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Title: Development and evaluation of aperture-based complexity metrics using film and EPID measurements of static MLC openings

Abstract

Purpose: Complexity metrics have been suggested as a complement to measurement-based quality assurance for intensity modulated radiation therapy (IMRT) and volumetric modulated arc therapy (VMAT). However, these metrics have not yet been sufficiently validated. This study develops and evaluates new aperture-based complexity metrics in the context of static multileaf collimator (MLC) openings and compares them to previously published metrics. Methods: This study develops the converted aperture metric and the edge area metric. The converted aperture metric is based on small and irregular parts within the MLC opening that are quantified as measured distances between MLC leaves. The edge area metric is based on the relative size of the region around the edges defined by the MLC. Another metric suggested in this study is the circumference/area ratio. Earlier defined aperture-based complexity metrics—the modulation complexity score, the edge metric, the ratio monitor units (MU)/Gy, the aperture area, and the aperture irregularity—are compared to the newly proposed metrics. A set of small and irregular static MLC openings are created which simulate individual IMRT/VMAT control points of various complexities. These are measured with both an amorphous silicon electronic portal imaging device and EBT3 film. The differences between calculated and measured dose distributions are evaluatedmore » using a pixel-by-pixel comparison with two global dose difference criteria of 3% and 5%. The extent of the dose differences, expressed in terms of pass rate, is used as a measure of the complexity of the MLC openings and used for the evaluation of the metrics compared in this study. The different complexity scores are calculated for each created static MLC opening. The correlation between the calculated complexity scores and the extent of the dose differences (pass rate) are analyzed in scatter plots and using Pearson’s r-values. Results: The complexity scores calculated by the edge area metric, converted aperture metric, circumference/area ratio, edge metric, and MU/Gy ratio show good linear correlation to the complexity of the MLC openings, expressed as the 5% dose difference pass rate, with Pearson’s r-values of −0.94, −0.88, −0.84, −0.89, and −0.82, respectively. The overall trends for the 3% and 5% dose difference evaluations are similar. Conclusions: New complexity metrics are developed. The calculated scores correlate to the complexity of the created static MLC openings. The complexity of the MLC opening is dependent on the penumbra region relative to the area of the opening. The aperture-based complexity metrics that combined either the distances between the MLC leaves or the MLC opening circumference with the aperture area show the best correlation with the complexity of the static MLC openings.« less

Authors:
 [1]; ;  [2]
  1. Department of Radiation Physics, University of Gothenburg, Göteborg 413 45 (Sweden)
  2. Department of Therapeutic Radiation Physics, Sahlgrenska University Hospital, Göteborg 413 45 (Sweden)
Publication Date:
OSTI Identifier:
22413610
Resource Type:
Journal Article
Journal Name:
Medical Physics
Additional Journal Information:
Journal Volume: 42; Journal Issue: 7; Other Information: (c) 2015 American Association of Physicists in Medicine; Country of input: International Atomic Energy Agency (IAEA); Journal ID: ISSN 0094-2405
Country of Publication:
United States
Language:
English
Subject:
60 APPLIED LIFE SCIENCES; APERTURES; COLLIMATORS; COMPARATIVE EVALUATIONS; FILM DOSIMETRY; QUALITY ASSURANCE; RADIATION DOSE DISTRIBUTIONS; RADIATION DOSES; RADIOTHERAPY

Citation Formats

Götstedt, Julia, Karlsson Hauer, Anna, and Bäck, Anna. Development and evaluation of aperture-based complexity metrics using film and EPID measurements of static MLC openings. United States: N. p., 2015. Web. doi:10.1118/1.4921733.
Götstedt, Julia, Karlsson Hauer, Anna, & Bäck, Anna. Development and evaluation of aperture-based complexity metrics using film and EPID measurements of static MLC openings. United States. https://doi.org/10.1118/1.4921733
Götstedt, Julia, Karlsson Hauer, Anna, and Bäck, Anna. 2015. "Development and evaluation of aperture-based complexity metrics using film and EPID measurements of static MLC openings". United States. https://doi.org/10.1118/1.4921733.
@article{osti_22413610,
title = {Development and evaluation of aperture-based complexity metrics using film and EPID measurements of static MLC openings},
author = {Götstedt, Julia and Karlsson Hauer, Anna and Bäck, Anna},
abstractNote = {Purpose: Complexity metrics have been suggested as a complement to measurement-based quality assurance for intensity modulated radiation therapy (IMRT) and volumetric modulated arc therapy (VMAT). However, these metrics have not yet been sufficiently validated. This study develops and evaluates new aperture-based complexity metrics in the context of static multileaf collimator (MLC) openings and compares them to previously published metrics. Methods: This study develops the converted aperture metric and the edge area metric. The converted aperture metric is based on small and irregular parts within the MLC opening that are quantified as measured distances between MLC leaves. The edge area metric is based on the relative size of the region around the edges defined by the MLC. Another metric suggested in this study is the circumference/area ratio. Earlier defined aperture-based complexity metrics—the modulation complexity score, the edge metric, the ratio monitor units (MU)/Gy, the aperture area, and the aperture irregularity—are compared to the newly proposed metrics. A set of small and irregular static MLC openings are created which simulate individual IMRT/VMAT control points of various complexities. These are measured with both an amorphous silicon electronic portal imaging device and EBT3 film. The differences between calculated and measured dose distributions are evaluated using a pixel-by-pixel comparison with two global dose difference criteria of 3% and 5%. The extent of the dose differences, expressed in terms of pass rate, is used as a measure of the complexity of the MLC openings and used for the evaluation of the metrics compared in this study. The different complexity scores are calculated for each created static MLC opening. The correlation between the calculated complexity scores and the extent of the dose differences (pass rate) are analyzed in scatter plots and using Pearson’s r-values. Results: The complexity scores calculated by the edge area metric, converted aperture metric, circumference/area ratio, edge metric, and MU/Gy ratio show good linear correlation to the complexity of the MLC openings, expressed as the 5% dose difference pass rate, with Pearson’s r-values of −0.94, −0.88, −0.84, −0.89, and −0.82, respectively. The overall trends for the 3% and 5% dose difference evaluations are similar. Conclusions: New complexity metrics are developed. The calculated scores correlate to the complexity of the created static MLC openings. The complexity of the MLC opening is dependent on the penumbra region relative to the area of the opening. The aperture-based complexity metrics that combined either the distances between the MLC leaves or the MLC opening circumference with the aperture area show the best correlation with the complexity of the static MLC openings.},
doi = {10.1118/1.4921733},
url = {https://www.osti.gov/biblio/22413610}, journal = {Medical Physics},
issn = {0094-2405},
number = 7,
volume = 42,
place = {United States},
year = {Wed Jul 15 00:00:00 EDT 2015},
month = {Wed Jul 15 00:00:00 EDT 2015}
}