Effect of the Mexico City's air density in low-energy X-ray spectra and its influence in the air kerma and absorbed depth-dose to water

Abstract

Mexico City is situated at around 2,200 m relative to the sea-level, so the use of computed X-ray spectra or measured from other laboratories might affect the air-kerma or the absorbed-dose. This work aims at investigating the effect of the air-density in Mexico City on the air-kerma and the absorbed-depth-dose to water using low-energy X-ray spectra. Air-kerma was calculated for both standard reference (SRC) and laboratory conditions (LC) using unfiltered X-ray spectra of 20 kV, 50 kV, 100 kV and 120 kV measured at PTB. Absorbed depth-dose measurements in water were performed for the same beam-qualities using an ionization-chamber. Independently, depth-dose has been calculated through Monte-Carlo (MC) DOSRZnrc. X-ray spectra from SpekCalc were also generated and used to calculate the absorbed-depth-dose for comparison. For the air-kerma, differences up to 7% were found between the LC and SRC. For the absorbed-depth-dose, the experimental results agree quite well (0.3%–0.7%) with MC for kilovoltages over 50 kV. For 20 kV, the experimental depth-dose curve decreases smoother than the calculated one showing a difference of 25%–50% depending on the depth, which is possibly associated to the output of the X-ray tube. Comparing the absorbed-depth-dose curves between LC and SRC, discrepancies of more than 50% are observed at 20 kV. Concerning the data from SpekCalc, disagreements are observed for all the energies studied. The air-density affects mainly the air-kerma and the absorbed-depth-dose at energies below 50 kV. Based on this work, one can suggest the use of unfiltered X-ray spectra published by PTB and corrected under LC to calculate the absorbed-dose using MC.