title: Portal dose image verification: formalism and application of the collapsed cone superposition method.
abstract: A formalism tailored for portal dose image verification is proposed to facilitate the comparison of calculated and measured portal dose distributions. Each portal image is converted into a dose proportional image and normalized to the reference beam calibration dose per monitor unit. The calculated or measured dose to a detector phantom is accordingly normalized so as to enable direct comparison. The collapsed cone kernel superposition method is adapted and evaluated for calculation of portal dose distributions in a water-equivalent detector phantom through comparisons with Monte Carlo calculations and with measurements. The deviation compared with Monte Carlo calculations for 6 and 15 MV was between +0.9% (the 0.9 quantile) and -2.1% (the 0.1 quantile) for a range of investigated geometries. Collapsed cone calculations compared with measurements for clinical fields agreed within [-1.9%, +2.4%] for 15 MV and [-0.9%, +3.2%] for 6 MV for the 0.1 and 0.9 quantiles, respectively. Hence, the absolute portal dose to a detector phantom could be calculated and verified well within the present accuracy requirements for clinical dose calculations.
authors.affiliation: MDS Nordion AB, Box 1704, SE-751 47 Uppsala, Sweden.
authors.forename: Christina Vallhagen
authors.initials: CV
authors.lastname: Dahlgren
authors.forename: Anders
authors.initials: A
authors.lastname: Ahnesjö
authors.forename: Anders
authors.initials: A
authors.lastname: Montelius
authors.forename: Göran
authors.initials: G
authors.lastname: Rikner
mesh_terms: D000465:Algorithms; D002138:Calibration; D003198:Computer Simulation; D006801:Humans; D009010:Monte Carlo Method; D019047:Phantoms, Imaging; D011786:Quality Control; D011856:Radiographic Image Enhancement / Q000379:methods* / Q000592:standards; D011874:Radiometry / Q000379:methods* / Q000592:standards; D011878:Radiotherapy / Q000379:methods; D011879:Radiotherapy Dosage; D011880:Radiotherapy Planning, Computer-Assisted / Q000379:methods* / Q000592:standards; D012542:Scattering, Radiation