Source Description:
Dimensions and internal source configurations for the Med3631 source 1 are taken from the study by Rivard 1. The Med3631 125I seed consists of polystyrene spheres (0.560 mm diameter), coated with a negligible thickness of radioactive material, with two radioactive spheres located on each side of two 0.560 mm diameter 80% Au / 20% Cu alloy spheres. The encapsulating titanium cylinder has an outside diameter of 0.810 mm and an inner diameter of 0.710 mm. The source has an average weld thickness of 0.10 mm. The end welds are modelled using a 0.405 mm radius Ti hemisphere overlapped with a 0.355 mm radius air sphere with its center shifted by 0.050 mm relative to the Ti sphere. Calculations are done with the internal spheres arranged in the "ideal" configuration (center of source spheres located at ± 1.807 mm and ± 1.084 mm, center of marker spheres located at ± 0.361 mm) given in the study of the Med3631 125I seed by Rivard 1. Rivard's study also contains a discussion on how the internal movement of the source spheres effects dosimetry parameters. However, we assume the six spheres are always centred. The overall length is 4.70 mm and the active length is assumed to be 4.20 mm. The mean photon energy calculated on the surface of the source is 28.35 keV with statistical uncertainties < 0.010%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 cm3 water phantom, by the air-kerma strength per history (scored in vacuo). As described in ref. 2 , dose-rate constants are provided for air-kerma strength calculated using voxels of 2.66x2.66x0.05 cm3 (WAFAC) and 0.1x0.1x0.05 cm3 (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 3,4 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 and includes a small 1/r2 correction (0.5%) 2. egs_brachy and BrachyDose MC uncertainties are statistical uncertainties only (k=1).
| Author | Method | Λ (cGy h-1 U-1) | Abs. Uncertainty |
| Safigholi et al 5 | WAFAC | 0.9957 | 0.0001 |
| Safigholi et al 5 | Point | 0.9972 | 0.0017 |
| Taylor, Rogers 6 | WAFAC | 0.978* | 0.003 |
| Taylor, Rogers 6 | Point | 0.977* | 0.003 |
| Rivard 1 | Point (MCNP) | 1.011 | 0.03 |
| Li et al 7 | TLD (1999 NIST calibration) | 1.067 | |
| Wallace, Fan 8 | TLD (1999 NIST calibration) | 1.056 | |
| Rodriguez, Rogers 9 | WAFAC (BrachyDose) | 0.995 | 0.002 |
| Rodriguez, Rogers 10 | TLD (Revised Wallace) | 1.032 | 0.055 |
| Rivard et al 11 | TG43U1 Consensus Value | 1.036 |
* Due to the loss of MC geomtry input it is impossible to identify the prevoius DRC discrepency
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 12. The mean residual deviations from the actual data for the best fit are < 0.08%.
| Fitting coefficients for g L (r) = (a0 r-2 + a1 r-1 + a2 + a3r + a4r2 + a5 r3) e-a6r | |||
| Fit range | Coefficients | ||
| r min (cm) | r max (cm) | ||
| 0.10 | 10.00 | a0 / cm2 | (3.7+/-0.4)E-04 |
| a1 / cm | (-5.89+/-0.05)E-02 | ||
| a2 | (1.1461+/-0.0023)E+00 | ||
| a3 / cm-1 | (3.67+/-0.11)E-01 | ||
| a4 / cm-2 | (-1.89+/-0.22)E-02 | ||
| a5 / cm-3 | (7.7+/-0.4)E-04 | ||
| a6 / cm-1 | (3.63+/-0.08)E-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 5o. The anisotropy factor, φ an (r), was calculated by integrating the solid angle weighted dose rate over 0 o ≤ ϑ ≤ 90 o .
F(0.25,θ)  |
F(0.50,θ)  |
F(1.00,θ)  |
F(5.00,θ)  |
Tabulated data:
Tabulated data are available in .xlsx format: Excel
References:
1. M J Rivard, Monte Carlo calculations of AAPM Task Group Report No. 43 dosimetry parameters for the MED3631-A/M 125I source, Med. Phys., 28 , 629-637, 2001
2. 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
3. R. Loevinger, Wide-angle free-air chamber for calibration of low-energy brachytherapy sources, Med. Phys., 20 , 907, 1993
4. S. M Seltzer et al , New National Air-Kerma-Strength Standards for 125I and 103Pd Brachytherapy Seeds, J. Res. Natl. Inst. Stand. Technol., 108 , 337 - 358, 2003
5. H. Safigholi, M. J. P. Chamberland, R. E. P. Taylor, C. H. Allen, M. P. Martinov, D. W. O. Rogers, and R. M. Thomson, Update of the Carleton Laboratory for Radiotherapy Physics (CLRP) TG-43 parameter database for brachytherapy, to be published (Current calculation).
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. Z. Li et al, Experimental measurements of dosimetric parameters on the transverse axis of a new 125I source, Med.Phys., 27,1275-1280,2000 8.R.E.Wallace, J.J.Fan, Report on the dosimetry of a new design 125Iodine brachytherapy source, Med. Phys.,26, 1925-1931, 1999
9.M. Rodriguez, D. W. O. Rogers, On determining dose rate constants spectroscopically, Med. Phys.40,011713-10,2013. 10. M. Rodriguez , D. W. O. Rogers, Effect of improved TLD dosimetry on the determination of dose rate constants for 125I and 103Pd brachytherapyseeds,Med.Phys.41, 114301-15, 2014.
11. 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
12. R. E. P. Taylor, D. W. O. Rogers, More accurate fitting of 125I and 103Pd radial dose functions, Med. Phys., 35 , 4242-4250, 2008