Source Description:
Dimensions for the Braquibac source 1 are taken from the study by Pirchio et al . The source element for the Braquibac seed is a cylindrical silver rod with an outer diameter of 0.500 mm and a length of 3.07 mm. The rod is coated with a silver halide layer that is 1.00 μm thick. The titanium encapsulation has a 0.779 mm outer diameter and is 0.0900 mm thick. End welds are 0.390 mm thick and approximated here as being hemispherical in shape. The overall source length is 4.68 mm and the active length of the source is 3.07 mm. The cylindrical source element is free to move 0.415 mm along the seed axis and 0.05 mm radially from the center of the seed, however, we assume the it is always centred. The mean photon energy calculated on the surface of the source is 27.42 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 at 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. MC uncertainties are only statistical uncertainties (k=1).
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 7 .The mean residual deviations from the actual data for the best fit were < 0.18%.
| 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) | ||
| 0.05 | 10.00 | a0 / cm2 | (7.10+/-0.19)E-04 |
| a1 / cm | (-2.05+/-0.04)E-02 | ||
| a2 | (1.1801+/-0.0024)E+00 | ||
| a3 / cm-1 | (3.21+/-0.17)E-01 | ||
| a4 / cm-2 | (-1.58+/-0.28)E-02 | ||
| a5 / cm-3 | (9.54+/-0.34)E-04 | ||
| a6 / cm-1 | (3.83+/-0.12)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 0o ≤ ϑ ≤ 90o .
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:
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. R. Loevinger, Wide-angle free-air chamber for calibration of low--energy brachytherapy sources, Med. Phys., 20 , 907, 1993.
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 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. R. E. P. Taylor, D. W. O. Rogers, More accurate fitting of 125I and 103Pd radial dose functions, Med. Phys., 35 , 4242-4250, 2008