title: Output factors for blocked fields.
abstract: This study investigates the output from blocked treatment fields with 6 MV and 18 MV photons. Square fields, defined by collimators, were blocked down to smaller symmetrical square, rectangular and asymmetrical rectangular fields using blocks on a tray beneath the collimators. Up to 7.5% difference was found between the output measured for the blocked field and the output calculated using a collimator output factor for the surface field size. A method is described by which output factors for the blocked fields can be measured and used in practice. Three methods of approximating these output factors are also presented.
authors.affiliation: Department of Radio-Oncology, University Hospital Zurich, Switzerland.
authors.forename: E M
authors.initials: EM
authors.lastname: Dean
authors.forename: J B
authors.initials: JB
authors.lastname: Davis
mesh_terms: D006801:Humans; D008961:Models, Structural; D011874:Radiometry / Q000379:methods*; D011879:Radiotherapy Dosage; D011882:Radiotherapy, High-Energy