Extending TOPAS with an analytical microdosimetric function: application and benchmarking with nBio track structure simulations

Microdosimetric distributions are important to accurately describe the biological impact of ionizing radiation, particularly in ion therapy. The computational demands of track structure simulations, a gold standard for modeling microscopic energy deposition, limit their practicality for large-scale or clinical applications. The analytical microdosimetric function (AMF) is a computationally efficient function that reproduces track structure simulation results. First introduced in the particle and heavy ion transport code system Monte Carlo code in 2006 and updated in 2023, the AMF offers a promising alternative for calculating microdosimetric spectra. This study implements the AMF within the Geant4-based Tool for Particle Therapy (TOPAS) platform, enabling efficient calculation of microdosimetric spectra and radiobiological metrics, including dose-mean lineal energy (y¯D) and relative biological effectiveness (RBE) using clinically relevant models, including the modified microdosimetric kinetic model and Mayo Clinic Florida MKM. Using OpenTOPAS (v4.0.0), the AMF extension was benchmarked against TOPAS-nBio track structure simulations for ions relevant to radiotherapy and space applications (¹H,⁴He,⁷Li,¹²C,¹⁶O,²⁰Ne,⁴⁰Ar,⁵⁶Fe). AMF results were further compared with TOPAS-nBio at different depths within the mixed radiation field of a carbon spread-out Bragg peak (SOBP). The AMF extension demonstrated reasonable agreement with TOPAS-nBio track structure simulations for most ions and energies. Microdosimetric spectra and derived metrics,y¯Dand RBE, showed average discrepancies under 10% for most cases. Deviations were largely attributed to differences in Monte Carlo stopping power models and ionization cross-sections. In an SOBP, the RBE calculated using TOPAS-nBio and AMF consistently agreed within 5%. Additionally, the AMF achieved significant computational efficiency, reducing simulation times by over 98% compared to TOPAS-nBio at discrete depths in an SOBP. The AMF extension in TOPAS provides a computationally efficient alternative to track structure simulations for microdosimetric analysis and RBE modeling. Its integration with advanced RBE models enables rapid, accurate calculations critical for particle therapy research and clinical treatment planning.