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Time dependence of energy spectra of brachytherapy sources and its impact on the half and the tenth value layers
Author(s) -
Yue Ning J.,
Chen Zhe,
Hearn Robert A.,
Rodgers Joseph J.,
Nath Ravinder
Publication year - 2009
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 - 10.1118/1.3246843
Subject(s) - photon , electromagnetic shielding , spectral line , formalism (music) , photon energy , brachytherapy , dosimetry , impurity , atomic physics , half life , radioactive decay , energy spectrum , computational physics , physics , chemistry , materials science , optics , nuclear physics , nuclear medicine , visual arts , medicine , art , musical , quantum mechanics , astronomy , radiation therapy , pharmacokinetics
Purpose: Several factors including radionuclide purity influence the photon energy spectra from sealed brachytherapy sources. The existence of impurities and trace elements in radioactive materials as well as the substrate and encapsulation may not only alter the spectrum at a given time but also cause change in the spectra as a function of time. The purpose of this study is to utilize a semiempirical formalism, which quantitatively incorporates this time dependence, to calculate and evaluate the shielding requirement impacts introduced by this time dependence for a Pd103 source. Methods: The formalism was used to calculate the N th VLthicknesses in lead for a Pd103 model 200 seed. Prior to 2005, the Pd103 in this source was purified to a level better than 0.006% of the total Pd103 activity, the key trace impurity consisting of Zn65 . Because Zn65 emits higher energy photons and has a much longer half‐life of 244 days compared to Pd103 , its presence in Pd103 seeds led to a time dependence of the photon spectrum and other related physical quantities. This study focuses on the time dependence of the NthVL and the analysis of the corresponding shielding requirements. Results: The results indicate that the first HVL and the first TVL in lead steadily increased with time for about 200 days and then reached a plateau. The increases at plateau were more than 1000 times compared to the corresponding values on the zeroth day. The second and third TVLs in lead reached their plateaus in about 100 and 60 days , respectively, and the increases were about 19 and 2.33 times the corresponding values on the zeroth day, respectively. All the TVLs demonstrated a similar time dependence pattern, with substantial increases and eventual approach to a plateau. Conclusions: The authors conclude that the time dependence of the emitted photon spectra from brachytherapy sources can introduce substantial variations in the values of the N th VLwith time if certain impurities are present. The contribution of Zn65 to the dose rate constant was less than 0.03% in the earlier Pd103 seeds, and because of the use of new processing technologies since 2005, this impurity has been essentially eliminated, as demonstrated in the measured spectra of current Pd103 model 200 seeds. This study illustrates the importance of performing photon spectroscopy of the manufactured radioactive sources as a quality assurance test for an assessment over time of both the radiation protection and the dosimetric properties.