### Source Description:

Dimensions for the IAI-125A source^{ 1,2 }are taken from the study by Meigooni

*et al*. The IsoAid Advantage seed contains a 3 mm long silver rod with a diameter of 0.500 mm. The silver rod is coated with a 1 μm thick layer of AgI containing

^{ 125 }I (coating assumed to be the same thickness on the cylindrical and end face surfaces). The source is encapsulated in a titanium casing 0.050 mm thick with an outside diameter of 0.800 mm. The end welds have a maximum thickness of 0.100 mm and are modelled using a 0.400 mm radius Ti hemisphere overlapped with a 0.350 mm radius air sphere with its center shifted by 0.0500 mm relative to the Ti sphere. The overall source length is 4.50 mm and the active length is 3.00 mm. The cylindrical source element is free to move approximately 0.350 mm along the seed axis and 0.100 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. 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 | 0.925 | 0.002 |

R. E. P. Taylor, D. W. O. Rogers ^{ 6 } |
point | 0.959 | 0.002 |

A. S. Meigooni et al ^{ 1 } |
point (PTRAN) | 0.98 | 0.03 |

A. S. Meigooni et al ^{ 1 } |
TLD | 0.99 | 0.08 |

T. D. Solberg et al ^{ 2 } |
point (MCNP) | 0.962 | 0.005 |

T. D. Solberg et al ^{ 2 } |
TLD | 0.96 | 0.05 |

M. J. Rivard et al ^{ 7 } |
Consensus Value | 0.981 |

### 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 | 6.7921E-04 | -1.5741E-02 | 1.1828E+00 | 2.5623E-01 | -2.5011E-02 | 1.0344E-03 | 3.3650E-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. A. S. Meigooni * et al * , Experimental and theoretical determination of dosimetric characteristics of IsoAid Advantage ^{ 125 } I brachytherapy source, Med. Phys., ** 29 ** , 2152 -- 2158, 2002

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. 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.