Source Description:
Dimensions and internal source positions for the ProstaSeed source 1,2 are taken from the nominal configuration in the study by Li. The ProstaSeed contains five silver spheres (0.500 mm diameter) which are coated in 125 I. As in the study by Li the 125 I is assumed to be of negligible thickness. The five spheres are free to move within the Ti encapsulation which consists of walls with a thickness of 0.050 mm and 0.300 mm thick end welds. The end welds are modelled using a 0.400 mm radius Ti hemisphere overlapped with a 0.350 mm radius air sphere with its center shifted by 0.250 mm relative to the Ti sphere. The overall length is 4.50 mm and the active length is 3.00 mm. The maximum possible displacement of a source sphere is 0.850 mm along the seed axis and 0.100 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 | 0.930 | 0.002 |
| R. E. P. Taylor, D. W. O. Rogers 6 | point | 0.932 | 0.003 |
| Z. Li 1 | point (PTRAN) | 0.925 | 0.04 |
| R. E. Wallace 2 | TLD | 0.980 | 0.03 |
| M. J. Rivard et al 7 | Consensus Value | 0.953 |
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 | 6.1867E-04 | -1.5851E-02 | 1.1656E+00 | 4.6505E-01 | -4.1911E-03 | 3.6906E-03 | 4.7930E-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 .
References:
1. Z. Li, Monte Carlo calculations of dosimetry parameters of the Urocor Prostaseed 125 I source, Med. Phys., 29 , 1029--1034, 2002
2. R. E. Wallace, Empirical dosimetric characterization of model I125-SL 125 Iodine brachytherapy source in phantom, Med. Phys., 27 , 2796--2802, 2000
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.