Answer to Question #11729 Submitted to "Ask the Experts"

Category: Instrumentation and Measurements — Surveys and Measurements (SM)

The following question was answered by an expert in the appropriate field:


I would like to research and measure the anisotropy and radial dose function of an IMC OncoSeed Model 6711 iodine-125 (125I) brachytherapy source. However, I am struggling to find the energy dependence of a thermoluminescent dosimeter (TLD)-100 when I calibrate it with cobalt-60 (60Co). Can you please assist me?


There have been numerous publications related to evaluation of TLD energy efficiencies associated with exposure to brachytherapy sources. A 2014 paper by J.L. Reed, et al., discusses the energy dependence of TLD-100 for 125I exposure vs. 60Co exposure. Results showed relative efficiencies, compared to 60Co, for two 125I seeds of 0.883 and 0.870 (the latter was for the same OncoSeed Model 6711 that is of interest to you). The relative efficiency for the 125I compared to 60Co was evaluated by determining the (tissue) dose rate constants for the two radionuclides; the dose rate constant is defined as the tissue dose rate per unit air kerma rate at the location of interest. Efficiency results can vary depending on changes in certain physical characteristics such as the thickness and geometry of the TLD material and the characteristics of the radiation field; e.g., the degree of scatter contributing to the response of the dosimeter.

A number of authors have carried out the kind of study to which you refer, in which the intent is to evaluate anisotropy and radial dose distribution. See, for example, the 2002 paper by Timothy D. Solberg, et al., which includes a number of possibly useful references. You can find other examples by doing an internet search. You should also review the American Association of Physicists in Medicine (AAPM) 2004 document which is the basis for many of the types of measurements of concern.

George Chabot, CHP, PhD


  • Reed JL, Rasmussen BE, Davis SD, Micka JA, Culberson WS, DeWerd LA. Determination of the intrinsic energy dependence of LiF:Mg,Ti thermoluminescent dosimeters for 125I and 103Pd brachytherapy sources relative to 60Co. Med Phys 41(12); December 2014. Available at; accessed 10 September 2016.
  • Rivard MJ, Coursey BM, DeWerd LA, Hanson WF, Huq MS, Ibbott GS, Mitch MG, Nath R, Williamson JF. Update of AAPM Task Group No. 43 Report: A revised AAPM protocol for brachytherapy dose calculations. Med Phys 31(3):633–674; March 2004. Available at; accessed 10 September 2016.
  • Solberg TD, DeMarco JJ, Hugo G, Wallace RE. Dosimetric parameters of three new solid core I-125 brachytherapy sources. J Appl Clin Med Phys 3(2):119–134; 2002. Available at; accessed 10 September 2016.
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