Brachytherapy
TG-43 formalism
The dose rate is D(r,θ) = S_k · Λ · [G(r,θ)/G(r0,θ0)] · g(r) · F(r,θ). Air kerma strength S_k is expressed in U, where 1 U = 1 cGy·cm²/h = 1 µGy·m²/h; Λ is the dose-rate constant in cGy/h/U at the reference point; g(r) is the radial dose function (attenuation and scatter); F(r,θ) is the anisotropy function. TG-43 assumes an infinite water medium, so TG-186 model-based methods are used for heterogeneity such as shielded applicators, bone, or breast implants.
Practice points
LDR uses I-125 or Pd-103 seeds; HDR uses an Ir-192 afterloader with dwell-position verification and redundant interlocks. Source strength is verified with a well chamber. Fractionation is compared using EQD2 with α/β = 10 for tumor and 3 for late-responding organs. For cervix, dose was historically reported to Point A (2 cm superior to the os and 2 cm lateral to the tandem), but contemporary reporting uses HR-CTV D90 and OAR D2cc.
Traps
Confusing mCi with air kerma strength in U, assuming TG-43 remains valid in heterogeneous tissue, and forgetting that HDR QA includes timer error, dwell-position accuracy, and applicator reconstruction accuracy.
Worked example
HDR 6 Gy × 5 fractions to tumor with α/β = 10: EQD2 per fraction = d(d + α/β)/(2 + α/β) = 6 × 16/12 = 8 Gy, so the course is 40 Gy EQD2.