title: X-ray source and the output factor.
abstract: When the collimator setting of a linear accelerator is made sufficiently small, the output factor in air, R, is greatly reduced because the collimators obstruct the periphery of the x-ray source. This has been utilized to examine the size of the source by varying the width y of a narrow field and determining how R(y) varies. The sources diameters in the two principal directions were clearly influenced by the design of the accelerators. The x-ray sources of two accelerators with bending magnets were found to be noncircular while that of a linear accelerator without a magnet showed circular symmetry. The position of the source relative to the axis of collimator rotation was determined by measuring R for offset narrow fields. For one of the accelerators, the source was initially moving and off the central axis by about 2 mm for the first five monitor units. The results correlated well with sharpness in portal-film images. The technique can serve to evaluate the major source characteristics in acceptance testing and quality control.
authors.affiliation: Department of Radiation Oncology, Roger Williams Medical Center, Brown University, Providence, Rhode Island 02908, USA.
authors.forename: T C
authors.initials: TC
authors.lastname: Zhu
authors.forename: B E
authors.initials: BE
authors.lastname: Bjärngard
authors.forename: H
authors.initials: H
authors.lastname: Shackford
mesh_terms: D004867:Equipment Design; D006801:Humans; D008433:Mathematics; D008961:Models, Structural; D010315:Particle Accelerators; D011878:Radiotherapy / Q000295:instrumentation; D014965:X-Rays