Source Description:
Dimensions for the Isotope Products Laboratories (IPL) 137Cs source model 67-6520 are taken from the study by Meigooni et al 1. The radioactive part of the IPL source is a cylindrical cesium oxide ceramic (Cs2O) rod (density of 1.47 g/cm3) with an outer diameter of 1.52 mm and a length of 14.8 mm. The radioisotope is uniformly distributed in the core of the IPL source. The source encapsulation has two layers of 304 stainless steel (density of 1.47 g/cm3) with a total thickness of 0.584 mm which includes an inner and outer thickness of 0.254 mm and 0.33 mm, respectively, separated by an air gap (0.09 mm). A colour-coded aluminum ring with inner (outer) diameters of 1.52 mm (3.05 mm) and thickness of 0.8 mm identifies the source activity and source tip. The active core is placed unsymmetrically within the capsule with tip and end located at 3 and 2 mm, respectively, from the capsule. A small thickness of 0.09 mm air is occupied around the active portion of the source. The overall source length (outer diameter) is 20 mm (3.05 mm) and the active length of both sources is 14.8 mm. The mean photon energy calculated on the surface of the source is 646.30 keV with statistical uncertainties < 0.001% .
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 80x80x80 cm3 water phantom, by the air-kerma strength per history factor (scored in vacuo). Air kerma per history is always calculated using a tracklength estimator in a 10x10x0.05 cm3 air voxel located in vacuo on the transverse axis 100 cm away from the source and then corrected (kr2 = 1.00217) for the lateral and thickness dimensions of the scoring voxel to give the air kerma per history on the central axis at a point 100 cm from the source’s mid-point as described in our previous study 2,3. Low-energy photons emitted from the source encapsulation are suppressed in the air-kerma calculations by discarding all photons with energy less than 10 keV (i.e., PCUT set to 10 keV in EGSnrc). Electron transport is not included in MC simulations for this source. egs_brachy 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 0.2 cm to 20 cm.
Anisotropy function - F(r,θ):
Anisotropy functions are calculated using the line source approximation and tabulated at 12 radii from 0.25 cm to 20 cm and 47 unique polar angles with a resolution of 5° or better. The anisotropy factor, φan (r), is calculated by integrating the solid angle weighted dose rate over 0° = ϑ = 180° .
F(0.50,θ) |
F(1.00,θ) |
F(5.00,θ) |
Along-Away Dose Data:
Along-away dose data are tabulated at 16 away distances from 0 cm to 20 cm and 31 along points from -20 cm to 20 cm. Doses are normalized to SK, the air kerma strength.
Primary and Scatter Separated (PSS) Dose Data: D ii (r,θ):
Primary and Scatter Separated (PSS) dose data are tabulated at 12 radii from 0.25 cm to 20 cm and 47 unique polar angles with a resolution of 5°or better. High resolution (Δr = 1 mm, ΔΘ = 1°) primary scatter dose data are also available in .csv files. For the purposes of these calculations, any photon escaping the source encapsulation is considered a primary. Only photons that scatter within the phantom are counted in the scatter tallies. Doses are normalized to the total photon energy escaping the encapsulation. The "ii" subscript labeled in the Dii(r, θ) represent the total scatter as Dto(r, θ), the primary photons as Dpr(r, θ), the single scatter photon as Dss(r, θ), and the multiple scatter photons as Dms (r, θ) .
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Dii (r,90°)*r 2
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Dii (1.00,θ)
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Dii (10.00,θ)  |
Dii (20.00,θ) |
High resolution (1mm/1°) Tabulated D ii (r,θ) data in .csv format: Zipped archive
Photon Energy Spectra
Photon energy spectra generated by the source model are calculated using the egs_brachy surface count scoring option to get the spectrum on the surface of the source. The plotted values are the counts per MeV in 1 keV bins, normalized to 1 count total in the spectrum. The MC calculations have a statistical uncertainty less than 0.001% on the mean energy. The spectrum data are available in xmgrace format below.
Photon energy spectrum on the source surface: xmgrace
Tabulated data:
Tabulated data are available in .xlsx format: Excel
References:
1. J. Pérez-Calatayud, et al , Dosimetric characteristics of the CDC-type miniature cylindrical 137Cs brachytherapy sources, Med Phys , 29 , 538-543, 2002
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. D. W. O. Rogers, Inverse square corrections for FACs and WAFACs, Appl. Radiat. Isot.,153 ,108638, 2019
4. H. Safigholi, M. J. P. Chamberland, R. E. P. Taylor, M. P. Martinov, D. W. O. Rogers, and R. M. Thomson, Update of the CLRP TG-43 parameter database for high-energy brachytherapy sources, to be published (Current calculation)
5. Perez-Calatayud et al , Dose Calculation for Photon-Emitting Brachytherapy Sources with Average Energy Higher than 50 keV: Full Report of the AAPM and ESTRO, 2012 by AAPM, ISBN: 978-1-936366-17-0