TG-43 Brachytherapy Dose Calculator

Dose Distribution Visualization

100% isodose
50% isodose
20% isodose
10% isodose
5% isodose

Source Parameters

Source Properties

Radionuclide: I-125
Half-life: 59.4 days
Active Length (L): 0.35 cm
Dose Rate Constant (Λ): 0.965 cGy·h⁻¹·U⁻¹

Point Dose Calculation

Click on the visualization to calculate dose at any point

Distance (r): 1.00 cm
Angle (θ): 90.0°
GL(r,θ): 1.0000
GL(r₀,θ₀): 1.0000
gL(r): 1.000
F(r,θ): 1.000
Dose Rate: 0.965 cGy/h

TG-43U1 Formula

Ḋ(r,θ) = SK · Λ · [GL(r,θ) / GL(r₀,θ₀)] · gL(r) · F(r,θ)
Reference: r₀ = 1 cm, θ₀ = 90°
Point source: GP = 1/r²
Line source: GL = β / (L·r·sin θ)
where β = subtended angle (radians)

Understanding TG-43 Parameters

Key Parameters:

  • SK (Air Kerma Strength): Source strength in units of U (μGy·m²·h⁻¹)
  • Λ (Dose Rate Constant): Dose rate at reference point per unit air kerma strength
  • GL (Geometry Function): Accounts for spatial distribution and inverse square law
  • gL(r) (Radial Dose Function): Accounts for scatter and attenuation along transverse axis
  • F(r,θ) (Anisotropy Function): Accounts for angular dose variation

Anisotropy Effects:

  • θ = 0° (along source): Reduced dose due to self-attenuation and encapsulation
  • θ = 90° (perpendicular): Maximum dose, reference orientation
  • Distance dependence: Anisotropy effects more pronounced at close distances
  • Source length: Longer sources show more anisotropy
  • Energy dependence: Low-energy sources (I-125, Pd-103) show greater anisotropy than high-energy (Ir-192)