WE‐C‐BRB‐03: Effect of Scattered Photons When Determining Dose Rate Constants Spectroscopically
Purpose: To determine the effect of scatter from the seed when establishing the dose rate constant of 125I and 103Pd seeds using the spectroscopic method of Chen and Nath (Med Phys 34 (2007) 1412) which is based on the main peaks in the spectrum. Methods: The energy spectra generated by two 125I and one 103Pd seeds were calculated in vacuo at 10 cm from the source using the EGSnrc BrachyDose Monte Carlo code. Energy bin width was 0.2 keV and statistical uncertainty was 0.02%. The energy spectra generated by the seeds were then used as a 3 mm line source in a Monte Carlo simulation to calculate the dose rate constant. Two simulation were performed; one using the full energy spectrum and the other using only the main peaks. Statistical uncertainty in air kerma strength and dose rate was 0.1%. Results: The differences between the average energy of the full spectrum and the average energy of the main peaks generated by the three seeds were 0.07% or less. Differences in air kerma strength and dose rate constant were 0.5% or less and the difference in the dose rate was lower than the statistical uncertainty (0.1%). Conclusions: Air kerma strength, dose rate at the reference point and dose rate constant were calculated by Monte Carlo simulation using both the full energy spectrum and the main peaks only of three brachytherapy seeds. The differences in the calculation of these clinical parameters were less than 0.5%. This difference is substantially lower than the reported uncertainty in establishing the dose rate constant spectroscopically, which can reach up to 3.9% and 2.6% for the 125I and 103Pd seeds respectively. Therefore, the effects of scatter on determining the dose rate constant spectroscopically is absorbed by the other uncertainties in the method.