title: Experimentally validated pencil beam scanning source model in TOPAS.
abstract: The presence of a low-dose envelope, or 'halo', in the fluence profile of a proton spot can increase the output of a pencil beam scanning field by over 10%. This study evaluated whether the Monte Carlo simulation code, TOPAS 1.0-beta 8, based on Geant4.9.6 with its default physics list, can predict the spot halo at depth in phantom by incorporating a halo model within the proton source distribution. Proton sources were modelled using three 2D Gaussian functions, and optimized until simulated spot profiles matched measurements at the phantom surface out to a radius of 100 mm. Simulations were subsequently compared with profiles measured using EBT3 film in Solidwater® phantoms at various depths for 100, 115, 150, 180, 210 and 225 MeV proton beams. Simulations predict measured profiles within a 1 mm distance to agreement for 2D profiles extending to the 0.1% isodose, and within 1 mm/1% Gamma criteria over the integrated curve of spot profile as a function of radius. For isodose lines beyond 0.1% of the central spot dose, the simulated primary spot sigma is smaller than the measurement by up to 15%, and can differ by over 1 mm. The choice of particle interaction algorithm and phantom material were found to cause ~1 mm range uncertainty, a maximal 5% (0.3 mm) difference in spot sigma, and maximal 1 mm and ~2 mm distance to agreement in isodoses above and below the 0.1% level, respectively. Based on these observations, therefore, the selection of physics model and the application of Solidwater® as water replacement material in simulation and measurement should be used with caution.
authors.affiliation: Department of Radiation Oncology, University of Pennsylvania, Philadelphia, PA, USA.
authors.forename: Liyong
authors.initials: L
authors.lastname: Lin
authors.forename: Minglei
authors.initials: M
authors.lastname: Kang
authors.forename: Timothy D
authors.initials: TD
authors.lastname: Solberg
authors.forename: Christopher G
authors.initials: CG
authors.lastname: Ainsley
authors.forename: James E
authors.initials: JE
authors.lastname: McDonough
mesh_terms: D000465:Algorithms; D005336:Fiber Optic Technology / Q000295:instrumentation*; D006801:Humans; D008962:Models, Theoretical; D009010:Monte Carlo Method; D019047:Phantoms, Imaging; D061766:Proton Therapy / Q000295:instrumentation* / Q000379:methods*; D011785:Quality Assurance, Health Care; D011879:Radiotherapy Dosage; D014867:Water / Q000737:chemistry
chemical_list: D014867:Water