Photon activation therapy: a Monte Carlo study on dose enhancement by various sources and activation media

Mahdi Bakhshabadi, Mahdi Ghorbani, Ali Soleimani Meigooni

In the present study, a number of brachytherapy sources and activation media were simulated using MCNPX code and the results were analyzed based on the dose enhancement factor values. Furthermore, two new brachytherapy sources (¹³¹Cs and a hypothetical ¹⁷⁰Tm) were evaluated for their application in photon activation therapy (PAT). ¹²⁵I, ¹⁰³Pd, ¹³¹Cs and hypothetical ¹⁷⁰Tm brachytherapy sources were simulated in water and their dose rate constant and the radial dose functions were compared with previously published data. The sources were then simulated in a soft tissue phantom which was composed of Ag, I, Pt or Au as activation media uniformly distributed in the tumour volume. These simulations were performed using the MCNPX code, and dose enhancement factor (DEF) was obtained for 7, 18 and 30 mg/ml concentrations of the activation media. Each source, activation medium and concentration was evaluated separately in a separate simulation. The calculated dose rate constant and radial dose functions were in agreement with the published data for the aforementioned sources. The maximum DEF was found to be 5.58 for a combination of the ¹⁷⁰Tm source with 30 mg/ml concentration of I. The DEFs for ¹³¹Cs and ¹⁷⁰Tm sources for all the four activation media were higher than those for other sources and activation media. From this point of view, these two sources can be more useful in photon activation therapy with photon emitter sources. Furthermore, ¹³¹Cs and ¹⁷⁰Tm brachytherapy sources can be proposed as new options for use in the field of PAT.