ABSTRACT
As of the year 2012, the international standard for brachytherapy dose calculations is the TG-43 formalism. is formalism was developed by the American Association of Physicists in Medicine (AAPM) Task Group No. 43 and published in 1995 (Nath et al. 1995). Prior to the introduction of TG-43 dose calculation algorithm, brachytherapy dosimetry formalisms were based on calculating the exposure rate in air at the point of interest and making a correction for the attenuation of photons in water. e simplest version of these protocols assumed a point isotropic source resulting in a 1D formulation such as
D r A r
en( ) = ⋅
µ ρ air
, where the product of source activity and
the exposure rate constant was simply corrected by ratio of the mass-energy absorption coecients for water and air. is was improved through accounting for dose fallo based on physical properties of medium and capsule, such as exponential attenuation, by the Sievert integral
D r S e A
en t( , ) cos
secθ µ ρ
= ⋅ ⋅
( ) ⋅
θ θ φ φ
2∫ −T r t[( cos )sec ]d as described in Appendix B of the 1995 TG-43 report, which provides a detailed description of this process (see also Chapter
7). TG-43 formalism is instead based upon the dose rate at a reference point in water from the actual source geometry, which is then corrected for relative attenuation in water at all other points using factors such as radial dose function and anisotropy function. Deviation from the physics-oriented model of the Sievert integral toward the formalism recommended in the TG-43 report was a bold move. Given that the goals of a brachytherapy dose calculation formalism are accuracy and consistency of clinical implementation, this was the right decision to overcome the limitations of the Sievert integral to model radiological interactions and needless complication and specication of various terms. is is discussed in greater detail in Chapter 7.
