Source Description:
The PharmaSeed BT-125-1 source 1,2,3 dimensions are taken from studies by Popescu et al 1 , DeMarco et al 2 and Solberg et al 3. The source element for the PharmaSeed BT-125-1 is a cylindrical palladium rod with an outer diameter of 0.50 mm and a length of 3.25 mm. The rod is coated with a 0.50 μm thick layer of 125I. The titanium encapsulation has a 0.80 mm outer diameter and is 0.060 mm thick. End welds are 0.50 mm thick and are modelled as a cylinder and have a 0.40 mm radius (Note:The CLRPv1 database mistakenly modeled the endcaps as hemisphers). The BT-125-2 source model is identical to the BT-125-1 source with the exception that the cylindrical rod is made of Ag rather than Pd. The overall source length is 4.50 mm and the active length of both sources is 3.25 mm. The cylindrical source element can move 0.124 mm along the seed axis and 0.090 mm radially from the seed center, however, we assume the source element is always centred. The mean photon energy calculated on the surface of the source is 27.28 keV with statistical uncertainties < 0.014%.
(Note: The 2D anisotropy function , F(r ≤ 5 cm, θ ≤ 15 °), values differ by up to 35% (on average 10%) from the CLRPv1 database due to changes in the geometry modelled (i.e., end caps were modelled as hemispheres); in this update they are modelled as full cylinders according to the descriptions in Popescu et al 1 DeMarco et al 2 and Solberg et al 3.
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 the 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).
| Author | Method | Λ (cGy h-1 U-1) | Abs. Uncertainty |
| Safigholi et al 7 | WAFAC | 0.9041 | 0.00025 |
| Safigholi et al 7 | Point | 0.9413 | 0.002 |
| Taylor, Rogers 8 | WAFAC | 0.901 | 0.002 |
| Taylor, Rogers 8 | Point | 0.938 | 0.003 |
| Popescu et al 1 | Point (PTRAN) | 0.95 | 0.030 |
| Popescu et al 1 | TLD | 0.90 | 0.060 |
| Solberg et al 3 | Point (MCNP) | 0.955 | 0.005 |
| Solberg et al 3 | TLD | 0.95 | 0.070 |
| DeMarco et al 2 | Point (MCNP) | 0.955 | 0.005 |
| Rodriguez, Rogers 9 | WAFAC (BrachyDose) | 0.906 | 0.015 |
| Rodriguez, Rogers 9 | TLD (Revised Popescu) | 0.880 | 0.063 |
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 10. The mean residual deviations from the actual data for the best fit are < 0.12%.
| 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 | (6.88+/-0.24)E-04 |
| a1 / cm | (-1.62+/-0.05)E-02 | ||
| a2 | (1.1795+/-0.0020)E+00 | ||
| a3 / cm-1 | (4.15+/-0.04)E-01 | ||
| a4 / cm-2 | (1.22+/-0.21)E-03 | ||
| a5 / cm-3 | (2.45+/-0.09)E-03 | ||
| a6 / cm-1 | (4.601+/-0.018)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), was calculated by integrating the solid angle weighted dose rate over 0o ≤ ϑ ≤ 90o .
F(0.50,θ)  |
F(1.00,θ)  |
F(5.00,θ)  |
Tabulated data:
Tabulated data are available in .xlsx format: Excel
References:
1. C. C. Popescu et al , Dosimetric characteristics of the PharmaSeed model BT-125-1 source, Med. Phys., 27 , 2174 - 2181, 2000
2. J. J. DeMarco et al , Dosimetric characteristics for three low-energy brachytherapy sources using the Monte Carlo N-Particle code, Med. Phys., 29 , 662 - 668, 2002
3. T. D. Solberg et al , Dosimetric parameters of three new solid core 125I brachytherapy sources, J. Appl. Clin. Med. Phys, 3 , 119 -134, 2002
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 the brachytherapy, to be published (Current calculation)
8. R. E. P. Taylor, D. W. O. Rogers, An EGSnrc Monte Carlo-calculated database of TG-43 parameters, Med. Phys, 35, 4228-4241, 2008 9. 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.
10. R. E. P. Taylor, D. W. O. Rogers, More accurate fitting of 125I and 103Pd radial dose functions, Med. Phys., 35 , 4242-4250, 2008