Convex hull-based microdosimetry in Geant4-DNA: linking electron track structures to radiobiological effectiveness

A new Convex Hull microdosimetric technique has been incorporated in Geant4-DNA to enhance geometrical modeling of microdosimetry at nanometric scales. This new microdosimetric technique aims to provide a direct relationship between electron track structures and their effectiveness in radiobiology. Track structures in liquid water were simulated using monoenergetic electrons ranging from 0.1 keV to 1000 keV, including clinical electron beams between 6 MeV and 18 MeV. The CH technique, based on the Jarvis-March gift wrapping algorithm, has been incorporated in Geant4-DNA to increase computational efficiency by geometrically encapsulating track-associated microdosimetric quantities. Dose mean lineal energy and frequency mean lineal energy were used in conjunction with the Microdosimetric Kinetic Model to estimate radiobiological parameters and relative biological effectiveness. These results are compared with the reference results obtained using the KURBUC and FLUKA codes. The CH approach correctly predicts the reference microdosimetric quantities within 5% accuracy for the micrometer scale, while larger systematic differences are found on the nanometer scale due to the different definitions of the track volume. The<i>α</i>parameter increases with increasing incident particle energy, ranging from 0.094 Gy^-1for an incident particle energy of 0.1 keV to 0.265 Gy^-1for an incident particle energy of 5 keV, which results in an RBE of approximately 1.04, showing the effect of ionization clustering. A comparison of the results with the reference results reveals differences of 7%-12%. The RBE results for clinical electron beams are found to be nearly independent of the incident particle energy and remain close to 0.85 when normalized to the results obtained for x-rays. The CH-based approach provides an efficient computational scheme for the calculation of the micro- and nanodosimetric quantities, which allows the model-based calculation of the RBE and the quantification of the quality of the radiation on the nanometric scale.