Effect of improved TLD dosimetry on the determination of dose rate constants for (125)I and (103)Pd brachytherapy seeds.

PURPOSE

To more accurately account for the relative intrinsic energy dependence and relative absorbed-dose energy dependence of TLDs when used to measure dose rate constants (DRCs) for (125)I and (103)Pd brachytherapy seeds, to thereby establish revised "measured values" for all seeds and compare the revised values with Monte Carlo and consensus values.

METHODS

The relative absorbed-dose energy dependence, f(rel), for TLDs and the phantom correction, Pphant, are calculated for (125)I and (103)Pd seeds using the EGSnrc BrachyDose and DOSXYZnrc codes. The original energy dependence and phantom corrections applied to DRC measurements are replaced by calculated (f(rel))(-1) and Pphant values for 24 different seed models. By comparing the modified measured DRCs to the MC values, an appropriate relative intrinsic energy dependence, kbq (rel), is determined. The new Pphant values and relative absorbed-dose sensitivities, SAD (rel), calculated as the product of (f(rel))(-1) and (kbq (rel))(-1), are used to individually revise the measured DRCs for comparison with Monte Carlo calculated values and TG-43U1 or TG-43U1S1 consensus values.

RESULTS

In general, f(rel) is sensitive to the energy spectra and models of the brachytherapy seeds. Values may vary up to 8.4% among (125)I and (103)Pd seed models and common TLD shapes. Pphant values depend primarily on the isotope used. Deduced (kbq (rel))(-1) values are 1.074 ± 0.015 and 1.084 ± 0.026 for (125)I and (103)Pd seeds, respectively. For (1 mm)(3) chips, this implies an overall absorbed-dose sensitivity relative to (60)Co or 6 MV calibrations of 1.51 ± 1% and 1.47 ± 2% for (125)I and (103)Pd seeds, respectively, as opposed to the widely used value of 1.41. Values of Pphant calculated here have much lower statistical uncertainties than literature values, but systematic uncertainties from density and composition uncertainties are significant. Using these revised values with the literature's DRC measurements, the average discrepancies between revised measured values and Monte Carlo values are 1.2% and 0.2% for (125)I and (103)Pd seeds, respectively, compared to average discrepancies for the original measured values of 4.8%. On average, the revised measured values are 4.3% and 5.9% lower than the original measured values for (103)Pd and (125)I seeds, respectively. The average of revised DRCs and Monte Carlo values is 3.8% and 2.8% lower for (125)I and (103)Pd seeds, respectively, than the consensus values in TG-43U1 or TG-43U1S1.

CONCLUSIONS

This work shows that f(rel) is TLD shape and seed model dependent suggesting a need to update the generalized energy response dependence, i.e., relative absorbed-dose sensitivity, measured 25 years ago and applied often to DRC measurements of (125)I and (103)Pd brachytherapy seeds. The intrinsic energy dependence for LiF TLDs deduced here is consistent with previous dosimetry studies and emphasizes the need to revise the DRC consensus values reported by TG-43U1 or TG-43U1S1.