Source Description:
The dimensions used by Kirov and Williamson in their study of the STM seed 1,2,3,4 are used here. The STM1251 source consists of a cylindrical gold rod (0.180 mm diameter) inside of a 3.81 mm long aluminum wire with a diameter of 0.510 mm. The aluminum wire is coated with 1.90 μm of nickel, 2.50 μm of copper and 17.0nm of radioactive iodine. The source is encapsulated in a titanium tube with 0.080 mm thick walls, 0.810 mm outer diameter and 0.130 mm thick solid cylindrical end welds. The overall source length is 4.50 mm and the active length is 3.80 mm. The cylindrical source element is free to move 0.240 mm along the seed axis and 0.066 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. 4 , 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 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.
| Author | Method | Λ (cGy h-1 U-1) | Abs. Uncertainty |
| R. E. P. Taylor, D. W. O. Rogers 7 | WAFAC | 1.012 | 0.002 |
| R. E. P. Taylor, D. W. O. Rogers 7 | point | 1.045 | 0.003 |
| A. S. Kirov, J. F. Williamson 1 | extrap (PTRAN) | 1.041 | |
| A. S. Kirov, J. F. Williamson 1 | WAFAC (PTRAN) | 0.980 | 0.024 |
| Z. Li, J. F. Williamson 3 | TLD | 1.039 | 0.073 |
| S. Chiu-Tsao et al 2 | TLD | 1.07 | 0.06 |
| M. J. Rivard et al 8 | Consensus Value | 1.018 |
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 .
| 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 | 9.0798E-04 | -2.4072E-02 | 1.0947E+00 | 5.1514E-01 | 1.1439E-02 | 3.4677E-03 | 4.7100E-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. A. S. Kirov, J. F. Williamson, Monte Carlo-aided dosimetry of the Source Tech Medical Model STM1251 I-125 interstitial brachytherapy source, Med. Phys., 28 , 764 -- 772, 2001
2. S. Chiu-Tsao et al , , Med. Phys., 30 , 1735 -- 1732} , 2003
3. Z. Li, J. F. Williamson, Measured transverse-axis dosimetric parameters of the model STM1251 125 I interstitial source, J. Appl. Clin. Med. Phys, 3 , 212 -- 217, 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 125 I and 103 Pd Brachytherapy Seeds, J. Res. Natl. Inst. Stand. Technol., 108 , 337 -- 358, 2003
7. R. E. P. Taylor, D. W. O. Rogers, An EGSnrc Monte Carlo-calculated database of TG-43 parameters, Med. Phys., 35 , 4228--4241, 2008
8. M. J. Rivard et al , Supplement to the 2004 update of the AAPM Task Group No. 43 Report, Med. Phys., 34 , 2187 -- 2205, 2007
9. 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.