SU-E-T-482: A More Accurate Bare 12 5I Spectrum.

Abstract

To compare measured energy spectra of 125 I brachytherapy seeds with Monte Carlo calculated energy spectra when using the bare 125 I energy spectrum recommended by TG43 and the one presented in NCRP Report 58 and to determine the impact of these different spectra on the calculation of the dose rate constant. The energy spectrum, absorbed dose at 1 cm on the bisector and air kerma strength of 10 different 125 I seeds were calculated using the EGSnrc BrachyDose Monte Carlo code. Energy bin width was 0.2 keV. The simulations were done with the bare 125 I energy spectrum recommended by TG43 or NCRP Report 58. Statistical uncertainties for the energy spectra were 0.02% and 0.1% or less for the other clinical parameters. Results are compared with published measuredvalues. There is close agreement between the measured and calculated branching ratio generated by 125 I seeds when simulated using the spectrum presented in NCRP58. However, for all seeds a 7% lower value is observed in the 31 keV peak when using data suggested by TG43. On the other hand, differences in the average energy, air kerma, absorbed dose and dose rate constant are undetectable (less than statistical uncertainty, <0.1%) when performed with either spectrum. For dosimetric parameter calculations, the bare 125 I energy spectra presented in TG43 and NCRP58 produce indistinguishable results. However, for branching ratio investigation purposes, there is a clear difference in the 31 keV peak between values obtained using the different 125 I bare energy spectra. Measured 125 I branching ratios are in close agreement with those calculated using the bare 125 I energy spectrum presented in NCRP58. Reassessment of the bare 125 I energy spectrum recommended in TG43 is suggested.