A comparison of methods to calculate biological effectiveness (RBE) from Monte Carlo simulations

The relative biological effectiveness (RBE) of radiation is assessed and easily calculated by Monte Carlo simulations of the passage of radiation through matter. The expression to calculate the RBE provided by microdosimetry requires the use of the energy spectrum of charged particles. This paper compares the RBE values obtained for Palladium-103 (103Pd) and iodine-125 (125I) when calculated with 2 different spectra: the electron slowing-down spectrum and the ejection spectrum. The former yields a value of 10.6%, twice the value obtained with the latter (4.5%). Which spectrum to use is an open question. A theoretical argument is presented in favor of the ejection spectrum.