Comparison of filtered x-ray spectra and depth doses derived from a hybrid Monte Carlo model of an orthovoltage x-ray unit with experimental measurements

The Monte Carlo N-Particle Version 5 (MCNP5) code system was used to create an MC model of the head of a Philips RT-250 unit and a 30 × 30 × 30 cm³ water phantom. For the x-ray machine head, this MC model includes the vacuum region, beryllium window, collimators, inherent filters, and exterior steel housing. For increased computational efficiency of MC simulations, the primary x-ray spectrum from the target was pre-calculated using a well-validated analytical software package. The current hybrid MC model was tested for the three common kilovoltage x-ray beams available from th machine: 250 kVp (0.35 mm Cu), 125 kVp (0.1 mm Cu), and 75 kVp (0.1 mm Cu). Specifically, calculated percentage-depth-dose (PDD) values and x-ray spectra were compared with experimental data from film/ionization chamber and Compton-scatter spectrum measurements, respectively. A comparison between the calculated PDD and measured data for all three x-ray beams showed overall agreement within 3%. The measured x-ray spectra were also compared with the MC results showing reasonable agreement between the two, in terms of the spectral shapes (or effects of filtration on the final spectra). In conclusion, despite its simplicity, the current hybrid MC model was capable of predicting the dosimetric and spectral characteristics of each beam from the RT-250 orthovoltage unit, demonstrating its applicability as a convenient and efficient computational tool for various preclinical and clinical applications.