title: A novel technique for measuring the low-dose envelope of pencil-beam scanning spot profiles.
abstract: To investigate the profile measurement capabilities of an IBA-Dosimetry scintillation detector and to assess its feasibility for determining the low-intensity tails of pencil-beam scanning spots, the responses of the scintillation detector and Gafchromic EBT2 film to a 115 MeV proton spot were measured in-air at the isocenter. Pairs of irradiations were made: one lower-level irradiation insufficient to cause saturation, and one higher-level irradiation which deliberately saturated the central region of the spot, but provided magnification of the tails. By employing the pair/magnification technique, agreement between the film and scintillation detector measurements of the spot profile can be extended from 4% of the central spot dose down to 0.01%. Gamma analysis between these measurements shows 95% and 99% agreement within a ±9 cm bound using criteria of 3 mm/3% and 5 mm/5%, respectively. Above 4%, our 115 MeV proton spot can be well-described by Gaussian function; below 4%, non-Gaussian, diamond-shaped tails predominate.
authors.affiliation: Department of Radiation Oncology, University of Pennsylvania, 3400 Civic BLvd, 2326W TRC, PCAM, Philadelphia, PA 19104, USA. liyonglin8@gmail.com
authors.forename: Liyong
authors.initials: L
authors.lastname: Lin
authors.forename: Christopher G
authors.initials: CG
authors.lastname: Ainsley
authors.forename: Thierry
authors.initials: T
authors.lastname: Mertens
authors.forename: Olivier
authors.initials: O
authors.lastname: De Wilde
authors.forename: Patrick T
authors.initials: PT
authors.lastname: Talla
authors.forename: James E
authors.initials: JE
authors.lastname: McDonough
mesh_terms: D005373:Film Dosimetry / Q000379:methods*; D061766:Proton Therapy