Transit dose comparisons for ⁶⁰Co and ¹⁹²Ir HDR sources

The goal of this study is to evaluate the ambient dose due to the transit of high dose rate (HDR) ⁶⁰Co sources along a transfer tube as compared to ¹⁹²Ir ones in a realistic clinical scenario. This goal is accomplished by evaluating air-kerma differences with Monte Carlo calculations using PENELOPE2011. Scatter from both the afterloader and the patient was not taken into account. Two sources, mHDR-v2 and Flexisource Co-60, (Elekta Brachytherapy, Veenendaal, the Netherlands) have been considered. These sources were simulated within a standard transfer tube located in an infinite air phantom. The movement of the source was included by displacing their positions along the connecting tube from z  =  -75 cm to z  =  +75 cm and combining them. Since modern afterloaders like Flexitron (Elekta) or Saginova (BEBIG GmbH) are able to use equally ¹⁹²Ir and ⁶⁰Co sources, it was assumed that both sources are displaced with equal speed. Typical HDR source activity content values were provided by the manufacturer. 2D distributions were obtained with type-A uncertainties (k  =  2) less than 0.01%. From those, the air-kerma ratio ⁶⁰Co/¹⁹²Ir was evaluated weighted by their corresponding typical activities. It was found that it varies slowly with distance (less than 10% variation at 75 cm) but strongly in time due to the shorter half-life of the ¹⁹²Ir (73.83 d). The maximum ratio is located close to the tube. It reaches a value of 0.57 when the typical activity of the sources at the time when they were installed by the vendor was used. Such ratio increases up to 1.28 at the end of the recommended working life (90 d) of the ¹⁹²Ir source. ⁶⁰Co/¹⁹²Ir air-kerma ratios are almost constant (0.51-0.57) in the vicinity of the source-tube with recent installed sources. Nevertheless, air-kerma ratios increase rapidly (1.15-1.29) whenever the ¹⁹²Ir is approaching the end of its life. In case of a medical event requiring the medical staff to access the treatment room, these ratios indicate that the dosimetric impact on the medical team will be lower, with a few exceptions, in the case of ⁶⁰Co-based HDR brachytherapy as compared to ¹⁹²Ir-based one when typical air-kerma strength values are considered.