Conversion Coefficients of equivalent and effective doses in terms of air kerma for computational scenarios of Total Body Irradiation in lying-down patients

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This study aimed to calculate the Conversion Coefficients (CC) of Equivalent and Effective doses by air kerma considering Total Body Irradiation scenarios with Varian linear accelerator with photon beams energy of 4, 6, 10, 15, 18, and 25 MV. The simulations were performed in the MCNPX code and the University of Florida (UF) phantoms were used to represent exposed lying down adult patients in the AP, PA, RLAT, and LLAT irradiation geometries. Lead attenuators were inserted in the scenarios for the preservation of organs of risk and their contribution were analyzed for CC. For most counts, the statistical uncertainty was approximately 5%. For the gonads, CC values for the male phantom decreased with the increase of energy in the AP geometry, which did not occur for the female phantom. As the beam becomes more penetrating, the ovary absorbs more energy because of its internal position. Considering the lung, an organ of risk in TBI, the insertion of the attenuators in the scenarios caused the CC values to reduce by more than 30%. For organs and tissues such as skin and male breasts, the attenuators caused the dose to increase. As for the active bone marrow, which is the TBI target tissue, it was not possible to obtain a good estimate for CC at 15, 18, and 25 MV due to a limitation of the method used to calculate the dose in the bone areas. Nevertheless, for lower energies the CC values for the marrow were valid.