In this action the relative dose rate distribution around the new 125I brachytherapy source IsoSeed I25.S17plus were determined and reported in a form suitable for clinical use. Results for the new source were also compared to corresponding results for other commercially available 125I sources of similar design.

Monte Carlo simulations were performed using the MCNP5 v.1.6 general purpose code. The model of the new source was prepared from information provided by the manufacturer and verified by imaging a sample of ten non-radioactive sources (see figure). Corresponding simulations were also performed for the 6711 125I brachytherapy source, using updated geometric information presented recently in the literature. 

I25S17plus im

Left: I25.S17plus source geometry, nominal dimensions and tolerances provided by the manufacturer. All dimensions are in mm. The marker’s radioactive coating (not shown) is AgI (ρ = 5.675g/cm3) of 1 μm nominal thickness (±30%). End weld thickness tolerance (not shown) is 0.35-0.40 mm. Right: Digital radiograph of 10 non-radioactive I25.S17plus sources.

 

 

Results were presented in the form of the quantities defined in the update of the Task Group 43 report, as well as a relative dose rate table in Cartesian coordinates. The dose rate distribution of the new source is comparable to that of sources of similar design (IsoSeed I25.S17, Oncoseed 6711, SelectSeed 130.002, Advantage IAI-125A, I-Seed AgX100, Thinseed 9011). Noticeable differences were observed only for the IsoSeed I25.S06 and Best 2301 sources.

 

I25S17plus DoseDiff

Percentage differences between the dose rate distribution calculated in this work for the new I25.S17plus source and corresponding data for the I25.S17 [2], I25.S06 [3], 6711 [7] and AgX [9] sources, presented on a pixel by pixel basis using an appropriate colormap. Dose rate distributions sources other than the I25.S17plus were calculated using linear interpolation on published TG43 data. Results are not presented at pixels where extrapolations to data in the literature had to be performed. Selected isodose-rate lines (in units of cGyh-1U-1) are superimposed to facilitate the comparison. Solid lines correspond to the I25.S17plus and dashed lines correspond to the source compared to it in each subplot.

 

The uncertainty of the dose distribution around the new source, as well as the dosimetric quantities derived from it according to the Task Group 43 formalism, were determined from the standard error of the mean of simulations for a sample of fifty source models. These source models were prepared by randomly selecting values of geometric parameters from uniform distributions defined by manufacturer stated tolerances.

 

I25S17plus uncertainty

(a) A color map representation of dose rate versus distance along and away from the new I25.S17plus source .

(b) Relative uncertainty of dose rate results in (a) due to source geometrical uncertainty and MC simulation type A uncertainty.

(c) Total relative uncertainty of dose rate results in (a) obtained from the quadrature combination of relative uncertainty in (b) with cross sections and water to air mass energy absorption ratio uncertainties.

Selected isodose rate lines (in units of cGyh-1U-1) are superimposed in all figures.