Evaluation of high-energy brachytherapy source electronic disequilibrium and dose from emitted electrons.
OBJECTIVE
The region of electronic disequilibrium near photon-emitting brachytherapy sources of high-energy radionuclides (60Co, 137CS, 192Ir, and 169Yb) 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 192Ir 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 60Co, 137CS, 192Ir, and 169Yb 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 60Co and 192Ir, respectively. Electron emissions become important (i.e., > 0.5%) within 3.3 mm of 60Co and 1.7 mm of 192Ir sources, yet are negligible over all distances for 137Cs and 169Yb. 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.
Ballester
Facundo
F
Department of Atomic, Molecular and Nuclear Physics, University of Valencia, C/Dr Moliner 50, E-46100 Burjassot, Spain. fballest@uv.es
Granero
Domingo
D
Pérez-Calatayud
José
J
Melhus
Christopher S
CS
Rivard
Mark J
MJ
D001918:Brachytherapy / Q000295:instrumentation; D002588:Cesium Radioisotopes; D003037:Cobalt Radioisotopes; D003198:Computer Simulation; D004583:Electrons / Q000627:therapeutic use*; D007496:Iridium Radioisotopes; D008962:Models, Theoretical; D009010:Monte Carlo Method; D017785:Photons; D011829:Radiation Dosage; D011868:Radioisotopes; D011874:Radiometry / Q000379:methods*; D011879:Radiotherapy Dosage; D012984:Software; D014867:Water / Q000737:chemistry; D014965:X-Rays; D015018:Ytterbium
D002588:Cesium Radioisotopes; D003037:Cobalt Radioisotopes; D007496:Iridium Radioisotopes; D011868:Radioisotopes; D014867:Water; D015018:Ytterbium