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

Area of Science:

• Medical Physics
• Nuclear Medicine
• Radiotherapy Physics

Background:

• ¹²⁵I brachytherapy seeds are widely used in cancer treatment.
• Accurate dosimetry relies on precise knowledge of the radionuclide's energy spectrum.
• Discrepancies exist between recommended energy spectra, potentially impacting treatment planning.

Purpose of the Study:

• To compare measured ¹²⁵I energy spectra with Monte Carlo simulations using TG43 and NCRP Report 58 spectra.
• To evaluate the impact of these spectra on dose rate constant calculations.
• To determine the best spectrum for accurate branching ratio calculations.

Main Methods:

• Monte Carlo simulations using EGSnrc BrachyDose code.
• Calculated energy spectra, absorbed dose, and air kerma strength for 10 ¹²⁵I seeds.
• Simulations performed with TG43 and NCRP Report 58 bare ¹²⁵I energy spectra.
• Results compared with published measured values.

Main Results:

• Monte Carlo simulations closely matched measured branching ratios using the NCRP Report 58 spectrum.
• The TG43 spectrum yielded a 7% lower 31 keV peak for branching ratios.
• Differences in average energy, air kerma, absorbed dose, and dose rate constant were negligible (<0.1%).

Conclusions:

• TG43 and NCRP58 spectra yield indistinguishable results for dosimetric parameter calculations.
• Significant differences in branching ratio calculations exist between the two spectra.
• Measured branching ratios align better with the NCRP58 spectrum, suggesting a reassessment of the TG43 spectrum is warranted.