title: An empirical model for calculation of the collimator contamination dose in therapeutic proton beams.
abstract: Collimators are used as lateral beam shaping devices in proton therapy with passive scattering beam lines. The dose contamination due to collimator scattering can be as high as 10% of the maximum dose and influences calculation of the output factor or monitor units (MU). To date, commercial treatment planning systems generally use a zero-thickness collimator approximation ignoring edge scattering in the aperture collimator and few analytical models have been proposed to take scattering effects into account, mainly limited to the inner collimator face component. The aim of this study was to characterize and model aperture contamination by means of a fast and accurate analytical model. The entrance face collimator scatter distribution was modeled as a 3D secondary dose source. Predicted dose contaminations were compared to measurements and Monte Carlo simulations. Measurements were performed on two different proton beam lines (a fixed horizontal beam line and a gantry beam line) with divergent apertures and for several field sizes and energies. Discrepancies between analytical algorithm dose prediction and measurements were decreased from 10% to 2% using the proposed model. Gamma-index (2%/1 mm) was respected for more than 90% of pixels. The proposed analytical algorithm increases the accuracy of analytical dose calculations with reasonable computation times.
authors.affiliation: Institut Curie: Centre de Protonthérapie d'Orsay, 91400 Orsay, France. Dosisoft, 94230 Cachan, France.
authors.forename: M
authors.initials: M
authors.lastname: Vidal
authors.forename: L
authors.initials: L
authors.lastname: De Marzi
authors.forename: H
authors.initials: H
authors.lastname: Szymanowski
authors.forename: L
authors.initials: L
authors.lastname: Guinement
authors.forename: C
authors.initials: C
authors.lastname: Nauraye
authors.forename: E
authors.initials: E
authors.lastname: Hierso
authors.forename: N
authors.initials: N
authors.lastname: Freud
authors.forename: R
authors.initials: R
authors.lastname: Ferrand
authors.forename: P
authors.initials: P
authors.lastname: François
authors.forename: D
authors.initials: D
authors.lastname: Sarrut
mesh_terms: D000465:Algorithms; D006801:Humans; D061766:Proton Therapy / Q000295:instrumentation / Q000379:methods* / Q000592:standards; D011522:Protons; D011879:Radiotherapy Dosage; D011880:Radiotherapy Planning, Computer-Assisted / Q000379:methods*; D012542:Scattering, Radiation
chemical_list: D011522:Protons