RBE of kV CBCT radiation determined by Monte Carlo DNA damage simulations

Abstract

Due to the higher LET of kilovoltage (kV) radiation, there is potential for an increase in relative biological effectiveness (RBE) of absorbed doses of radiation from kV cone beam computed tomography (CBCT) sources in reference to megavoltage or Co-60 doses. This work develops a method for accurately coupling a Monte Carlo (MC) radiation transport code (PENELOPE) with the damage simulation (MCDS) to predict relative numbers of DNA double strand breaks (DSBs). The MCDS accounts for slowing down of electrons and delta ray production within the cell nucleus; however, determining the spectrum of electrons incident on the cell nucleus from photons interacting in a larger region of tissue is not trivial. PENELOPE simulations were conducted with a novel tally algorithm invoked where electrons incident on a detection material were tracked and both the incident energy and the final deposited dose were recorded. The DSB yield predicted by a set of MCDS runs of monoenergetic electrons was then looked up in a table and weighted by the specific energy of the incident electron. Our results indicate that the RBE for DSB induction is 1.1 for diagnostic x-rays with energies from 80 to 125 kVp. We found no significant change in RBE with depth or filtration. The predicted absolute DSB yields are about three times lower for cells irradiated under anoxic conditions than the yield in cells irradiated under normoxic (5%) or fully aerobic (100%) conditions. However, oxygen concentration has a negligible (± 0.02) effect on the RBE of kV CBCT x-rays.

The effect of energy spectrum change on DNA damage in and out of field in 10-MV clinical photon beams

“…These differences in photon spectra are in conceptual agreement with other published data [10][11][12][13]23]. For shallow depths, the photon spectrum peak is lower in the out-of-field regions, but for deeper depths, the peak energy is independent of location in or out of the primary beam.…”

“…For example, studies have shown changes in RBE (or quality factor) of up to 40 % depending on the photon energy spectrum, location in or out of the field [9,[11][12][13]30]. Additionally, Hsiao and Stewart [7] reported changes in RBE and deoxyribonucleic acid (DNA) double-strand breaks (DSB) for various diagnostic X-ray sources and gamma-emitting isotopes, which compared well to measured values.…”

“…In this approach, the weight of any electron is proportional to its track length in the cell. Kirkby at el [11] used the doseweighting method, and they mentioned that DSB changed 3 % for cells with thicknesses from 1 to 50 μm. However, for investigating track length approach dependency on cell dimension, the electron spectra at depth of 3.5 cm were calculated for three cells with different thicknesses (1, 10 and 100 μm).…”

“…Photon and electron energy cutoffs were set to the minimum value of 1 keV to include delta rays created in neighboring cells. The limitation imposed by this cutoff if approximated to be on the order of 0.04 % [11]. At each Fig.…”

Radiation Safety Aspects of Linac Operation with Bremsstrahlung Converters

“…The Monte Carlo methods form a broad class of stochastic algorithms that proved successful in a variety of disciplines including genetics [14], space physics [15] and economics [16]. While problems might be solvable using deterministic methods, Monte Carlo methods utilize repetition of random sampling to arrive at a numerical result.…”