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

Aim of the study: The ⁵⁷Co 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 ⁵⁷Co source.

Material and methods: A hypothetical ⁵⁷Co 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 ⁵⁷Co 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 ¹⁹²Ir source.

Results: The results are presented as tables and figures. Air kerma strength per 1 mCi activity for the ⁵⁷Co source was 0.46 cGyh–1 cm 2 mCi–1. The dose rate constant for the ⁵⁷Co source was determined to be 1.215 cGyh–1U–1. The radial dose function for the ⁵⁷Co source has an increasing trend due to multiple scattering of low energy photons. The anisotropy function for the ⁵⁷Co source at various distances from the source is more isotropic than the ¹⁹²Ir source.

Conclusions: The ⁵⁷Co source has advantages over ¹⁹²Ir due to its lower energy photons, longer half-life, higher dose rate constant and more isotropic anisotropic function. However, the ¹⁹²Ir source has a higher initial air kerma strength and more uniform radial dose function. These properties make ⁵⁷Co a suitable source for use in brachytherapy applications.