Approaches to calculating AAPM TG-43 brachytherapy dosimetry parameters for 137Cs, 125I, 192Ir, 103Pd, and 169Yb sources

Underlying characteristics in brachytherapy dosimetry parameters for medical radionuclides Cs-137. I-125, Ir-192, Pd-103, and Yb-169 were examined using Monte Carlo methods. Sources were modeled as unencapsulated point or line sources in liquid water to negate variations due to materials and construction. Importance of phantom size, mode of radiation transport physics-i.e.. photon transport only or coupled photon:electron transport, phantom material, volume averaging. and Monte Carlo tally type were studied. For noninfinite media, g(r) was found to degrade as r approached R, the phantom radius. MCNP5 results were in agreement with those published using GEANT4. Brachytherapy dosimetry parameters calculated using coupled photon:electron radiation transport simulations did not differ significantly from those using photon transport only. Dose distributions from low-energy photon-emitting radionuclides I-125 and Pd-103 were sensitive to phantom material by upto a factor of 1.4 and 2.0, respectively, between tissue-equivalent materials and water at r =9 cm. In comparison, high-energy photons from Cs-137, Ir-192, and Yb-169 demonstrated +/- 5% differences in dose distributions 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 two-dimensional anisotropy function was largely within +/- 0.5% of unity for Cs-137, I-125, and Ir-192. However, an energy and geometry effect was noted for Pd-103 and Yb-169, with FPd-103(0.5,0 degrees)=1.05 and FYb-169(0.5,0 degrees)=0.98 for L=0.5 cm. Simulations of monoenergetic photons for L=0.5 cm produced energy-dependent variations in F(r, theta) having a maximum value at 10 keV, minimum at 50 keV. and similar to 1.0 for higher-energy photons up to 750 keV. Both the F6 cell heating and *F4 track-length estimators were employed to determine brachytherapy dosimetry parameters. F6 was found to be necessary for g(r), while both tallies provided equivalent results for F(r, theta). (C) 2006 American Association of Physicists in Medicine.