Energy Deposited by Electrons in DNA Following n+Gd Interaction

Neutron capture therapy treatment of brain tumors is generally done by introducing boron atoms into the tumor. The patient is then irradiated with thermal neutrons causing the emission of alpha particles and Li ions that produce high LET radiation damage. A newer approach is to use Auger electrons to impart similar damage. The superiority of Auger electrons over α-particles for cell lethality was demonstrated by Laster et al.,1 when Gd atoms were bound to the DNA of V79 Chinese Hamster cells. The dose values for 10% survival in Megawat-Minutes were as follows: 9.1 for control cells, 4.8 for gadolinium ambient, 3.3 for BOPP and 1.8 for GdBOPP. The range of Auger electrons in the cell is of the order of tens of nanometers, but because they originate immediately at the critical target (the DNA), and because their range is so short, they are very efficient in damaging the DNA. In the present work, the energy deposition from Auger electrons in DNA was calculated using the EGS4 Monte Carlo code. The DNA was modeled using six nucleosomes symmetrically arranged in an appropriately-dimensioned circular array. Each nucleosome was modeled as two DNA helices wrapped 1.5 times around each central histone cylinder. The model included chemical composition and density of each region. The DNA model is shown in Figure 1. Since the exact location of the Gd atoms in the DNA is not known, the calculations were done assuming a random electron source location over the model described. The model used for electron energy deposition calculation, was a set of six nucleosomes as shown in Figure 2. A, B, C, and are source locations used for the various calculations. Energy deposition was calculated for several different electron sources and ranged between 0.5 and 30keV.