WE‐C‐108‐12: Accounting for Intrinsic Energy Dependence of TLDs When Establishing Dose Rate Constants for Brachytherapy Seeds

Abstract

Purpose: To retrospectively account for the intrinsic energy dependence of TLDs when used to measure dose rate constants (DRC) for I–125 and Pd‐103 brachytherapy seeds, to establish new measured values for all seeds and compare the new values with Monte Carlo (MC) values and consensus values published by TG‐43U1. Methods: To calculate the relative absorbed‐dose energy dependence, f^rel, the ratio D(TLD)/D(w) at brachytherapy energies relative to Co‐60 is calculated using the EGSnrc BrachyDose code for TLD sized voxels at 1 cm from the seed centered in a water phantom. A similar geometry is used at high energy with the detector at 5 cm depth and DOSXYZnrc code. The original energy dependence applied to DRC measurements is replaced by the calculated f^rel values. By comparing the revised measured DRCs to the MC values, one can determine an appropriate relative intrinsic energy dependence, k^rel. The new relative absorbed‐dose sensitivity, S^rel, calculated as the product of f^rel and k^rel, is to give the measured DRC for comparison with Monte Carlo calculated values and TG‐43U1 consensus values. Results: The calculated f^rel values are 1.389 and 1.354 and the deduced k^rel values are 1.067 and 1.066 for I‐125 and Pd‐103 seed models, respectively. Using these new values with the literature DRC measurements, differences between calculated and revised measured DRCs are within 5% (typical measurement uncertainty) and the average discrepancy is 0.5%. The average of both DRC sets are on average 3.4% lower than the the consensus values reported in TG‐43U1. Conclusion: This work demonstrates that the energy response dependence, measured 25 years ago, applied to recent DRC measurements of I–125 and Pd‐103 brachytherapy seeds needs to be updated, thereby leading to a revision of the DRC consensus values reported by TG‐43U1.