1 A Review on Main Defects of TG-43 Mehdi Zehtabian 1 , Reza Faghihi 1,2 and Sedigheh Sina 1,2 1 Nuclear Engineering Department, School of Mechanical Engineering, Shiraz University, 2 Radiation Research Center, School of Mechanical Engineering, Shiraz University, Iran 1. Introduction The Sivert integral and modular dose calculation models (TG-43) are two methods used for calculation of dose distributions around a brachytherapy source (Khan, 2003). Among all methods used for calculation of dose distributions, TG-43 has become most popular and most promising method because it uses the quantities measured in the medium to calculate dose rates. In this chapter, we will explain the TG-43 dose calculation formalism and after verification of the main reason of its popularity, we will review main defects of this formalism. 1.1 TG-43 formalism In 1995, the American Association of Physicists in Medicine (AAPM)) published a report on the dosimetry of sources used in interstitial brachytherapy, Task Group No. 43 (TG-43) (Nath et al., 1995). This report introduces dose calculation formalism utilizing new quantities like air kerma strength (S K ), dose rate constant (Λ), geometry function (G (r,θ)), radial dose function (g (r)), and anisotropy function (F (r,θ)). These dosimetry parameters consider geometry, encapsulation and self-filtration of the source, the spatial distribution of radioactivity within the source, and scattering in water surrounding the source. According to this protocol, the absorbed dose rate distribution around a sealed brachytherapy source at point P with polar coordinates (r, ߠ) can be determined using the following formalism: ܦ . ሺߠ ,ݎሻൌΛ ௞ G ሺr, θሻ G ሺr ଴ , θ ଴ ሻ ሺݎሻ ܨሺߠ ,ݎሻ (1) where r is the distance to the point P and ߠis the angle with respect to the long axis of the source, and (r 0 , ߠ 0 ) is reference point that r 0 = 1 cm and ߠ 0 = ߨ/2 (Fig. 1). 1.1.1 Air kerma strength Air kerma strength, S k , acounts for brachytherapy source strength and is defined as the product of air kerma rate at a calibration distance (d) in free space, which is usually chosen to be 1 m, along the transverse axis of the source and the square of the distance (d 2 ), S k =K (d)d 2 (2) www.intechopen.com