title: Evaluating the accuracy of a three-term pencil beam algorithm in heterogeneous media.
abstract: The goal of this work was to evaluate the accuracy of our in-house analytical dose calculation code against MCNPX data in heterogeneous phantoms. The analytical model utilizes a pencil beam model based on Fermi-Eyges theory to account for multiple Coulomb scattering and a least-squares fit to Monte Carlo data to account for nonelastic nuclear interactions as well as any remaining, uncharacterized scatter (the 'nuclear halo'). The model characterized dose accurately (up to 1% of maximum dose in broad fields (4  ×  4 cm2and 10  ×  10 cm2) and up to 0.01% in a narrow field (0.1  ×  0.1 cm2) fit to MCNPX data). The accuracy of the model was benchmarked in three types of stylized phantoms: (1) homogeneous, (2) laterally infinite slab heterogeneities, and (3) laterally finite slab heterogeneities. Results from homogeneous phantoms and laterally infinite slab heterogeneities showed high levels of accuracy (>98% of points within 2% or 0.1 cm distance-to-agreement (DTA)). However, because range straggling and secondary particle production were not included in our model, central-axis dose differences of 2-4% were observed in laterally infinite slab heterogeneities when compared to Monte Carlo dose. In the presence of laterally finite slab heterogeneities, the analytical model resulted in lower pass rates (>96% of points within 2% or 0.1 cm DTA), which was attributed to the use of the central-axis approximation.
authors.affiliation: Department of Physics and Astronomy, Louisiana State University and Agricultural and Mechanical College, 202 Nicholson Hall, Tower Drive, Baton Rouge, LA 70803, USA.
authors.forename: J W
authors.initials: JW
authors.lastname: Chapman
authors.forename: N C
authors.initials: NC
authors.lastname: Knutson
authors.forename: J D
authors.initials: JD
authors.lastname: Fontenot
authors.forename: W D
authors.initials: WD
authors.lastname: Newhauser
authors.forename: K R
authors.initials: KR
authors.lastname: Hogstrom
mesh_terms: D000465:Algorithms; D019047:Phantoms, Imaging; D061766:Proton Therapy / Q000379:methods / Q000592:standards*; D011879:Radiotherapy Dosage