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Accounting for Intrinsic Energy Dependence of TLDs When Establishing Dose Rate Constants for Brachytherapy Seeds

M Rodriguez

M Rodriguez1,2*, D Rogers1, (1) Carleton University, Ottawa, ON, (2) Universidad Nacional Autonoma de Honduras

WE-C-108-12 Wednesday 10:30AM - 12:30PM Room: 108

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.

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.

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.

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.

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