title: Quantitative analysis of in-air output ratio.
abstract: Output factor (Scp) is one of the important factors required to calculate monitor unit (MU), and is divided into two components: phantom scatter factor (Sp) and in-air output ratio (Sc). Generally, Sc for arbitrary fields are calculated using several methods based on Sc determined by the absorbed dose measurement for several square fields. However, there are calculation errors when the treatment field has a large aspect ratio and the opening of upper and lower collimator are exchanged. To determine Sc accurately, scattered photons from the treatment head and backscattered particles into the monitor chamber must be analyzed individually. In this report, a simulation model that agreed well with measured Sc was constructed and dose variation by scattered photons from the treatment head and by backscattered particles into the monitor chamber was analyzed quantitatively. The results showed that the contribution of scattered photons from the primary collimator was larger than that of the flattening filter, and backscattered particles were affected by not only the upper jaw but also the lower jaw. In future work, a new Sc determination algorism based on the result of this report will be proposed.
authors.affiliation: Tokyo Metropolitan Geriatric Hospital and Institute of Gerontorogy, Sakaecho 35-2, Itabashi-ku, Tokyo 173-0015, Japan. miyashita-hisayuki@hs.tmu.ac.jp
authors.forename: Hisayuki
authors.initials: H
authors.lastname: Miyashita
authors.forename: Shogo
authors.initials: S
authors.lastname: Hatanaka
authors.forename: Yukio
authors.initials: Y
authors.lastname: Fujita
authors.forename: Shimpei
authors.initials: S
authors.lastname: Hashimoto
authors.forename: Atsushi
authors.initials: A
authors.lastname: Myojyoyama
authors.forename: Hidetoshi
authors.initials: H
authors.lastname: Saitoh
mesh_terms: D000388:Air / Q000032:analysis*; D000465:Algorithms; D003198:Computer Simulation; D019544:Equipment Failure Analysis / Q000379:methods; D008962:Models, Theoretical; D011874:Radiometry / Q000379:methods*; D011879:Radiotherapy Dosage; D011880:Radiotherapy Planning, Computer-Assisted / Q000295:instrumentation* / Q000379:methods*; D020266:Radiotherapy, Conformal / Q000295:instrumentation*; D012542:Scattering, Radiation
keywords: Monte Carlo method; backscattered particles; collision water kerma; in-air output ratio; scattered photon