Theragenics Co., I-Seed I-125, AgX100


Source Description:

Source dimensions for the AgX100 seed1 are taken from the paper by Mourtada et al. The AgX100 source consists of a radio-opaque silver marker coated with a 2 μm thick layer of radioactive AgI (density of 5.675 g/cm3). The silver marker is a 3.50 mm long cylindrical silver rod (density of 10.5 g/cm3) with a 0.293 mm radius (0.295 mm with coating). The silver marker is encapsulated in a 3.7 mm long titanium tube (density of 4.54 g/cm3)  with 0.05 mm thick walls, a 0.4 mm outer radius and 0.4 mm thick hemi-spherical end welds. The overall source length is 4.50 mm and the active length is 3.50 mm. The cylindrical source element is free to move aproximately 0.2 mm along the seed axis and 0.06 mm radially from the center of the seed, however, we assume the silver rod is always centred. The mean photon energy calculated on the surface of the source is 27.29 keV with statistical uncertainties < 0.010%

Dose-Rate Constant - Λ :

Dose-rate constants, Λ , are calculated by dividing the dose to water per history in a (0.1 mm)voxel centered on the reference position,        (1 cm,Π/2), in the 30x30x30 cmwater phantom, by the air-kerma strength per history (scored in vacuo). As described in ref. , dose-rate constants are provided for air-kerma strength calculated using voxels of 2.66x2.66x0.05 cm3 (WAFAC) and 0.1x0.1x0.05 cm3 (point) located 10 cm from the source. The larger voxel size averages the air-kerma per history over a region covering roughly the same solid angle subtended by the primary collimator of the WAFAC3,4 at NIST used for calibrating low-energy brachytherapy sources and is likely the most clinically relevant value. The small voxel serves to estimate the air kerma per history at a point on the transverse axis and includes a small 1/r2 correction (0.5%) 2. egs_brachy and BrachyDose MC uncertainties are  statistical uncertainties only (k=1).

(Note*: The CLRPv2  DRC value differs from that of Rodriguez_Rogers 7, as there is some unexplained change in the geometry model but we do not have the MC input file from those BrachyDose simulations).  

Author Method Λ (cGy h-1 U-1) Abs. Uncertainty
Safigholi et al 5 WAFAC 0.9233 0.0007
Safigholi et al 5 point 0.9545 0.008
Mourtada et al 1 MCNPX WAFAC 0.918 0.0245
Mourtada et al 1 MCNPX Point  0.942 0.0245
Chen et al 6 TLD 0.995 0.066
Rodriguez, Rogers 7 TLD Revised (Chen) 0.923 0.049
Rodriguez, Rogers 7 WAFAC (BrachyDose) 0.900* 0.0015
Rivard et al 8 TG43U1S2 consensus value 0.952 0.043

Radial dose function - g(r):

The radial dose function, g(r), is calculated using both line and point source geometry functions and tabulated at 36 different radial distances ranging from 0.05 cm to 10 cm. Fit parameters for a modified polynomial expression are also provided 9. The mean residual deviations from the actual data for the best fit are < 0.19%.  

Click image for higher res version

radial dose function

Fitting coefficients for g L (r) = (a0 r-2 + a1 r-1 + a2 + a3r + a4r2 + a5 r3) e-a6r
Fit range Coefficients
min (cm) max (cm)
0.05 10.0 a0 / cm2 (8.82+/-0.20)E-04
    a1 / cm (-1.96+/-0.05)E-02
    a2 (1.1859+/-0.0025)E+00
    a3 / cm-1 (3.40+/-0.13)E-01
    a4 / cm-2 (-1.11+/-0.21)E-02
    a5 / cm-3 (1.01+/-0.08)E-03
    a6 / cm-1 (4.04+/-0.09)E-01

Anisotropy function - F(r,θ):

Anisotropy functions are calculated using the line source approximation and tabulated at radii of 0.1, 0.15, 0.25, 0.5, 0.75, 1, 2, 3, 4, 5, 7.5 and 10 cm and 32 unique polar angles with a minimum resolution of 5o. The anisotropy factor, φan(r), is calculated by integrating the solid angle weighted dose rate over 0 ≤ ϑ ≤ 90o.

Click images for higher res versions
Anisotropy function
Anisotropy function
Anisotropy function
Anisotropy function

Tabulated data:

Tabulated data are available in .xlsx format: Excel


1. Firas Mourtada et al , Monte Carlo calculations of AAPM Task Group Report No. 43 dosimetry parameters for the 125I I-Seed AgX100 source model, Brachytherapy, 11, 237 - 244, 2012 
2. R. E. P. Taylor et al , Benchmarking BrachyDose: voxel-based EGSnrc Monte Carlo calculations of TG--43 dosimetry parameters, Med. Phys., 34 , 445 - 457, 2007 
3. R. Loevinger, Wide-angle free-air chamber for calibration of low--energy brachytherapy sources, Med. Phys., 20 , 907, 1993 
4. S. M Seltzer et al , New National Air-Kerma-Strength Standards for 125 I and 103 Pd Brachytherapy Seeds, J. Res. Natl. Inst. Stand. Technol., 108 , 337 - 358, 2003
5. H. Safigholi, M. J. P. Chamberland, R. E. P. Taylor, C. H. Allen, M. P. Martinov, D. W. O. Rogers, and R. M. Thomson, Update of the CLRP TG-43 parameter database for the brachytherapy, to be published (Current calculation)    
6. Chen et al, Experimental characterization of the dosimetric properties of a newly designed I-Seed model AgX100 125I interstitial brachytherapy source, Brachytherapy, 11, 476-482, 2012
7. M. Rodriguez , D. W. O. Rogers, Effect of improved TLD dosimetry on the determination of dose rate constants for 125I and 103Pd brachytherapyseeds, Med.Phys. 41, 114301-15, 2014      
8. M. J. Rivard et al ,  Supplement 2 for the 2004 update of the AAPM Task Group No. 43 Report: Joint recommendations by the AAPM and GEC-ESTRO, Med. Phys., 44 , e297-e338, 2017                                                                                                                                  
9. R. E. P. Taylor, D. W. O. Rogers, More accurate fitting of 125I and 103Pd radial dose functions, Med. Phys., 35 , 4242-4250, 2008

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Carleton Laboratory for Radiotherapy Physics 

CLRP TG-43 Parameter Database V2
 May 5, 2020

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