Dosimetric characterization of the M-15 high-dose-rate Iridium-192 brachytherapy source using the AAPM and ESTRO formalism

Abstract

The Source Production & Equipment Co. (SPEC) model M-15 is a new Iridium-192 brachytherapy source model intended for use as a temporary high-dose-rate (HDR) brachytherapy source for the Nucletron microSelectron Classic afterloading system. The purpose of this study is to characterize this HDR source for clinical application by obtaining a complete set of Monte Carlo calculated dosimetric parameters for the M-15, as recommended by AAPM and ESTRO, for isotopes with average energies greater than 50 keV. This was accomplished by using the MCNP6 Monte Carlo code to simulate the resulting source dosimetry at various points within a pseudoinfinite water phantom. These dosimetric values next were converted into the AAPM and ESTRO dosimetry parameters and the respective statistical uncertainty in each parameter also calculated and presented. The M-15 source was modeled in an MCNP6 Monte Carlo environment using the physical source specifications provided by the manufacturer. Iridium-192 photons were uniformly generated inside the iridium core of the model M-15 with photon and secondary electron transport replicated using photoatomic cross-sectional tables supplied with MCNP6. Simulations were performed for both water and air/vacuum computer models with a total of 4 × 109 sources photon history for each simulation and the in-air photon spectrum filtered to remove low-energy photons belowδ = 10 keV. Dosimetric data, including D·(r,θ), gL(r), F(r,θ), φan(r), and φ-an, and their statistical uncertainty were calculated from the output of an MCNP model consisting of an M-15 source placed at the center of a spherical water phantom of 100 cm diameter. The air kerma strength in free space, SK, and dose rate constant, Λ, also was computed from a MCNP model with M-15 Iridium-192 source, was centered at the origin of an evacuated phantom in which a critical volume containing air at STP was added 100 cm from the source center. The reference dose rate, D·(r0,θ0) ≡ D· (1cm,π/2), is found to be 4.038 ± 0.064 cGy mCi-1 h-1. The air kerma strength, SK, is reported to be 3.632 ± 0.086 cGy cm2 mCi-1 g-1, and the dose rate constant, Λ, is calculated to be 1.112 ± 0.029 cGy h-1 U-1. The normalized dose rate, radial dose function, and anisotropy function with their uncertainties were computed and are represented in both tabular and graphical format in the report. A dosimetric study was performed of the new M-15 Iridium-192 HDR brachytherapy source using the MCNP6 radiation transport code. Dosimetric parameters, including the dose-rate constant, radial dose function, and anisotropy function, were calculated in accordance with the updated AAPM and ESTRO dosimetric parameters for brachytherapy sources of average energy greater than 50 keV. These data therefore may be applied toward the development of a treatment planning program and for clinical use of the source.

Figure 1

Configuration of the $\mathrm{M}-15$ $\text{Iridium}-192$ HDR brachytherapy source modeled for the MCNP simulation. This model consists of a 3.5 mm long $\text{Iridium}-192$ source encapsulated in a steel shell.

Figure 2

Presentation of the normalized dose‐rate distribution of model $\mathrm{M}-15$ $\text{Iridium}-192$ from the MCNP output. Geometry effects were removed by multiplying the dose rate, $\dot{\mathrm{D}}(\mathrm{r},\theta )$, by the radial distance squared, r². Once corrected, the dose rate is then normalized to the dose rate at 1 cm along the seed's traverse's axis, $\dot{\mathrm{D}}(\mathrm{r},\theta )=4.038\pm 0.064\hspace{0.17em}\text{cGy}\hspace{0.17em}{\text{mCi}}^{-1}\hspace{0.17em}{\mathrm{h}}^{-1}$, to obtain the normalized dose rate.

Figure 3

Calculated $\mathrm{M}-15$ radial dose function, ${\mathrm{g}}_{\mathrm{L}}$(r), presented at distances between 0.5 and 10 cm. These data were fit to a 5th order polynomial, as recommended in TG‐43U1, and fit to a double exponential function, as discussed by Furhang and Anderson, ⁽¹⁶⁾ for better accuracy when extrapolating outside of the measured data.

Figure 4

Resulting anisotropy function, $\mathrm{F}(\mathrm{r},\theta )$, of the model $\mathrm{M}-15$ source at angles between 0° and 180° and for depths between 0.5 cm to 10 cm.