### 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 0.064 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. 2 , 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 ^{ 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.

Author |
Method |
Λ (cGy h-1 U-1) |
Abs. Uncertainty |

R. E. P. Taylor, D. W. O. Rogers ^{ 5 } |
WAFAC | 0.917 | 0.003 |

R. E. P. Taylor, D. W. O. Rogers ^{ 5 } |
point | 0.949 | 0.003 |

R. Pirchio et al ^{ 1 } |
extrap (MCNP) | 0.937 | 0.004 |

### 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 ^{ 6 } .

*Click image for higher res version*

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 | 7.0485E-04 | -2.1444E-02 | 1.1969E+00 | 2.5892E-01 | -1.9130E-02 | 7.4604E-04 | 3.4830E-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. R. Pirchio * et al * , On the physical, spectra, and dosimetric characteristics of a new ^{ 125 } I brachytherapy source, Med. Phys., ** 34 ** , 2801--2806, 2007

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 ^{ 125 } I and ^{ 103 } Pd Brachytherapy Seeds, J. Res. Natl. Inst. Stand. Technol., ** 108 ** , 337 -- 358, 2003

5. R. E. P. Taylor, D. W. O. Rogers, An EGSnrc Monte Carlo-calculated database of TG-43 parameters, Med. Phys., ** 35 ** , 4228--4241, 2008

6. 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.