Evaluation of high-energy brachytherapy source electronic disequilibrium and dose from emitted electrons

Purpose: The region of electronic disequilibrium near photon-emitting brachytherapy sources of high-energy radionuclides (Co-60, Cs-137, Ir-192, and Yb-169) and contributions to total dose from emitted electrons were studied using the GEANT4 and PENELOPE Monte Carlo codes. Methods: Hypothetical sources with active and capsule materials mimicking those of actual sources but with spherical shape were examined. Dose contributions due to source photons, x rays, and bremsstrahlung; source beta(-), Auger electrons, and internal conversion electrons; and water collisional kerma were scored. To determine if conclusions obtained for electronic equilibrium conditions and electron dose contribution to total dose for the representative spherical sources could be applied to actual sources, the Ir-192 mHDR-v2 source model (Nucletron B.V., Veenendaal, The Netherlands) was simulated for comparison to spherical source results and to published data. Results: Electronic equilibrium within 1% is reached for Co-60, Cs-137, Ir-192, and Yb-169 at distances greater than 7, 3.5, 2, and 1 mm from the source center, respectively, in agreement with other published studies. At 1 mm from the source center, the electron contributions to total dose are 1.9% and 9.4% for Co-60 and Ir-192, respectively. Electron emissions become important (i.e., >0.5%) within 3.3 mm of Co-60 and 1.7 mm of Ir-192 sources, yet are negligible over all distances for Cs-137 and Yb-169. Electronic equilibrium conditions along the transversal source axis for the mHDR-v2 source are comparable to those of the spherical sources while electron dose to total dose contribution are quite different. Conclusions: Electronic equilibrium conditions obtained for spherical sources could be generalized to actual sources while electron contribution to total dose depends strongly on source dimensions, material composition, and electron spectra. (C) 2009 American Association of Physicists in Medicine. [DOI: 10.1118/1.3194754]