Microdosimetric calculations for radionuclides emitting β and α particles and Auger electrons.

This work focuses on the calculation of S-values and radial energy profiles for radionuclides emitting high (Y-90, Sr-89), medium (Re-186, Sm-153) and low-energy (Er-169, Lu-177) β-particles, Auger electrons (In-111, Ga-67, I-123) and α-particles (At-211, Ac-225). Simulations were performed using the EGSnrc and GEANT4-DNA Monte Carlo (MC) codes for a spherical cell geometry. S-values were computed using decay spectra available in literature for Tc-99m and In-111. To investigate the effect on S-value when the same emission spectrum is used in two different MC codes. Internal modules of the MC codes were used to simulate the decay of other radionuclides mentioned above. Radial energy profiles for uniformly distributed radioactive sources in the cell nucleus and cytoplasm were calculated and results were compared with the literature. For S-values calculated using the same emission spectrum, the results showed good agreement with each other and with the literature. Whereas, the S-values calculated using the internal decay data of the MC codes, for instance, for Ga-67 and Y-90, showed discrepancies up to 40%. Radial energy profiles were also different from those reported in the literature. Our results show that well validated radiation emission spectra must be used for such calculations and internal decay spectra of MC codes should be used with caution. The normalized probability density functions must be used to sample points uniformly into spherical volumes and the methodology proposed here can be used to correctly determine radial energy profiles.