximum (15.4 cm depth), and at the BP distal 80% depth (15.6 cm) within a polyethylene (ρ=0.96 g/cm³) phantom. The samples were irradiated with a 150 MeV clinical proton beam that delivered 1 x 1011 protons/cm². Changes to the percentage of molecular bonds in the DNA phosphate backbone and base pairs were determined using XPS.
Results: In the entrance (~0.5 kev/μm) and build up (~0.9 kev/μm) regions the dominate form of DNA damage is through phosphate backbone bond breaks for both dry and hydrated DNA. At the depths of the BP maximum (~2.6 keV/μm linear energy transfer (LET)) and distal 80% falloff (LET ~3.1 keV/μm), backbone bond damage increased sharply for dry DNA, however for hydrated DNA base pair bond damage increases and becomes dominate.
Conclusion: This sharp increase in the base pair damage in the hydrated DNA in the high LET BP region could provide information as to the importance of reactive oxygen species created in water in the proves of DNA damage by proton irradiation. Additionally, the increasing base pair damage might provide information as to why the relative biological effectiveness of protons beams increases as LET increases near the end of range.