Abstract
Purpose:To develop and validate a Monte Carlo (MC) model of a Phillips RT‐250 orthovoltage unit to test various beam spectrum modulation strategies for in vitro/vivo studies. A model of this type would enable the production of unconventional beams from a typical orthovoltage unit for novel therapeutic applications such as gold nanoparticle‐aided radiotherapy.Methods:The MCNP5 code system was used to create a MC model of the head of RT‐250 and a 30 × 30 × 30 cm3 water phantom. For the x‐ray machine head, the current model includes the vacuum region, beryllium window, collimators, inherent filters and exterior steel housing. For increased computational efficiency, the primary x‐ray spectrum from the target was calculated from a well‐validated analytical software package. Calculated percentage‐depth‐dose (PDD) values and photon spectra were validated against experimental data from film and Compton‐scatter spectrum measurements.Results:The model was validated for three common settings of the machine namely, 250 kVp (0.25 mm Cu), 125 kVp (2 mm Al), and 75 kVp (2 mm Al). The MC results for the PDD curves were compared with film measurements and showed good agreement for all depths with a maximum difference of 4 % around dmax and under 2.5 % for all other depths. The primary photon spectra were also measured and compared with the MC results showing reasonable agreement between the two, validating the input spectra and the final spectra as predicted by the current MC model.Conclusion:The current MC model accurately predicted the dosimetric and spectral characteristics of each beam from the RT‐250 orthovoltage unit, demonstrating its applicability and reliability for beam spectrum modulation tasks. It accomplished this without the need to model the bremsstrahlung xray production from the target, while significantly improved on computational efficiency by at least two orders of magnitude.Supported by DOD/PCRP grant W81XWH‐12‐1‐0198
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