TU‐D‐224C‐03: TG‐43U1 Brachytherapy Dosimetry Parameters for Virtual, Unfiltered Sources

Purpose: Underlying brachytherapy dosimetry characteristics for 137Cs, 125I, 192Ir, 103Pd, and 169Yb were examined using Monte Carlo methods. Sources were modeled as unencapsulated point or line sources in liquid water to negate source‐specific effects of materials and construction. Importance of phantom size (R), radiation transport mode, phantom material, and volume averaging were studied. Method and Materials: Radiation transport simulations were performed with MCNP5 using the most recent photon cross‐section libraries. The AAPM TG‐43U1 brachytherapy dosimetry formalism was employed and extended to radionuclides with EAVG > 50 keV. Radiation spectra were taken from the National Nuclear Data Center and compared to those commonly referenced. Enough photon histories were simulated to maintain statistical uncertainties < 1%. Results and Discussion: For non‐infinite media, g(r) was found to degrade as r approached R, the phantom radius, and MCNP5 results were in agreement with those published using GEANT4. Dosimetry parameters calculated using coupled photon‐electron radiation transport simulations did not differ significantly from those using photon transport only. Low‐energy radionuclides 125I and 103Pd were sensitive to phantom material with up to a factor of 1.4 and 2, respectively, between tissue‐equivalent materials and water at r = 9 cm. In comparison, high‐energy photons from 137Cs, 192Ir, and 169Yb demonstrated ± 5% differences between water and tissue‐substitutes at r = 20 cm. Similarly, volume‐averaging effects were found to be more significant for low‐energy radionuclides. When modeling line sources with L ⩽ 0.5 cm, the 2‐D anisotropy function was largely within ± 0.5% of unity for 137Cs, 125I, and 192Ir. However, an energy and geometry effect was noted for 103Pd and 169Yb, with F(0.5,0°) = 1.05 and 0.98, respectively, for L = 0.5 cm. Additional radiation transport calculations using mono‐energetic photons showed energy‐dependent variations in F(r,θ) as a function of effective length and θ.