title: Dose contributions from large-angle scattered particles in therapeutic carbon beams.
abstract: In carbon therapy, doses at center of spread-out Bragg peaks depend on field size. For a small field of 5 x 5 cm2, the central dose reduces to 96% of the central dose for the open field in case of 400 MeV/n carbon beam. Assuming the broad beam injected to the water phantom is made up of many pencil beams, the transverse dose distribution can be reconstructed by summing the dose distribution of the pencil beams. We estimated dose profiles of this pencil beam through measurements of dose distributions of broad uniform beams blocked half of the irradiation fields. The dose at a distance of a few cm from the edge of the irradiation field reaches up to a few percent of the central dose. From radiation quality measurements of this penumbra, the large-angle scattered particles were found to be secondary fragments which have lower LET than primary carbon beams. Carbon ions break up in beam modifying devices or in water phantom through nuclear interaction with target nuclei. The angular distributions of these fragmented nuclei are much broader than those of primary carbon particles. The transverse dose distribution of the pencil beam can be approximated by a function of the three-Gaussian form. For a simplest case of mono-energetic beam, contributions of the Gaussian components which have large mean deviations become larger as the depth in the water phantom increases.
authors.affiliation: Tokai University Unified Graduate School, Graduate School of Science and Technology, 1117 Kitakaname, Hiratsuka, Kanagawa, 259-1292, Japan.
authors.forename: Yohsuke
authors.initials: Y
authors.lastname: Kusano
authors.forename: Tatsuaki
authors.initials: T
authors.lastname: Kanai
authors.forename: Yuki
authors.initials: Y
authors.lastname: Kase
authors.forename: Naruhiro
authors.initials: N
authors.lastname: Matsufuji
authors.forename: Masataka
authors.initials: M
authors.lastname: Komori
authors.forename: Nobuyuki
authors.initials: N
authors.lastname: Kanematsu
authors.forename: Atsushi
authors.initials: A
authors.lastname: Ito
authors.forename: Hirohisa
authors.initials: H
authors.lastname: Uchida
mesh_terms: D001822:Body Burden; D002247:Carbon Isotopes / Q000032:analysis* / Q000627:therapeutic use*; D003198:Computer Simulation; D006801:Humans; D008954:Models, Biological; D011874:Radiometry / Q000379:methods*; D011879:Radiotherapy Dosage; D020266:Radiotherapy, Conformal / Q000379:methods; D012062:Relative Biological Effectiveness; D012542:Scattering, Radiation
chemical_list: D002247:Carbon Isotopes