Quality indexes based on water measurements for low and medium energy x-ray beams: A theoretical study with PENELOPE

Purpose: To study the use of quality indexes based on ratios of absorbed doses in water at two different depths to characterize x-ray beams of low and medium energies. Methods: A total of 55 x-ray beam spectra were generated with the codes XCOMP5R and SPEKCALC and used as input of a series of Monte Carlo simulations performed with PENELOPE, in which the percentage depth doses in water and the k(Q), Q(0) factors, defined in the TRS-398 protocol, were determined for each beam. Some of these calculations were performed by simulating the ionization chamber PTW 30010. Results: The authors found that the relation between k(Q), Q(0) and the ratios of absorbed doses at two depths is almost linear. A set of ratios statistically compatible with that showing the best fit has been determined. Conclusions: The results of this study point out which of these ratios of absorbed doses in water could be used to better characterize x-ray beams of low and medium energies. (C) 2014 American Association of Physicists in Medicine.