title: Comparing stochastic proton interactions simulated using TOPAS-nBio to experimental data from fluorescent nuclear track detectors.
abstract: Whilst Monte Carlo (MC) simulations of proton energy deposition have been well-validated at the macroscopic level, their microscopic validation remains lacking. Equally, no gold-standard yet exists for experimental metrology of individual proton tracks. In this work we compare the distributions of stochastic proton interactions simulated using the TOPAS-nBio MC platform against confocal microscope data for Al2O3:C,Mg fluorescent nuclear track detectors (FNTDs). We irradiated [Formula: see text] mm3FNTD chips inside a water phantom, positioned at seven positions along a pristine proton Bragg peak with a range in water of 12 cm. MC simulations were implemented in two stages: (1) using TOPAS to model the beam properties within a water phantom and (2) using TOPAS-nBio with Geant4-DNA physics to score particle interactions through a water surrogate of Al2O3:C,Mg. The measured median track integrated brightness (IB) was observed to be strongly correlated to both (i) voxelized track-averaged linear energy transfer (LET) and (ii) frequency mean microdosimetric lineal energy, [Formula: see text], both simulated in pure water. Histograms of FNTD track IB were compared against TOPAS-nBio histograms of the number of terminal electrons per proton, scored in water with mass-density scaled to mimic Al2O3:C,Mg. Trends between exposure depths observed in TOPAS-nBio simulations were experimentally replicated in the study of FNTD track IB. Our results represent an important first step towards the experimental validation of MC simulations on the sub-cellular scale and suggest that FNTDs can enable experimental study of the microdosimetric properties of individual proton tracks.
authors.affiliation: Department of Radiation Oncology, Massachusetts General Hospital & Harvard Medical School, Boston, MA, United States of America. Department of Medical Engineering and Physics, University College London, London, United Kingdom.
authors.forename: T S A
authors.initials: TS
authors.lastname: Underwood
authors.forename: W
authors.initials: W
authors.lastname: Sung
authors.forename: C H
authors.initials: CH
authors.lastname: McFadden
authors.forename: S J
authors.initials: SJ
authors.lastname: McMahon
authors.forename: D C
authors.initials: DC
authors.lastname: Hall
authors.forename: A L
authors.initials: AL
authors.lastname: McNamara
authors.forename: H
authors.initials: H
authors.lastname: Paganetti
authors.forename: G O
authors.initials: GO
authors.lastname: Sawakuchi
authors.forename: J
authors.initials: J
authors.lastname: Schuemann
mesh_terms: D004583:Electrons; D018499:Linear Energy Transfer; D009010:Monte Carlo Method; D019047:Phantoms, Imaging; D011522:Protons; D011874:Radiometry / Q000295:instrumentation / Q000379:methods*; D013269:Stochastic Processes; D014867:Water / Q000737:chemistry
chemical_list: D011522:Protons; D014867:Water