170 research article Dosimetric characterizations of GZP6 60Co high dose rate brachytherapy sources: application of superimposition method Mohammad Taghi Bahreyni Toossi1, Mahdi Ghorbani2, Ali Asghar Mowlavi3 and Ali Soleimani Meigooni4 1 Medical Physics Research Center, Medical Physics Department, Faculty of Medicine, Mashhad University of Medical Sciences, Mashhad, Iran 2 North Khorasan University of Medical Sciences, Bojnurd, Iran 3 Physics Department, School of Sciences, Hakim Sabzevari University, Sabzevar, Iran 4 Comprehensive Cancer Center of Nevada, 3730 S. Eastern Avenue, Las Vegas, Nevada, USA Radiol Oncol 2012; 46(2): 170-178. Received 2 October 2011 Accepted 21 November 2011 Correspondence to: Mahdi Ghorbani, Vice Chancellery of Research and Technology, North Khorasan University of Medical Sciences, South Shariati No. 7, Bojnurd, North Khorasan, Iran. Phone: +98 584 2247124; Fax: +98 584 2247124; E-mail: mhdghorbani@gmail.com Disclosure: No potential conflicts of interest were disclosed. Background. Dosimetric characteristics of a high dose rate (HDR) GZP6 Co-60 brachytherapy source have been evaluated following American Association of Physicists in MedicineTask Group 43U1 (AAPM TG-43U1) recommenda- tions for their clinical applications. Materials and methods. MCNP-4C and MCNPX Monte Carlo codes were utilized to calculate dose rate constant, two dimensional (2D) dose distribution, radial dose function and 2D anisotropy function of the source. These param- eters of this source are compared with the available data for Ralstron 60Co and microSelectron192Ir sources. Besides, a superimposition method was developed to extend the obtained results for the GZP6 source No. 3 to other GZP6 sources. Results. The simulated value for dose rate constant for GZP6 source was 1.1040.03 cGyh-1U-1. The graphical and tabulated radial dose function and 2D anisotropy function of this source are presented here. The results of these inves- tigations show that the dosimetric parameters of GZP6 source are comparable to those for the Ralstron source. While dose rate constant for the two 60Co sources are similar to that for the microSelectron192Ir source, there are differences between radial dose function and anisotropy functions. Radial dose function of the 192Ir source is less steep than both 60Co source models. In addition, the 60Co sources are showing more isotropic dose distribution than the 192Ir source. Conclusions. The superimposition method is applicable to produce dose distributions for other source arrange- ments from the dose distribution of a single source. The calculated dosimetric quantities of this new source can be introduced as input data to the GZP6 treatment planning system (TPS) and to validate the performance of the TPS. Key words: brachytherapy; GZP6; TG-43; superimposition method; Monte Carlo simulation Introduction Recently, the GZP6 high dose rate (HDR) 60Co brachytherapy unit, manufactured byNuclear Power Institute of China (Chengdu, China), has been introduced for brachytherapy procedures.1 Although not as common as 192Ir source, 60Co has been used in past as a brachytherapy source in the treatment of various malignancies2-4, and a brachy- therapy technique remains a frequent treatment in clinical praxis.5 There are also potential logisti- cal advantages for 60Co sources for HDR systems over 192Ir6, including less frequency of source ex- change which provides longer duration of clini- cal use and reduced operating costs. Different geometric designs of 60Co radionuclide have been used in radiotherapy clinics in the past with lim- ited traditional dosimetric information.7,8 Dose cal- Radiol Oncol 2012; 46(2): 170-178. doi:10.2478/v10019-012-0005-3