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Effect of improved TLD dosimetry on the determination of dose rate constants for 125 I and 103 Pd brachytherapy seeds
Author(s) -
Rodriguez M.,
Rogers D. W. O.
Publication year - 2014
Publication title -
medical physics
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.473
H-Index - 180
eISSN - 2473-4209
pISSN - 0094-2405
DOI - 10.1118/1.4895003
Subject(s) - monte carlo method , dosimetry , thermoluminescent dosimeter , brachytherapy , absorbed dose , imaging phantom , physics , percentage depth dose curve , nuclear medicine , thermoluminescence , computational physics , materials science , atomic physics , nuclear physics , dosimeter , irradiation , ionization chamber , mathematics , optics , statistics , ion , radiation therapy , medicine , quantum mechanics , ionization
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, P phant , 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 P phant values for 24 different seed models. By comparing the modified measured DRCs to the MC values, an appropriate relative intrinsic energy dependence,k bq rel , is determined. The new P phant values and relative absorbed‐dose sensitivities,S AD rel , calculated as the product of ( f rel ) −1 and( k bq 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. P phant values depend primarily on the isotope used. Deduced( k bq rel ) − 1values 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 P phant 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.