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Medical Physics / Volume 37 / Issue 7 / RADIATION THERAPY PHYSICS

Med. Phys. 37, 3501 (2010); http://dx.doi.org/10.1118/1.3442028 (9 pages)

Wide field array calibration dependence on the stability of measured dose distributions

Thomas A. Simon

Department of Nuclear and Radiological Engineering, University of Florida, 202 Nuclear Science Building, Gainesville, Florida 32611-8300, Department of Radiation Oncology, Health Science Center, University of Florida, P.O. Box 100385, Gainesville, Florida 32610-0385, and Sun Nuclear Corporation, 425-A Pineda Court, Melbourne, Florida 32940

William E. Simon

Sun Nuclear Corporation, 425-A Pineda Court, Melbourne, Florida 32940

Darren Kahler, Jonathan Li, and Chihray Liu

Department of Radiation Oncology, Health Science Center, University of Florida, P.O. Box 100385, Gainesville, Florida 32610-0385

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(Received 22 March 2010; accepted 10 May 2010; revised 27 April 2010; published online 15 June 2010)

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Purpose: 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 (α, θ, and λ); each measurement uses the same radiation field, which is larger than the array. For measurement configuration θ, the array is rotated by 180° from its position in α; for λ, the array is shifted by one detector from its position in θ. The relative detector sensitivities are then determined through ratios of detector readings at the same field locations (using θ and λ). 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 α, θ, or λ, 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 θ and λ. 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 θ or λ resulted in calibration errors of up to 2%; α 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 θ and λ 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.

© 2010 American Association of Physicists in Medicine

ACKNOWLEDGMENTS

This work was supported in part by Sun Nuclear Corporation.

Article Outline

  1. INTRODUCTION
  2. MATERIALS AND METHODS
    1. Materials
    2. Methods
      1. Wide field calibration theory
      2. Limiting calibration error
    3. Effects of postulate failure
    4. Limiting violations of the first postulate
    5. Limiting violations of the second postulate
    6. Limiting violations of the third postulate
      1. Evaluating calibration factors
  3. RESULTS AND DISCUSSION
    1. Effects of postulate failure
    2. Limiting violations of the first calibration postulate
    3. Limiting violations of the third calibration postulate
    4. Evaluating calibration factors
    5. Other factors affecting the WF calibration
    6. Other arrays and IMRT
  4. CONCLUSION
Digital Object Identifier
http://dx.doi.org/10.1118/1.3442028

KEYWORDS and PACS

Keywords

calibration, dosimetry, linear accelerators, radiation therapy, calibration, linear accelerator, ionization chamber, detector array, relative dosimetry

PACS

  • 87.55.dk

    Dose-volume analysis

  • 87.53.Jw

    Therapeutic applications, including brachytherapy

PUBLICATION DATA

ISSN

0094-2405 (print)  

Publisher

$abstract.society.value is a Member of CrossRef American Association of Physicists in Medicine

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Departments of Radiology, Radiation Oncology, Biophysics, Community and Public Health, Medical College of Wisconsin
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