SU‐E‐T‐359: Emulation of Yb‐169 Gamma‐Ray Spectrum Using Metal‐Filtered 250 KVp X‐Rays for Pre‐Clinical Studies of Gold Nanoparticle‐Aided Radiation Therapy

Abstract

Purpose:To develop an external beam surrogate of the Yb‐169 brachytherapy source applying a filter‐based spectrum modulation technique to 250 kVp x‐rays. In‐vitro/vivo studies performed with the modulated 250 kVp beam will help gauge the benefits of implementing gold nanoparticle‐aided radiotherapy with the Yb‐169 source.Methods:A previously validated MCNP5 model of the Phillips RT‐250 orthovoltage unit was used to obtain the percentage depth dose (PDD) and filtered photon spectra for a variety of filtration and irradiation conditions. Photon spectra were obtained using the average flux F4 tally in air right after all collimation. A 30 × 30 × 30 cm3 water phantom was used to compute the PDD along the central axis (CAX) under the standards conditions of a 10 × 10 cm2 field size at 50 cm SSD. Cylindrical cells of 4 cm in diameter and the energy deposition F6 tally were used along the CAX to score the doses down to 20 cm depth. The number of particle history was set to 2 × 10⁸ in order to keep the relative uncertainty within each cell < 0.3%. The secondary electron spectrum within a gold‐loaded tissue due to each photon spectrum was also calculated using EGSnrc and compared with that due to Yb‐169 gamma rays.Results:Under the practical constraints for the spectrum modulation task, 250 kVp x‐rays filtered by a 0.25 mm Erbium (Er) foil produced the best match with Yb‐169 gamma rays, in terms of PDD and, more importantly, secondary electron spectrum.Conclusion:Modulation of 250kVp x‐ray spectrum by an Er‐filter was found effective in emulating the gamma ray spectrum of Yb‐169. Possible benefits as predicted from the current MC model such as enhanced radiosensitization with the Er‐filtered beam (as a surrogate of Yb‐169) was confirmed with a separate in‐vitro study.Supported by DOD/PCRP grant W81XWH‐12‐1‐0198