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Medical Physics / Volume 38 / Issue 12 / RADIATION THERAPY PHYSICS

Med. Phys. 38, 6730 (2011); http://dx.doi.org/10.1118/1.3659707 (12 pages)

Toward a better understanding of the gamma index: Investigation of parameters with a surface-based distance method a

a Partly presented at the 2010 Annual Meeting of the American Association of Physicists in Medicine (AAPM), July 18–22, Philadelphia, PA.
Heng Li, Lei Dong, Lifei Zhang, James N. Yang, Michael T. Gillin, and X. Ronald Zhu

Department of Radiation Physics, The University of Texas MD Anderson Cancer Center, Houston, Texas 77030

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(Received 25 July 2011; accepted 20 October 2011; revised 24 September 2011; published online 30 November 2011)

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Purpose: The purpose of this work was to clarify the interactions between the parameters used in the γ index with the surface-based distance method, which itself can be viewed as a generalized version of the γ index. The examined parameters included the distance to agreement (DTA)/dose difference (DD) criteria, the percentage used as a passing criterion, and the passing percentage for given DTA/DD criteria. The specific aims of our work were (1) to understand the relationships between the parameters used in the γ index, (2) to determine the detection limit, or the minimum detectable error, of the γ index with a given set of parameters, and (3) to establish a procedure to determine parameters that are consistent with the capacity of an IMRT QA system.
Methods: The surface-based distance technique with dose gradient factor was derived, and then the relationship between surface-based distance and γ index was established. The dose gradient factor for plans and measurements of 10 IMRT patients, 10 spine stereotactic radiosurgery (SRS) patients, and 3 Radiological Physics Center (RPC) head and neck phantom were calculated and evaluated. The detection limits of the surface-based distance and γ index methods were examined by introducing known shifts to the 10 IMRT plans.
Results: The means of the dose gradient factors were 0.434 mm/% and 0.956 mm/% for the SRS and IMRT plans, respectively. Key quantities (including the mean and 90th and 99th percentiles of the distance distribution) of the surface-based distance distribution between two dose distributions were linearly proportional to the actual shifts. However, the passing percentage of the γ index for a given set of DTA/DD criteria was not associated with the actual shift. For IMRT, using the standard quality assurance criteria of 3 mm/3% DTA/DD and a 90% passing rate, we found that the detection limit of the γ index in terms of global shift was 4.07 mm/4.07 % without noise.
Conclusions: Surface-based distance is a direct measure of the difference between two dose distributions and can be used to evaluate or determine parameters for use in calculating the γ index. The dose gradient factor represents the weighting between spatial and dose shift and should be determined before DTA/DD criteria are set. The authors also present a procedure to determine γ index parameters from measurements.

© 2011 American Association of Physicists in Medicine

ACKNOWLEDGMENTS

The authors thank the dosimetrists, graduate students, and physicist assistants who generated the plans, created the planar dose distributions, and made the measurements. Tamara Locke, Kathryn Carnes and Zachary Bohannan of the Department of Scientific Publications at The University of Texas MD Anderson Cancer Center are greatly appreciated for their editorial review of this manuscript. This research is supported in part by the National Institutes of Health through MD Anderson’s Cancer Center Support Grant CA016672.

Article Outline

  1. INTRODUCTION
  2. METHODS AND MATERIALS
    1. Surface-based dose comparison
    2. Relationship between surface-based distance and the γ index
    3. Materials
    4. Summary of analysis
  3. RESULTS
    1. Dose gradient factor
    2. Surface-based distance
    3. Simulation for IMRT planar dose distribution
  4. DISCUSSION
  5. CONCLUSIONS
Digital Object Identifier
http://dx.doi.org/10.1118/1.3659707

KEYWORDS and PACS

Keywords

dosimetry, phantoms, quality assurance, radiation therapy, radiology, surgery, γ index, surface-based distance, IMRT QA, DTA, DD

PACS

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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Medical Physics is an official science journal of the AAPM and of the COMP/CCPM/IOMP
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Departments of Radiology, Radiation Oncology, Biophysics, Community and Public Health, Medical College of Wisconsin
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