Monte Carlo dosimetry of the selectSeed interstitial brachytherapy seed

Abstract

This work provides full dosimetric data for the new selectSeed 125I prostate seed source to be distributed by Nucletron B.V. The AAPM TG-43 dosimetric formalism and the new 1999 NIST air kerma strength calibration standard have been followed. Air kerma strength, dose rate constant, radial dose functions, anisotropy functions, and anisotropy factors were calculated using Monte Carlo simulation. Corresponding calculations were also performed for the commercially available 6711 seed source, which is of similar design, for reasons of comparison. The calculated dose rate constant of the selectSeed was 0.954+/-0.005 cGy h(-1) U(-1) compared to 0.953+/-0.005 cGy h(-1) U(-1) for the 6711 source design. The latter value for the 6711 source suggests that the correction factor proposed by NIST for conversion of dose rate constants to the new 1999 NIST calibration standard may be overestimated by 2-3%. Radial dose functions of the two sources were found in good agreement for radial distances up to 4 cm, the selectSeed being less penetrating at greater radial distances (approximately 4% at 10 cm). The selectSeed source presents similar anisotropy characteristics with the 6711 source design. For both source designs, a distance and polar angle dependent discontinuity of anisotropy function values was observed owing to the dose contribution of radioactivity distributed on the ends of the cylindrical source cores. Variation of dosimetric parameters with possible variation in radioactive silver halide coating thickness of the silver source core of the new source was also investigated.

Supplement 2 for the 2004 update of theAAPMTask Group No. 43 Report: Joint recommendations by theAAPMandGEC‐ESTRO

“…To perform a direct comparison with Karaiskos et al MC results at the same positions, the authors were contacted 68 and a high-resolution dataset was obtained with sampling at the same radial resolution as their g L (r) results, but with 1°angular binning and centered on integer values of polar angles unlike in their Table III. 63 At r = 1 cm, the simulated results 4.6%, 4.5%, and 0.5% higher than the measured results at 10°, 31°, and 60°, respectively. At larger distances, agreement was within 5% and typically within 2%.…”

“…125 I source using a generalized MC code that was developed in-house specifically for brachytherapy dosimetry investigations. 63 While no identifying name or version of this code was declared, the MC code used by the University of Athens group has been well-validated under a variety of circumstances appropriate for radiation transport calculations for brachytherapy sources. Photoatomic interaction cross-sections from Scofield, 64 Hubbell et al, 65 and Hubbell and Øverbø 66 were used.…”

“…There was no volume averaging on the source transverse plane of 90°AE 7.6°to simulate the NIST WAFAC measurement geometry. Consequently, additional MC studies by Lymperopoulou et al 51 were used for estimation of MC Λ, as suggested by Papagiannis et al 62 MC estimations of dose to water by Karaiskos et al 63 utilized 10 9 photon histories. MC simulations were also performed by Taylor and Rogers 15 with L = 0.34 cm and similar methods as described for the model I25.S17…”

A dosimetric uncertainty analysis for photon‐emitting brachytherapy sources: Report of AAPM Task Group No. 138 and GEC‐ESTRO

“…A sophisticated MC dosimetric analysis would simulate the influence of varying each of these components and estimate the resultant effect of these uncertainties on the calculated dose distribution. Karaiskos et al 40 investigated the effect of varying the silver halide coating thickness ͑i.e., 1 -10 m͒ for an 125 I source; ⌳ and g͑r͒ were unchanged within 1%. Koona 41 assessed variable 125 I source capsule wall thickness ͑i.e., 30-100 m͒ and found an influence on ⌳ ranging from +16% to Ϫ1%.…”

NCS Report 20: Dosimetry and quality control of brachytherapy with low-energy photon sources (I-125)

“…This active element is encapsulated in a hollow titanium tube that is sealed by laser welding using hemispherical shaped end welds. The seeds are 4.5 mm long with an external diameter of 0.8 mm [18,19,20].…”