title: The "collimator monitoring fill factor" of a two-dimensional detector array, a measure of its ability to detect collimation errors.
abstract: OBJECTIVE
  Two-dimensional detector arrays are routinely used for constancy checks and treatment plan verification in photon-beam radiotherapy. In addition to the spatial resolution of the dose profiles, the "coverage" of the radiation field with respect to the detection of any beam collimation deficiency appears as the second characteristic feature of a detector array. The here proposed "collimator monitoring fill factor" (CM fill factor) has been conceived to serve as a quantitative characteristic of this "coverage".
  
  
  METHODS
  The CM fill factor is defined as the probability of a 2D array to detect any collimator position error. Therefore, it is represented by the ratio of the "sensitive area" of a single detector, in which collimator position errors are detectable, and the geometrical "cell area" associated with this detector within the array. Numerical values of the CM fill factor have been Monte Carlo simulated for 2D detector arrays equipped with air-vented ionization chambers, liquid-filled ionization chambers and diode detectors and were compared with the "FWHM fill factor" defined by Gago-Arias et al. (2012).
  
  
  RESULTS
  For arrays with vented ionization chambers, the differences between the CM fill factor and the FWHM fill factor are moderate, but occasionally the latter exceeds unity. For narrower detectors such as liquid-filled ionization chambers and Si diodes and for small sampling distances, large differences between the FWHM fill factor and the CM fill factor have been observed. These differences can be explained by the shapes of the fluence response functions of these narrow detectors.
  
  
  CONCLUSIONS
  A new parameter "collimator monitoring fill factor" (CM fill factor), applicable to quantitate the collimator position error detection probability of a 2D detector array, has been proposed. It is designed as a help in classifying the clinical performance of two-dimensional detector arrays in photon-beam radiotherapy.
authors.affiliation: University Clinic for Medical Radiation Physics, Medical Campus Pius Hospital, Carl von Ossietzky University Oldenburg, Germany.
authors.forename: Tenzin Sonam
authors.initials: TS
authors.lastname: Stelljes
authors.affiliation: University Clinic for Medical Radiation Physics, Medical Campus Pius Hospital, Carl von Ossietzky University Oldenburg, Germany.
authors.forename: Hui Khee
authors.initials: HK
authors.lastname: Looe
authors.affiliation: Prof. em., Medical Physics and Biophysics, Georg-August-University, Göttingen, Germany.
authors.forename: Dietrich
authors.initials: D
authors.lastname: Harder
authors.affiliation: University Clinic for Medical Radiation Physics, Medical Campus Pius Hospital, Carl von Ossietzky University Oldenburg, Germany.
authors.forename: Björn
authors.initials: B
authors.lastname: Poppe
mesh_terms: D000465:Algorithms; D003198:Computer Simulation; D009010:Monte Carlo Method; D011336:Probability; D061766:Proton Therapy / Q000295:instrumentation*; D055618:Radiation Equipment and Supplies; D011880:Radiotherapy Planning, Computer-Assisted / Q000295:instrumentation*; D061106:Radiotherapy Setup Errors / Q000517:prevention & control; D012825:Silicon
chemical_list: D012825:Silicon
keywords: 2D array; collimator error detection; coverage; fill factor