Exploring 57Co as a new isotope for brachytherapy applications

Purpose: The characteristics of the radionuclide Co-57 make it interesting for use as a brachytherapy source. Co-57 combines a possible high specific activity with the emission of relatively low-energy photons and a half-life (272 days) suitable for regular source exchanges in an afterloader. Co-57 decays by electron capture to the stable Fe-57 with emission of 136 and 122 keV photons. Methods: A hypothetical Co-57 source based on the Flexisource brachytherapy encapsulation with the active core set as a pure cobalt cylinder (length 3.5 mm and diameter 0.6 mm) covered with a cylindrical stainless-steel capsule (length 5 mm and thickness 0.125 mm) was simulated using Geant4 Monte Carlo (MC) code version 9.4. The radial dose function, g(r), and anisotropy function F(r,theta), for the line source approximation were calculated following the TG-43U1 formalism. The results were compared to well-known Ir-192 and 1(25)I radionuclides, representing the higher and the lower energy end of brachytherapy, respectively. Results: The mean energy of photons in water, after passing through the core and the encapsulation material was 123 keV. This hypothetical Co-57 source has an increasing g(r) due to multiple scatter of low-energy photons, which results in a more uniform dose distribution than Ir-192. Conclusions: Co-57 has many advantages compared to Ir-192 due to its low-energy gamma emissions without any electron contamination. Co-57 has an increasing g(r) that results in a more uniform dose distribution than Ir-192 due to its multiple scattered photons. The anisotropy of the Co-57 source is comparable to that of Ir-192. Furthermore, Co-57 has lower shielding requirements than Ir-192. (C) 2012 American Association of Physicists in Medicine. [http://dx.doi.org/10.1118/1.3700171]