Microdosimetric Evaluation of Current and Alternative Brachytherapy Sources-A Geant4-DNA Simulation Study

Area of Science:

• Medical Physics
• Radiation Oncology
• Nuclear Medicine

Background:

• High-dose-rate brachytherapy utilizes radioisotopes for targeted cancer treatment.
• ¹⁹²Ir is a common radionuclide, but alternatives are being explored.
• ⁷⁵Se, ¹⁶⁹Yb, and ¹⁵³Gd show promise due to their physical properties.

Purpose of the Study:

• To evaluate the relative biological effectiveness (RBE) of current and alternative brachytherapy radionuclides.
• To compare the radiation quality of ¹⁹²Ir, ¹²⁵I, ¹⁰³Pd with ⁷⁵Se, ¹⁶⁹Yb, and ¹⁵³Gd.
• To provide data for adjusting brachytherapy prescription doses based on radionuclide choice.

Main Methods:

• Monte Carlo simulations using Geant4 toolkit to model brachytherapy sources in a water phantom.
• Analysis of lineal energy distributions (y¯_D) to assess radiation quality.
• Electron track simulations down to 10 eV using Geant4-DNA models.
• Determination of scoring volume diameter by fitting ¹²⁵I y¯_D to its known RBE.

Main Results:

• Dose mean lineal energy (y¯_D) varied with distance for different isotopes: increasing for ¹⁹²Ir, ⁷⁵Se, ¹⁶⁹Yb; constant for ¹⁵³Gd, ¹²⁵I; decreasing for ¹⁰³Pd.
• The RBE for ¹²⁵I corresponded to a scoring diameter of approximately 25-40 nm.
• Calculated RBE values (relative to 1 MeV photons) ranged from 1.028-1.034 for ¹⁹²Ir to 1.17-1.22 for ¹⁰³Pd.

Conclusions:

• The investigated radionuclides exhibit higher radiation quality than high-energy photons.
• Significant variations in RBE exist among current and alternative brachytherapy isotopes.
• The findings support modifying brachytherapy prescription doses to account for differing radiation qualities.