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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.

international journal of medical physics, clinical engineering and radiation oncology

Comparative Study of Experimental Enhancement in Free Radical Generation against Monte Carlo Modeled Enhancement in Radiation Dose Deposition Due to the Presence of High Z Materials during Irradiation of Aqueous Media