Abstract
Purpose: To investigate conflicting results demonstrating higher cell-kill by irradiated high atomic number (Z) material, gold (Au) in tumor compared to Monte Carlo (MC) modeled enhancement in radiation dose deposition, and to compare the difference between radiosensitizing effects of gold and platinum. Methods and Materials: Since a majority of cell kill due to radiation is mediated by free radicals, evaluation of radicals generated from radiolysis of an aqueous medium can provide some insight into cell-kill. Here, free radicals generated due to the radiolysis of water by a clinical Iridium-192 (Ir-192) brachytherapy source in the presence and absence of thin and pure gold or platinum wires were quantified with electron paramagnetic/spin resonance (EPR/ESR) spectrometry and enhancements in free radical generation due to the presence of the wires during radiolysis were calculated. Those enhancements were compared against MC modeled enhancement in radiation dose deposition obtained from the geometry replicating the experimental setup. Results: Enhancements in free radical generation due to 100 and 127 μm diameter gold wires, and 127 μm diameter platinum wire were more than two times higher than the corresponding MC modeled enhancements in radiation dose deposition. Enhancement in hydroxyl free radical (OH?) generation due to thicker wires of gold and platinum were close to the enhancements in radiation dose deposition. The effects were similar for gold and platinum wires of equal diameter. Conclusions: Higher enhancement in radical generation compared to MC modeled enhancement in radiation dose deposition due to micron-size pure gold and platinum wires demonstrates that the surfaces of high Z materials in aqueous media become a secondary source of radicals under radiation field. High surface-to-volume ratio of nanoparticles can make this effect more pronounced, leading to higher cell kill than the predictions based on pure dose enhancement.
Highlights
Ionizing radiation produces most of the biological damage in tumors through the action of free radicals, especially hydroxyl [1]
We examine the conflicting results of radiosensitization and radiation dose enhancement by measuring the intensity of radiolysis induced OH radical signal with EPR spectrometry and comparing enhancement in OH radical generation due to micron thick pure gold and platinum wires against corresponding Monte Carlo (MC) modeled enhancement in radiation dose deposition in aqueous media under Ir-192 high dose rate (HDR) brachytherapy source irradiation
DMPO can capture hydrogen radical (H) to form DMPO-H. Both of these adducts can be identified with their unique spectra and detected with EPR spectrometry but DMPO-H was excluded in our study as its spectra from few samples were noisy
Summary
Ionizing radiation produces most of the biological damage in tumors through the action of free radicals, especially hydroxyl [1]. The effect of high Z materials, extensively studied through MC simulations as well as experiments clearly show significant enhancement in radiation dose deposition in the photoelectric effect dominant low energy range [7] [8]. This effect is mediated by increased production of low energy secondary electrons at their interfaces [9] [10]. Analogous to the results with low energy radiation, biological experiments with megavoltage (MV) beam show higher radiosensitization of high Z nanoparticles compared to MC modeled dose enhancement [16]-[18] These results suggest some effects other than pure physical dose deposition
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