CALCULATION OF CONVERSION COEFFICIENTS FOR AIR KERMA TO AMBIENT DOSE EQUIVALENT USING TRANSMITTED SPECTRA OF MEGAVOLTAGE X-RAYS THROUGH CONCRETE

摘要

With the fast advancement of technology, ~(60)Co teletherapy units are largely being replaced with medical linear accelerators. In most cases, the linear accelerator tends to be installed in the same room in which the ~(60)Co teletherapy unit was previously placed. If in-depth structural remodelling is out of the question, high-density concrete is usually used to improve shielding against primary, scatter and leakage radiation originating in the new equipment. This work presents a study based on Monte Carlo simulations of the transmission of some clinical photon spectra (from 6, 10, 15, 18 and 25 MV accelerators) through concrete, considering two different densities. Concrete walls with thickness ranging from 0.70 to 2.0 m were irradiated with 30 cmx30 cm primary beam spectra. The results show that the thickness of the barrier decreases up to ～65 % when barite (high-density concrete) is used instead of ordinary concrete. The average energies of primary and transmitted beam spectra were also calculated. In addition, conversion coefficients from air kerma to ambient dose equivalent, Η~*(d)/A_(air), and air kerma to effective dose, E/K_(air) for photon spectra from the transmitted spectra were calculated and compared. The results suggest that the 10-mm depth is not the best choice to represent the effective dose.