Source Description:
The PharamSeed BT-125-1 source 1,2 dimensions are taken from the study by Popescu et al . The source element for the PharmaSeed is a cylindrical palladium rod with an outer diameter of 0.500 mm and a length of 3.25 mm. The rod is coated with a 0.500 μm thick layer of 125 I. The titanium encapsulation has a 0.800 mm outer diameter and is 0.0600 mm thick. End welds are 0.500 mm thick and were modelled in this study as 0.400 mm hemispheres on top of 0.100 mm cylinders. The BT-125-2 source is identical to the BT-125-1 source with the exception that the cylindrical rod is made of silver rather than palladium. 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 center of the seed.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 cm 3 water phantom, by the air kerma strength per history (scored in vacuo ). As described in ref. 3 , dose rate constants are provided for air kerma strenth calculated using voxels of 2.7x2.7x0.05 cm 3 (WAFAC) and 0.1x0.1x0.05 cm 3 (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 4,5 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.
| Author | Method | Λ (cGy h-1 U-1) | Abs. Uncertainty |
| R. E. P. Taylor, D. W. O. Rogers 6 | WAFAC | 0.916 | 0.002 |
| R. E. P. Taylor, D. W. O. Rogers 6 | point | 0.958 | 0.003 |
| T. D. Solberg et al 1 | point (MCNP) | 0.962 | 0.005 |
| J. J. DeMarco et al 2 | point (MCNP) | 0.967 | 0.005 |
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 7 .
| Fitting coefficients for g L (r) = (a 0 r -2 + a 1 r -1 +a 2 + a 3 r +a 4 r 2 + a 5 r 3 ) e -a 6 r | ||||||||
| Fit range | Coefficients | |||||||
| r min (cm) | r max (cm) | a 0 / cm 2 | a 1 / cm | a 2 | a 3 / cm -1 | a 4 / cm -2 | a 5 / cm -3 | a 6 / cm -1 |
| 0.05 | 10.00 | 4.8089E-04 | -1.1519E-02 | 1.1389E+00 | 5.0233E-01 | 2.7405E-03 | 4.4628E-03 | 4.9310E-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(00.50,θ)  |
F(01.00,θ)  |
F(05.00,θ)  |
References:
1. T. D. Solberg et al , Dosimetric parameters of three new solid core I-125 brachytherapy sources, J. Appl. Clin. Med. Phys, 3 , 119--134, 2002
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. 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
4. R. Loevinger, Wide-angle free-air chamber for calibration of low--energy brachytherapy sources, Med. Phys., 20 , 907, 1993
5. 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
6. R. E. P. Taylor, D. W. O. Rogers, An EGSnrc Monte Carlo-calculated database of TG-43 parameters, Med. Phys., 35 , 4228--4241, 2008
7. R. E. P. Taylor, D. W. O. Rogers, More accurate fitting of 125 I and 103 Pd radial dose functions, Med. Phys., 35 , 4242--4250, 2008