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Medical Physics / Volume 38 / Issue 2 / RADIATION THERAPY PHYSICS

Med. Phys. 38, 1037 (2011); http://dx.doi.org/10.1118/1.3544657 (8 pages)

Per-beam, planar IMRT QA passing rates do not predict clinically relevant patient dose errors a

a Conflict of interest: Dr. Nelms serves as a paid consultant to Sun Nuclear Corporation. However, this work was neither funded nor requested as part of that consultancy.
Benjamin E. Nelms

Canis Lupus LLC and Department of Human Oncology, University of Wisconsin, Merrimac, Wisconsin 53561

Heming Zhen

Department of Medical Physics, University of Wisconsin, Madison, Wisconsin 53705

Wolfgang A. Tomé

Departments of Human Oncology, Medical Physics, and Biomedical Engineering, University of Wisconsin, Madison, Wisconsin 53792

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(Received 29 September 2010; accepted 30 December 2010; revised 28 December 2010; published online 31 January 2011)

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Purpose: The purpose of this work is to determine the statistical correlation between per-beam, planar IMRT QA passing rates and several clinically relevant, anatomy-based dose errors for per-patient IMRT QA. The intent is to assess the predictive power of a common conventional IMRT QA performance metric, the Gamma passing rate per beam.
Methods: Ninety-six unique data sets were created by inducing four types of dose errors in 24 clinical head and neck IMRT plans, each planned with 6 MV Varian 120-leaf MLC linear accelerators using a commercial treatment planning system and step-and-shoot delivery. The error-free beams/plans were used as “simulated measurements” (for generating the IMRT QA dose planes and the anatomy dose metrics) to compare to the corresponding data calculated by the error-induced plans. The degree of the induced errors was tuned to mimic IMRT QA passing rates that are commonly achieved using conventional methods.
Results: Analysis of clinical metrics (parotid mean doses, spinal cord max and D1cc, CTV D95, and larynx mean) vs IMRT QA Gamma analysis (3%/3 mm, 2/2, 1/1) showed that in all cases, there were only weak to moderate correlations (range of Pearson’s r-values: −0.295 to 0.653). Moreover, the moderate correlations actually had positive Pearson’s r-values (i.e., clinically relevant metric differences increased with increasing IMRT QA passing rate), indicating that some of the largest anatomy-based dose differences occurred in the cases of high IMRT QA passing rates, which may be called “false negatives.” The results also show numerous instances of false positives or cases where low IMRT QA passing rates do not imply large errors in anatomy dose metrics. In none of the cases was there correlation consistent with high predictive power of planar IMRT passing rates, i.e., in none of the cases did high IMRT QA Gamma passing rates predict low errors in anatomy dose metrics or vice versa.
Conclusions: There is a lack of correlation between conventional IMRT QA performance metrics (Gamma passing rates) and dose errors in anatomic regions-of-interest. The most common acceptance criteria and published actions levels therefore have insufficient, or at least unproven, predictive power for per-patient IMRT QA.

© 2011 American Association of Physicists in Medicine

ACKNOWLEDGMENTS

The authors wish to thank Sun Nuclear Corporation for allowing us the use of a prerelease version of the 3DVH analysis software to generate the patient/anatomy DVHs and metrics. Partial support of this work through a grant (Grant No. R01-109656) from the National Cancer Institute is gratefully acknowledged.

Article Outline

  1. INTRODUCTION
    1. Published studies on IMRT QA acceptance criteria
    2. Are the standard acceptance criteria adequate?
  2. MATERIALS AND METHODS
    1. Experimental design and data acquisition
    2. Correlation of IMRT QA metrics vs clinical metrics
  3. RESULTS
  4. DISCUSSION
  5. CONCLUSIONS
Digital Object Identifier
http://dx.doi.org/10.1118/1.3544657

KEYWORDS and PACS

Keywords

dosimetry, quality assurance, radiation therapy, statistical analysis, IMRT QA, IMRT, quality assurance

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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Figures (8) Tables (2)

Figures (click on thumbnails to view enlargements)

FIG.1
Schematic of the methodology of data generation for the correlation study.

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FIG.2
Sample DVH differences between the induced-error beam models (dashed lines) and the virtual measurement beam models (solid lines). These are the results for patient plan no. 22 (of 24).

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FIG.3
Magnitude of errors in the maximum cord dose and the cord D1cc vs the conventional IMRT QA performance metric of passing rate (%) averaged over all beams per plan, shown for three different sets of Gamma parameters.

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FIG.4
(a) Magnitude of errors in the mean contralateral parotid dose vs conventional IMRT QA performance metric of Gamma passing rate (%) averaged over all beams per plan. (b) Magnitude of errors in the mean ipsilateral parotid dose vs conventional IMRT QA performance metric of Gamma passing rate (%) averaged over all beams per plan. Data are shown for three different sets of Gamma parameters.

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FIG.5
Magnitude of errors in the mean larynx dose vs the conventional IMRT QA performance metric of passing rate (%) averaged over all beams per plan, shown for three different sets of Gamma parameters

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FIG.6
Magnitude of errors in CTV60’s D95 dose vs the conventional IMRT QA performance metric of passing rate (%) averaged over all beams per plan, shown for three different sets of Gamma parameters

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FIG.7
Distribution of errors in two critical anatomy dose metrics for three types of errors induced. (a) Errors in CTV D95 (low range of errors, overall) and (b) errors in contralateral parotid mean dose (higher range of errors).

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FIG.8
Generalized illustration of regions of false negatives (high passing rates despite critical patient dose errors) and false positives (low QA passing rates but with noncritical patient dose errors) when correlating critical patient dose errors to conventional IMRT QA Gamma passing rates. In this schematic, the critical dose error threshold is “E” and the standard acceptance criteria for Gamma passing rates is “C.”

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Tables

Table I. Pearson correlation values (r) and two-tailed p-values correlating the magnitude of anatomy dose errors to three IMRT QA Gamma passing rate performance metrics. Significant p-values (p<0.01) are italicized for emphasis. (Note: The statistically significant correlations have positive r-value (positive slope) indicating that the highest critical dose errors happen at the higher Gamma passing rates.)

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Table II. Range of errors (%) and mean absolute errors (%) for clinically relevant metrics in the case of all plans (N) meeting a specified threshold Gamma passing rate for three sets of Gamma parameters

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