### Source Description:

The PharmaSeed BT-125-1 source^{ 1,2,3 }dimensions are taken from the study by Popescu

*et al*. The source element for the PharmaSeed BT-125-1 is a cylindrical palladium rod with an outer diameter of 0.500 mm and a length of 3.25 mm. The rod is coated with a 0.500 μm thick layer of

^{ 125 }I. The titanium encapsulation has a 0.800 mm outer diameter and is 0.060 mm thick. End welds are 0.500 mm thick and were modelled in this study as 0.400 mm hemispheres on top of solid cylinders that are 0.100 mm thick and have a 0.400 mm radius. The BT-125-2 source is identical to the BT-125-1 source with the exception that the cylindrical rod is made of Ag rather than Pd. The overall source length is 4.50 mm and the active length of both sources is 3.25 mm. The cylindrical source element can move 0.124 mm along the seed axis and 0.090 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. 4 , 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 ^{ 5,6 } 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 ^{ 7 } |
WAFAC | 0.901 | 0.002 |

R. E. P. Taylor, D. W. O. Rogers ^{ 7 } |
point | 0.938 | 0.003 |

C. C. Popescu et al ^{ 1 } |
point (PTRAN) | 0.95 | 0.03 |

C. C. Popescu et al ^{ 1 } |
TLD | 0.90 | 0.06 |

T. D. Solberg et al ^{ 2 } |
point (MCNP) | 0.955 | 0.005 |

T. D. Solberg et al ^{ 2 } |
TLD | 0.95 | 0.07 |

J. J. DeMarco et al ^{ 3 } |
point (MCNP) | 0.955 | 0.005 |

### 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 } .

*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 | 6.8761E-04 | -1.6662E-02 | 1.1849E+00 | 4.6299E-01 | 1.3971E-02 | 4.6737E-03 | 5.0110E-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. C. C. Popescu * et al * , Dosimetric characteristics of the PharmaSeed model BT-125-1 source, Med. Phys., ** 27 ** , 2174 -- 2181, 2000

2. T. D. Solberg * et al * , Dosimetric parameters of three new solid core I-125 brachytherapy sources, J. Appl. Clin. Med. Phys, ** 3 ** , 119--134, 2002

3. J. J. DeMarco * et al * , Dosimetric characteristics for three low-energy brachytherapy sources using the Monte Carlo N-Particle code, Med. Phys., ** 29 ** , 662 -- 668, 2002

4. 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

5. R. Loevinger, Wide-angle free-air chamber for calibration of low--energy brachytherapy sources, Med. Phys., ** 20 ** , 907, 1993

6. 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

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

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.