172 Toossi MTB et al. / GZP6-TG43 geometries reproducible, at the same time flexible as it moves within the transfer tube and applicator, the active and non-active pellets are fitted within a steel spring cover. Dimensions and components of the active and non-active pellets of these sources are provided in the schematic diagram of source number 3 (Figure 2). In their first publication on this system, Mesbahi et al. measured the air kerma strengthsof source numbers 1, 2 and 5.11 In a sepa- rate investigation, Mesbahi has also calculated ra- dial dose function for these three sources.12 Naseri et al. have examined the accuracy of the dose dis- tributions calculated by the GZP6 treatment plan- ning system by Monte Carlo simulation of these sources.13 Monte Carlo simulation is widely used in radiophysics14, however, to our knowledge the GZP6 unit has not been studied before, based on a comprehensive determination of TG-43U1 dosi- metric parameters.15 Since the GZP6 unit has six sources with different fixed configurations, each source has been individually evaluated for their clinical applications. The goal of this project is to investigate the dosi- metric parameters (i.e. dose rate constant, radial dose function and anisotropy function) of source number 3, which has not been studied before, fol- lowing the TG-43U1 recommendation through Monte Carlo simulations. The results of these investigations will be compared with the corre- sponding data available for Ralstron (type 2) 60Co and microSelectron 192Ir sources.4,16 The dose distri- bution for the source No. 3 is used to produce dose distributions for the other GZP6 sources using a superimposition method developed in this study. Materials and methods Radioactive source structure The GZP6 high dose rate afterloading unit compris- es of different six 60Co sources affixed to six differ- ent channels in the system. In this study dosimetric characteristic of the source number 3 of the unit, which is a non-stepping source and includes one active cylindrical 60Co pellet, is being evaluated. As illustrated schematically in Figure 2, this source braid consists of an active nickel-plated cobalt pel- let and a number of inactive spherical pellets which are made of steel. The active 60Co cylinder has a ra- dius of 0.5 mm and length of 2 mm (including the nickel-plating), in which the 60Co radionuclide was distributed uniformly throughout the core. The nickel plating has a thickness of 0.05 mm, which has not been illustrated in the Figure 2. The active Radiol Oncol 2012; 46(2): 170-178. cobalt pellet is additionally encapsulated in a tita- nium capsule, with 0.25 mm in thickness and con- sisting of two hemispheres with 0.75 mm in radius at the two ends, sealed by Argon arc welding. The whole pellets are accommodated in a steel spring cover with an external diameter of 2.7 mm. 60Co is emitting two gamma photons with 1.17 and 1.33 MeV energies and β- particles with Emax=0.318 MeV. It has a half-life of 5.271 years. The latter particle is attenuated effectively with the titanium capsule. TG 43 formalism The formalism of TG-43U1 was followed to com- pute the dosimetric parameters of the source.15 According to this formalism the dose rate at the point P(r, θ) from the source center is defined by the following equation (as illustrated in Figure 2): Where r is the radial distance (in cm) from the source center and θ is the polar angle. r0is the ref- erence radial distance of 1 cm and θ0 is the refer- ence polar angle of 90º. Sk is air kerma strength of the source (in U, where 1 U=1 μGym2h-1), Λ is dose rate constant, G(r,θ) is geometry function, g(r,θ) is radial dose function and F(r,θ) is two-dimensional (2D) anisotropy function. The dose rate constant is the ratio of dose rate at the reference point (r0,θ0) and air kerma strength: Dose rate constant has a unit of cGyh-1U-1. In this study the line-source approximation was used for calculation of geometry function. By using this approximation, for the geometry function can be obtained from the following equation: If the geometry function is obtained from equation [4]: [1] [2] [3] [4] [5] As denoted by Awan et al.17, considering the source active length L, the radial distance r and the angles θ and ß as showed in the Figure 2, equation [5] can be resulted from the above equation: Thus if , the geometry function can be calcu- lated directly in terms of r and θ from equation [5].