Source Description:
Source dimensions for the 6711 seed 1,2 are taken from the paper by Dolan et al which presents a more realistic geometry than has been used in previous studies. The 6711 source consists of radioactive AgI and AgBr (2.5:1 molecular ratio of AgI:AgBr and a density of 6.2 g/cm 3 ) coated on a 2.80 mm long cylindrical silver rod with a 0.250 mm radius. The ends of the silver rod are conical sections bevelled at 45.0 o and the end faces of the rod have a radius of 0.175 mm. The radioactive coating is assumed to have a thickness of 1.75 μm over the entire surface of the rod. The silver rod is encapsulated in a 3.75 mm long titanium tube with 0.0700 mm thick walls, a 0.800 mm outer diameter and 0.375 mm thick hemi-spherical end welds. The overall source length is 4.55 mm and the active length is 2.80 mm. The cylindrical source element is free to move 0.475 mm along the seed axis and 0.080 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 | 0.924 | 0.002 |
| R. E. P. Taylor, D. W. O. Rogers 6 | point | 0.942 | 0.003 |
| J. Dolan et al 2 | extrap/WAFAC (PTRAN) | 0.942 | 0.017 |
| J. Dolan et al 2 | TLD | 0.971 | 0.059 |
| M. J. Rivard et al 7 | TG-43 Consensus Value | 0.965 | - |
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) = (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 | 5.9638E-04 | -1.3360E-02 | 1.1634E+00 | 4.0710E-01 | -5.5487E-03 | 1.7421E-03 | 4.4080E-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(00.25,θ)  |
F(00.50,θ)  |
F(01.00,θ)  |
F(05.00,θ)  |
References:
1. J. F. Williamson, Comparison of measured and calculated dose rates in water near I-125 and Ir-192 seeds, Med. Phys., 18 , 776 -- 786, 1991
2. J. Dolan et al , Monte Carlo and experimental dosimetry of an 125-I brachytherapy seed, Med. Phys., 33 , 4675 -- 4684, 2006
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. M. J. Rivard et al , Update of AAPM Task Group No. 43 Report: A revised AAPM protocol for brachytherapy dose calculations, Med. Phys., 31 , 633 -- 674, 2004
8. 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
Carleton Laboratory for Radiotherapy Physics
December 17 2007.