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

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.