IBt, InterSource, 1251L

Source Description: 

Dimensions for the InterSource seed^1,2 are taken from the study by Meigooni et al. The InterSource consists of two concentric hollow titanium cylindrical tubes which are laser welded together at the ends. Each tube is 0.040 mm thick and the outer diameter of the inner and outer tubes are 0.500 mm and 0.810 mm, respectively. There is a thin band (0.045 mm thick and 1.27 mm long) of 90% Pt / 10% Ir alloy deposited on top of the inner cylinder at its center. The radioactive ¹²⁵I is distributed uniformly throughout three cylindrical bands of an organic material (85.7% carbon 14.3% hydrogen with a density of 1.00g/cm³, the ¹²⁵I content is assumed to be negligble). The outer two bands of radioactive material are deposited on the inner cylinder and are 0.800 mm long, 0.015 mm thick and have their centers along the seed axis offset 1.45 mm from the middle of the seed. The center band is 0.500 mm long, 9.00x10^-3 mm thick and is deposited on top of the Pt/Ir alloy. The overall length is 4.5 mm and the active length of the seed is 4.35 mm.

Dose rate constants, Λ , are calculated by dividing the dose to water per history in a (0.1 mm)³voxel 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. 3, dose rate constants are provided for air kerma strenth calculated using voxels of 2.7x2.7x0.05 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^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.

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 provided⁸.

Click image for higher res version

Tabulated g(r) data: html Excel 

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 ° . The anisotropy factor, φan(r), was calculated by integrating the solid angle weighted dose rate over 0° ≤ ϑ ≤ 90 ° .

Click images for higher res versions

F(00.50,θ)	F(01.00,θ)
F(05.00,θ)

Tabulated F(r,θ) data: html Excel

References: 

1. A. S. Meigooni et al, Determination of the dosimetric characteristics of InterSource ¹²⁵I brachytherapy source, Appl. Radiat. Isotopes, 56, 589--599, 2002
2. B. Reniers et al, Dosimetric study of the new InterSource¹²⁵ iodine seed, Med. Phys., 28, 2285--2299, 2001
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 ¹²⁵I and ¹⁰³Pd 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 ¹²⁵I and ¹⁰³Pd radial dose functions, Med. Phys., 35, 4242--4250, 2008

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Carleton Laboratory for Radiotherapy Physics 
December 17 2007.