Monte Carlo Dosimetry of the (60)Co BEBIG High Dose Rate for Brachytherapy

INTRODUCTION: The use of high-dose-rate brachytherapy is currently a widespread practice worldwide. The most common isotope source is (192)Ir, but (60)Co is also becoming available for HDR. One of main advantages of (60)Co compared to (192)Ir is the economic and practical benefit because of its longer half-live, which is 5.27 years. Recently, Eckert & Ziegler BEBIG, Germany, introduced a new afterloading brachytherapy machine (MultiSource(®)); it has the option to use either the (60)Co or (192)Ir HDR source. The source for the Monte Carlo calculations is the new (60)Co source (model Co0.A86), which is referred to as the new BEBIG (60)Co HDR source and is a modified version of the (60)Co source (model GK60M21), which is also from BEBIG. OBJECTIVE AND METHODS: The purpose of this work is to obtain the dosimetry parameters in accordance with the AAPM TG-43U1 formalism with Monte Carlo calculations regarding the BEBIG (60)Co high-dose-rate brachytherapy to investigate the required treatment-planning parameters. The geometric design and material details of the source was provided by the manufacturer and was used to define the Monte Carlo geometry. To validate the source geometry, a few dosimetry parameters had to be calculated according to the AAPM TG-43U1 formalism. The dosimetry studies included the calculation of the air kerma strength S (k), collision kerma in water along the transverse axis with an unbounded phantom, dose rate constant and radial dose function. The Monte Carlo code system that was used was EGSnrc with a new cavity code, which is a part of EGS++ that allows calculating the radial dose function around the source. The spectrum to simulate (60)Co was composed of two photon energies, 1.17 and 1.33 MeV. Only the gamma part of the spectrum was used; the contribution of the electrons to the dose is negligible because of the full absorption by the stainless-steel wall around the metallic (60)Co. The XCOM photon cross-section library was used in subsequent simulations, and the photoelectric effect, pair production, Rayleigh scattering and bound Compton scattering were included in the simulation. Variance reduction techniques were used to speed up the calculation and to considerably reduce the computer time. The cut-off energy was 10 keV for electrons and photons. To obtain the dose rate distributions of the source in an unbounded liquid water phantom, the source was immersed at the center of a cube phantom of 100 cm(3). The liquid water density was 0.998 g/cm(3), and photon histories of up to 10(10) were used to obtain the results with a standard deviation of less than 0.5% (k = 1). The obtained dose rate constant for the BEBIG (60)Co source was 1.108±0.001 cGyh(-1)U(-1), which is consistent with the values in the literature. The radial dose functions were compared with the values of the consensus data set in the literature, and they are consistent with the published data for this energy range.