Comparison of the hypothetical Co-57 brachytherapy source with the Ir-192 source

Aim of the study: The Co-57 radioisotope has recently been proposed as a hypothetical brachytherapy source due to its high specific activity, appropriate half-life (272 days) and medium energy photons (114.17 keV on average). In this study, Task Group No. 43 dosimetric parameters were calculated and reported for a hypothetical Co-57 source. Material and methods: A hypothetical Co-57 source was simulated in MCNPX, consisting of an active cylinder with 3.5 mm length and 0.6 mm radius encapsulated in a stainless steel capsule. Three photon energies were utilized (136 keV [10.68%], 122 keV [85.60%], 14 keV [9.16%]) for the Co-57 source. Air kerma strength, dose rate constant, radial dose function, anisotropy function, and isodose curves for the source were calculated and compared to the corresponding data for a Ir-192 source. Results: The results are presented as tables and figures. Air kerma strength per 1 mCi activity for the Co-57 source was 0.46 cGyh(-1) cm 2 mCi(-1). The dose rate constant for the Co-57 source was determined to be 1.215 cGyh(-1)U(-1). The radial dose function for the Co-57 source has an increasing trend due to multiple scattering of low energy photons. The anisotropy function for the Co-57 source at various distances from the source is more isotropic than the Ir-192 source. Conclusions: The Co-57 source has advantages over Ir-192 due to its lower energy photons, longer half-life, higher dose rate constant and more isotropic anisotropic function. However, the Ir-192 source has a higher initial air kerma strength and more uniform radial dose function. These properties make Co-57 a suitable source for use in brachytherapy applications.