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Effect of Air Density in Low-Energy X-Ray Spectra and Its Influence On Absorbed Dose to Water

G Massillon-JL*, A Moreno Ramirez, Instituto de Fisica, UNAM, Mexico City, Mexico City, MX


(Sunday, 7/12/2020)   [Eastern Time (GMT-4)]

Room: AAPM ePoster Library

Purpose: Due to the altitude in Mexico City (above 2250 m), the use of X-ray spectra measured in other laboratory or calculated through some computational method to evaluate absorbed dose in the clinic can be misleading. This work aims at investigating the effect of air density correction in measured low-energy X-ray spectra as a function of the beam quality and studying its influence in the absorbed dose to water using Monte Carlo simulation.
Methods: We used unfiltered X-ray spectra measured at the Physikalisch-Technische Bundesanstalt in the 20-150 kV energy range that were previously normalized to the reference conditions (RC) of pressure (P), temperature (T) and relative humidity (R) and those generated from the SpekCalc computational code. To assess the influence of the air density (?) on the filtered X-ray spectra under our laboratory conditions (LC), the exponential attenuation law as well as the Drake-Böhn’s equation for air density was used. Corrected and non-corrected spectra were used as input source in the EGSnrc/DOSRZnrc Monte Carlo code to calculate the absorbed depth-dose curves in liquid water induced by nine different moderately filtered X-ray beam qualities (NIST M-series) which are generally used for our experiments.
Results: Corrected and non-corrected unfiltered spectra have shown differences of 128.5%-10.3% at photon energies below 7 keV and 10%- 1% at photon energies below 14 keV, independent of the beam quality. However, the difference in the absorbed dose to water imparted by a filtered 20 kV X-ray beam (13.48 keV effective energy) under our LC is about 50% smaller than the RC.
Conclusion: This work studied the influence of the Mexico City’s air density on the low-energy X-ray spectra and revealed the importance of knowing the environmental conditions under which a spectrum is measured or calculated before using it for dose evaluation in the clinic.


X Rays, Low Energy X-rays, Monte Carlo


TH- External Beam- Photons: Kilovoltage therapy

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