Source Description:
Dimensions for the 2335 source 1,2 are taken from studies by Meigooni et al and Peterson and Thomadsen. The Best 2335 source consists of a cylindrical tungsten marker which is 1.20 mm long with a diameter assumed to be approximately 0.500 mm. On either side of the marker are three 0.500 mm diameter spheres located 0.900 mm, 1.50 mm and 2.10 mm from the middle of the seed. The spheres are made of a polymer (composition by weight of 89.73% C, 7.85% H, 1.68% O and 0.740% N, and density of the polymer is assumed to be 1.00g/cm 3 here). The polymer spheres are coated in 103 Pd which is assumed to have negligible thickness. The spheres and titanium marker are encapsulated with the same Ti casing described above for the Best 2301 125 I source. The overall source length is 5.00 mm and the active length is 4.76 mm. The maximum possible displacement of a source sphere is 0.570 mm along the seed axis and up to 0.070 mm in the radial direction.
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 | 0.650 | 0.002 |
| R. E. P. Taylor, D. W. O. Rogers 6 | point | 0.652 | 0.002 |
| A. S. Meigooni et al 1 | point (PTRAN) | 0.67 | 0.02 |
| A. S. Meigooni et al 1 | TLD | 0.69 | 0.06 |
| S. W. Peterson, B. Thomadsen 2 | TLD | 0.71 | 0.07 |
| M. J. Rivard et al 7 | Consensus Value | 0.685 |
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 |
| 0.10 | 10.00 | 1.5101E-03 | -1.3800E-01 | 1.9004E+00 | 6.1474E-02 | -2.7870E-02 | 2.1866E-03 | 5.8760E-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° . The anisotropy factor, φan (r), was calculated by integrating the solid angle weighted dose rate over 0° ≤ ϑ ≤ 90°.
References:
1. A. S. Meigooni et al , Dosimetric Characteristics of the Best double-wall 103 Pd brachytherapy source, Med. Phys., 28 , 2567--2575, 2001
2. S. W. Peterson, B. Thomadsen, Measurements of the dosimetric constants for a new 103 Pd brachytherapy source, Brachytherapy, 1 , 110--119, 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. M. J. Rivard et al , Supplement to the 2004 update of the AAPM Task Group No. 43 Report, Med. Phys., 34 , 2187 -- 2205, 2007
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.