title: Wide field array calibration dependence on the stability of measured dose distributions.
abstract: OBJECTIVE
  The aim of this work was to simulate the effect of dose distribution changes on detector array calibrations and to explore compensatory methods that are used during calibration measurements.
  
  
  METHODS
  The array calibration technique that was investigated is known as wide field (WF) calibration. Using this method, a linear array [y-axis (65 detectors) of the IC PROFILER (Sun Nuclear Corporation, Melbourne, FL)] is calibrated with three measurements (alpha, theta, and lamda); each measurement uses the same radiation field, which is larger than the array. For measurement configuration theta, the array is rotated by 180 degrees from its position in a; for lamda, the array is shifted by one detector from its position in theta. The relative detector sensitivities are then determined through ratios of detector readings at the same field locations (using theta and lamda). This method results in error propagation that is proportional to the number of detectors in the array. During the procedure, the calibration protocol operates under three postulates, which state that (a) the beam shape does not change between measurements; (b) the relative sensitivities of the detectors do not change; and (c) the scatter to the array does not change as the array is moved. The WF calibration's sensitivity to a postulate (a) violation was quantified by applying a sine shaped perturbation (of up to 0.1%) to a, theta, or lamda, and then determining the change relative to a baseline calibration. Postulate (a) violations were minimized by using a continuous beam and mechanized array movement during theta and lamda. A continuously on beam demonstrated more stable beam symmetry as compared to cycling the beam on and off between measurements. Additional side-scatter was also used to satisfy postulate (c).
  
  
  RESULTS
  Simulated symmetry perturbations of 0.1% to theta or lamda resulted in calibration errors of up to 2%; alpha was relatively immune to perturbation (<0.1% error). Wide field calibration error on a linear accelerator with similar symmetry variations was +/- 1.6%. Using a continuous beam during theta and lamda with additional side-scatter reduced the calibration error from +/- 1.6% to +/- 0.48%.
  
  
  CONCLUSIONS
  This work increased the reproducibility of WF calibrations by limiting the effect of measurement perturbations primarily due to linear accelerator symmetry variations. The same technique would work for any array using WF calibration.
authors.affiliation: Department of Nuclear and Radiological Engineering, University of Florida, 202 Nuclear Science Building, Gainesville, Florida 32611-8300, USA.
authors.forename: Thomas A
authors.initials: TA
authors.lastname: Simon
authors.forename: William E
authors.initials: WE
authors.lastname: Simon
authors.forename: Darren
authors.initials: D
authors.lastname: Kahler
authors.forename: Jonathan
authors.initials: J
authors.lastname: Li
authors.forename: Chihray
authors.initials: C
authors.lastname: Liu
mesh_terms: D002138:Calibration; D011874:Radiometry / Q000295:instrumentation*; D011879:Radiotherapy Dosage; D050397:Radiotherapy, Intensity-Modulated; D015203:Reproducibility of Results