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Medical Physics / Volume 39 / Issue 2 / RADIATION THERAPY PHYSICS

Med. Phys. 39, 623 (2012); http://dx.doi.org/10.1118/1.3673958 (13 pages)

Detection and correction for EPID and gantry sag during arc delivery using cine EPID imaging

Pejman Rowshanfarzad, Mahsheed Sabet, and Daryl J. O’Connor

School of Mathematical and Physical Sciences, University of Newcastle, Newcastle, NSW 2308, Australia

Peter M. McCowan

Department of Physics and Astronomy, University of Manitoba, Winnipeg, Manitoba, R3T 2N2, Canada and Division of Medical Physics, CancerCare Manitoba, 675 McDermot Avenue, Winnipeg, Manitoba R3E 0V9, Canada

Boyd M. C. McCurdy

Division of Medical Physics, CancerCare Manitoba, 675 McDermot Avenue, Winnipeg, Manitoba R3E 0V9, Canada; Department of Physics and Astronomy, University of Manitoba,Winnipeg, Manitoba R3T 2N2, Canada; and Department of Radiology, University of Manitoba, Winnipeg, Manitoba R3A 1R9, Canada

Peter B. Greer

Department of Radiation Oncology, Calvary Mater Newcastle Hospital, Newcastle, NSW 2310, Australia and School of Mathematical and Physical Sciences, University of Newcastle, Newcastle, NSW 2308, Australia

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(Received 4 October 2011; accepted 12 December 2011; revised 10 December 2011; published online 11 January 2012)

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Purpose: Electronic portal imaging devices (EPIDs) have been studied and used for pretreatment and in-vivo dosimetry applications for many years. The application of EPIDs for dosimetry in arc treatments requires accurate characterization of the mechanical sag of the EPID and gantry during rotation. Several studies have investigated the effects of gravity on the sag of these systems but each have limitations. In this study, an easy experiment setup and accurate algorithm have been introduced to characterize and correct for the effect of EPID and gantry sag during arc delivery.
Methods: Three metallic ball bearings were used as markers in the beam: two of them fixed to the gantry head and the third positioned at the isocenter. EPID images were acquired during a 360° gantry rotation in cine imaging mode. The markers were tracked in EPID images and a robust in-house developed MATLAB code was used to analyse the images and find the EPID sag in three directions as well as the EPID + gantry sag by comparison to the reference gantry zero image. The algorithm results were then tested against independent methods. The method was applied to compare the effect in clockwise and counter clockwise gantry rotations and different source-to-detector distances (SDDs). The results were monitored for one linear accelerator over a course of 15 months and six other linear-accelerators from two treatment centers were also investigated using this method. The generalized shift patterns were derived from the data and used in an image registration algorithm to correct for the effect of the mechanical sag in the system. The Gamma evaluation (3%, 3 mm) technique was used to investigate the improvement in alignment of cine EPID images of a fixed field, by comparing both individual images and the sum of images in a series with the reference gantry zero image.
Results: The mechanical sag during gantry rotation was dependent on the gantry angle and was larger in the in-plane direction, although the patterns were not identical for various linear-accelerators. The reproducibility of measurements was within 0.2 mm over a period of 15 months. The direction of gantry rotation and SDD did not affect the results by more than 0.3 mm. Results of independent tests agreed with the algorithm within the accuracy of the measurement tools. When comparing summed images, the percentage of points with Gamma index <1 increased from 85.4% to 94.1% after correcting for the EPID sag, and to 99.3% after correction for gantry + EPID sag.
Conclusions: The measurement method and algorithms introduced in this study use cine-images, are highly accurate, simple, fast, and reproducible. It tests all gantry angles and provides a suitable automatic analysis and correction tool to improve EPID dosimetry and perform comprehensive linac QA for arc treatments.

© 2012 American Association of Physicists in Medicine

ACKNOWLEDGMENTS

This work was supported by the National Health and Medical Research Council Grant (Grant No. 569211). One of the authors (PR) gratefully acknowledges the award of the UNIPRS scholarship from the University of Newcastle.

Article Outline

  1. INTRODUCTION
  2. METHODS AND MATERIALS
    1. Materials
    2. Measurement Methods
      1. Measurement of the EPID sag
      2. Measurement of the combined EPID and gantry sag
    3. Data processing algorithms
      1. Detection algorithm
      2. Correction algorithm
    4. Evaluation of the method
      1. Test of the algorithm results for EPID and gantry sag
      2. Test of corrections for marker misalignment at the isocenter
  3. RESULTS
    1. The combined effect of gantry and EPID sag
    2. Effect of EPID sag
    3. Changes in the SDD
    4. Measurements on other linacs
    5. Evaluation of the method
      1. Test of the algorithm results for EPID and gantry sag
      2. Test of corrections for marker misalignment at the isocenter
    6. Correction algorithm results
  4. DISCUSSION
  5. CONCLUSIONS
Digital Object Identifier
http://dx.doi.org/10.1118/1.3673958

KEYWORDS and PACS

Keywords

biomechanics, biomedical equipment, dosimetry, image registration, linear accelerators, medical image processing, radiation therapy, EPID dosimetry, gantry sag, EPID sag, linac, quality assurance

PACS

  • 87.53.Bn

    Dosimetry/exposure assessment

  • 87.57.nj

    Registration

  • 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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