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 (¹⁹²Ir) 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 (⁶⁰Co) sources have been commercialized as an alternative to ¹⁹²Ir sources for high-dose-rate (HDR) brachytherapy, since ⁶⁰Co source have an advantage of longer half-life comparing with ¹⁹²Ir source. One of them is the HDR ⁶⁰Co Flexisource manufactured by Elekta. The purpose of this study was to compare the TG-43 dosimetric parameters of HDR flexi ⁶⁰Co and HDR microSelectron 192Ir 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 ⁶⁰Co and HDR microSelectron ¹⁹²Ir 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^-1U^-1 for HDR microSelectron 192Ir, and 1.097 cGy h^-1U^-1 for HDR flexi ⁶⁰Co 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 ⁶⁰Co source were higher than that of the other source. The anisotropic values sharply increased to the longitudinal sides of HDR flexi ⁶⁰Co source, and the rise was comparatively sharper to that of the other source.

Conclusions:
The primary photons from the lower-energy HDR microSelectron ¹⁹²Ir 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 ⁶⁰Co radionuclide could be used to treat tumors beyond the source compared with a HDR microSelectron ¹⁹²Ir source, despite the fact that ¹⁹²Ir has a lower exit dose than HDR flexi ⁶⁰Co radionuclide source.