ABSTRACT
Due to a number of distinct advantages resulting from the relatively low energy gamma ray spectrum of Yb-169, various designs of Yb-169 sources have been developed over the years for brachytherapy applications. Lately, Yb-169 has also been suggested as an effective and practical radioisotope option for a novel radiation treatment approach often known as gold nanoparticle-aided radiation therapy (GNRT). In a recently published study, the current investigators used the Monte Carlo N-Particle Version 5 (MCNP5) code to design a novel titanium-encapsulated Yb-169 source optimized for GNRT applications. In this study, the original MC source model was modified to accurately match the specifications of the manufactured Yb-169 source. The modified MC model was then used to obtain a complete set of the AAPM TG-43 parameters for the new titanium-encapsulated Yb-169 source. The MC-calculated dose rate constant for this titanium-encapsulated Yb-169 source was 1.19 ± 0.03 cGy·h-1·U-1, indicating no significant change from the values reported for stainless steel-encapsulated Yb-169 sources. The source anisotropy and radial dose function for the new source were also found similar to those reported for the stainless steel-encapsulated Yb-169 sources. The current results suggest that the use of titanium, instead of stainless steel, to encapsulate the Yb-169 core would not lead to any major change in the dosimetric characteristics of the Yb-169 source. The results also show that the titanium encapsulation of the Yb-169 source could be accomplished while meeting the design goals as described in the current investigators' published MC optimization study for GNRT applications.
Subject(s)
Brachytherapy , Monte Carlo Method , Radiotherapy Dosage , Ytterbium/therapeutic use , Anisotropy , Equipment Design , Humans , Radiometry , TitaniumABSTRACT
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.