Source Description:
The IsoSeed I25.S17plus 1,2 seed dimensions are taken from the study by Pantelis et al 1 and Moutsatsos et al 2.The source element for the I25.S17plus is a cylindrical silver rod (density of 10.5 g/cm3) with an outer diameter of 0.51 mm and a length of 3.40 mm. The rod is coated with a silver iodide layer (AgI, density of 5.675 g/cm3) that is 1.0 μm thick. The titanium encapsulation (grade 2, elemental composition by mass: 0.015% H, 0.1% C, 0.03% N, 0.25% O, 0.2% Fe, 99.405% Ti, density of 4.512 g/cm3) has a 0.80 mm outer diameter and is 0.055 mm thick. End welds are 0.40 mm thick and hemispherical in shape. The overall length is 4.50 mm and the active length of the source is 3.40 mm, as recomended by TG43U1S2 3 (Note: in CLRPv1 version of this database, the active length was assumed to be 3.402 mm). The cylindrical source element is free to move approximately 0.149 mm along the seed axis and 0.0895 mm radially from the center of the seed although this possible movement is not modelled. The mean photon energy calculated on the surface of the source is 27.30 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)3 voxel centered on the reference position, (1 cm,Π/2), in a 30x30x30 cm3 water phantom, by the air-kerma strength per history (scored in vacuo). As described in ref. 4 , 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 WAFAC 5,6 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%) 4. egs_brachy and BrachyDose MC uncertainties are statistical uncertainties only(k=1).
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 8.
| Fitting coefficients for g L (r) = (a0 r-2 + a1 r-1 + a2 + a3r + a4r2 + a5 r3) e-a6r | |||
| Fit range | Coefficients | ||
| r min (cm) | r max (cm) | ||
| 0.05 | 10.00 | a0 / cm2 | (6.94+/-0.17)E-04 |
| a1 / cm | (-1.72+/-0.04)E-02 | ||
| a2 | (1.1703+/-0.0019)E+00 | ||
| a3 / cm-1 | (3.93+/-0.11)E-01 | ||
| a4 / cm-2 | (-0.66+/-0.18)E-02 | ||
| a5 / cm-3 | (1.59+/-0.16)E-03 | ||
| a6 / cm-1 | (4.34+/-0.07)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 5 o . The anisotropy factor, φ an (r), was calculated by integrating the solid angle weighted dose rate over 0 o ≤ ϑ ≤ 90 o .
F(0.50,θ)  |
F(1.00,θ)  |
F(5.00,θ)  |
Tabulated data:
Tabulated data are available in .xlsx format: Excel
References:
1. E. Pantelis, P. Papagiannis, G. Anagnostopoulos, and D. Baltas, New 125 I brachytherapy source IsoSeed I25.S17plus: Monte Carlo dosimetry simulation and comparison to sources of similar design, J. Contemp. Brachytherapy, 5(4), 240 – 249, 2013
2. A. Moutsatsos, et al, Experimental determination of the Task Group-43 dosimetric parameters of the new I25.S17plus 125I brachytherapy source, beachytherapy, 13, 618-626, 2014
3. 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
4. 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
5. R. Loevinger, Wide-angle free-air chamber for calibration of low-energy brachytherapy sources, Med. Phys., 20, 907, 1993 6.S. M Seltzer et al , New National Air-Kerma-Strength Standards for 125I and 103Pd Brachytherapy Seeds, J. Res. Natl. Inst. Stand. Technol., 108, 337 – 358, 2003
7.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 brachytherapy, to be published (Current calc)
8. R. E. P. Taylor, D. W. O. Rogers, More accurate fitting of 125I and 103Pd radial dose functions, Med. Phys., 35 , 4242-4250, 2008