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SU‐E‐T‐98: VMAT Quality Assurance: Improving the Angular Correction Factors for Ion Chamber Arrays

Publication ,  Conference
Kishore, M; O'daniel, J; Bowsher, J; Yin, F
Published in: Medical Physics
January 1, 2011

Purpose: Two‐dimensional (2D) detector arrays for QA of volumetric‐modulated‐arc‐therapy (VMAT) plans show angular dependence. An angular correction‐factor‐table (CFs) minimize this effect, but assumes: (1) the response of all ion chambers (IC) is identical per angle, (2) the ICA response from 0–180° = 180–360°, and (3) the ICA response is independent of the direction of rotation. We investigated whether these assumptions are correct and developed improved CFs. Methods: Measurements were done with a 2D ICA (Matrixx®, IBA Dosimetry). Dose was calculated with the AAA algorithm (Eclipse v8.6, Varian Medical Systems). Default angular CF tables were available for 0–180° and 0–360°. Custom CFs were created by dividing the calculated dose to each IC by the measured dose from static open‐fields delivered every 5°, for both coronal (couch included) and 2 sagittal (couch avoided) orientations. The CFs were tested with open‐field arcs and VMAT plans, comparing results using gamma analysis (3%, 3mm). Additionally, measurements of 1cm‐wide fields delivered clockwise (CW) vs. counter‐clockwise (CCW) were compared to examine effects of ICA structure. Results: The angular response of individual IC significantly varies (1stdev<=4.6%). The response from 0–180° vs. 180–360° is significantly different (paired t‐test, p<0.0001). Custom CFs generated greater agreement between measurement and calculation for open‐field arcs and VMAT plans than the default CFs. Custom CFs also improved agreement compared to no CF for open‐field arcs, but not for VMAT plans. The CW vs. CCW tests demonstrated that the dose in the penumbra region of narrow fields is affected by the direction of rotation (<=1%); the cumulative effect of this may be significant for VMAT. Conclusion: Custom CF tables, using measurements from 0–360° and individual chamber CFs rather than a global CF per angle, can improve the measurement accuracy of an ICA. Even with these thorough corrections, discrepancies were still observed for VMAT plans. © 2011, American Association of Physicists in Medicine. All rights reserved.

Duke Scholars

Published In

Medical Physics

DOI

ISSN

0094-2405

Publication Date

January 1, 2011

Volume

38

Issue

6

Start / End Page

3508

Related Subject Headings

  • Nuclear Medicine & Medical Imaging
  • 5105 Medical and biological physics
  • 4003 Biomedical engineering
  • 1112 Oncology and Carcinogenesis
  • 0903 Biomedical Engineering
  • 0299 Other Physical Sciences
 

Citation

APA
Chicago
ICMJE
MLA
NLM
Kishore, M., O’daniel, J., Bowsher, J., & Yin, F. (2011). SU‐E‐T‐98: VMAT Quality Assurance: Improving the Angular Correction Factors for Ion Chamber Arrays. In Medical Physics (Vol. 38, p. 3508). https://doi.org/10.1118/1.3612049
Kishore, M., J. O’daniel, J. Bowsher, and F. Yin. “SU‐E‐T‐98: VMAT Quality Assurance: Improving the Angular Correction Factors for Ion Chamber Arrays.” In Medical Physics, 38:3508, 2011. https://doi.org/10.1118/1.3612049.
Kishore M, O’daniel J, Bowsher J, Yin F. SU‐E‐T‐98: VMAT Quality Assurance: Improving the Angular Correction Factors for Ion Chamber Arrays. In: Medical Physics. 2011. p. 3508.
Kishore, M., et al. “SU‐E‐T‐98: VMAT Quality Assurance: Improving the Angular Correction Factors for Ion Chamber Arrays.” Medical Physics, vol. 38, no. 6, 2011, p. 3508. Scopus, doi:10.1118/1.3612049.
Kishore M, O’daniel J, Bowsher J, Yin F. SU‐E‐T‐98: VMAT Quality Assurance: Improving the Angular Correction Factors for Ion Chamber Arrays. Medical Physics. 2011. p. 3508.

Published In

Medical Physics

DOI

ISSN

0094-2405

Publication Date

January 1, 2011

Volume

38

Issue

6

Start / End Page

3508

Related Subject Headings

  • Nuclear Medicine & Medical Imaging
  • 5105 Medical and biological physics
  • 4003 Biomedical engineering
  • 1112 Oncology and Carcinogenesis
  • 0903 Biomedical Engineering
  • 0299 Other Physical Sciences