TG-43 Dose Calculation Formalism: Development, Insights, and Modernization

The current international standard for brachytherapy dose calculations is the TG-43 formalism. This 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 the TG-43 dose calculation algorithm, brachytherapy dosimetry formalisms were based upon calculating the exposure rate in air at the point of interest and making a correction for the absorption of photons in water relative to air. The simplest version of these protocols assumed a point isotropic source resulting in a one-dimensional (1D) formulation such as https://www.w3.org/1998/Math/MathML" display="inline"> D ˙ r = Γ A r 2 . μ e n ρ a i r w a t e r , where the product of source activity (A) and the exposure rate constant (Γ) was simply corrected by the ratio of the mass-energy absorption coefficients for water and air. This approach was improved for 2D dose calculations by accounting for the radioactivity distribution throughout the source capsule and the dose falloff based on the physical properties of the medium and capsule, such as exponential attenuation, by the Sievert integral: https://www.w3.org/1998/Math/MathML" display="block"> D ˙ ( r , θ ) = S K ⋅ e μ " t ⋅ ( μ e n ¯ / ρ ) air water Γ A L r cos θ ⋅ ∫​θ 1 θ 2 e − μ " t sec ϕ T [ ( r cos θ − t ) sec ϕ ] d ϕ.