Dosimetric comparison between microSelectron iridium-192 and flexi cobalt-60 sources in high-dose-rate brachytherapy using Geant4 Monte Carlo code

PURPOSE: Manufacturing of miniaturized high activity iridium-192 ((192)Ir) sources have been made a market preference in modern brachytherapy. Smaller dimensions of the sources are flexible for smaller diameter of the applicators, and it is also suitable for interstitial implants. Presently, cobalt-60 ((60)Co) sources have been commercialized as an alternative to (192)Ir sources for high-dose-rate (HDR) brachytherapy, since (60)Co source have an advantage of longer half-life comparing with (192)Ir source. One of them is the HDR (60)Co Flexisource manufactured by Elekta. The purpose of this study was to compare the TG-43 dosimetric parameters of HDR flexi (60)Co and HDR microSelectron (192)Ir sources. MATERIAL AND METHODS: Monte Carlo simulation code of Geant4 (v.11.0) was applied. Following the recommendations of AAPM TG-43 formalism report, Monte Carlo code of HDR flexi (60)Co and HDR microSelectron (192)Ir was validated by calculating radial dose function, anisotropy function, and dose-rate constants in a water phantom. Finally, results of both radionuclide sources were compared. RESULTS: The calculated dose-rate constants per unit air-kerma strength in water medium were 1.108 cGy h(-1)U(-1) for HDR microSelectron (192)Ir, and 1.097 cGy h(-1)U(-1) for HDR flexi (60)Co source, with the percentage uncertainty of 1.1% and 0.2%, respectively. The values of radial dose function for distances above 22 cm for HDR flexi (60)Co source were higher than that of the other source. The anisotropic values sharply increased to the longitudinal sides of HDR flexi (60)Co source, and the rise was comparatively sharper to that of the other source. CONCLUSIONS: The primary photons from the lower-energy HDR microSelectron (192)Ir source have a limited range and are partially attenuated when considering the results of radial and anisotropic dose distribution functions. This implies that a HDR flexi (60)Co radionuclide could be used to treat tumors beyond the source compared with a HDR microSelectron (192)Ir source, despite the fact that (192)Ir has a lower exit dose than HDR flexi (60)Co radionuclide source.