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The dosimetric impact of inversely optimized arc radiotherapy plan modulation for real-time dynamic MLC tracking delivery

Falk, Marianne ; Larsson, Tobias LU ; Keall, Paul ; Chul Cho, Byung ; Aznar, Marianne ; Korreman, Stine ; Poulsen, Per and Af Rosenschöld, Per Munck LU orcid (2012) In Medical Physics 39(3). p.1588-1594
Abstract

Purpose: Real-time dynamic multileaf collimator (MLC) tracking for management of intrafraction tumor motion can be challenging for highly modulated beams, as the leaves need to travel far to adjust for target motion perpendicular to the leaf travel direction. The plan modulation can be reduced by using a leaf position constraint (LPC) that reduces the difference in the position of adjacent MLC leaves in the plan. The purpose of this study was to investigate the impact of the LPC on the quality of inversely optimized arc radiotherapy plans and the effect of the MLC motion pattern on the dosimetric accuracy of MLC tracking delivery. Specifically, the possibility of predicting the accuracy of MLC tracking delivery based on the plan... (More)

Purpose: Real-time dynamic multileaf collimator (MLC) tracking for management of intrafraction tumor motion can be challenging for highly modulated beams, as the leaves need to travel far to adjust for target motion perpendicular to the leaf travel direction. The plan modulation can be reduced by using a leaf position constraint (LPC) that reduces the difference in the position of adjacent MLC leaves in the plan. The purpose of this study was to investigate the impact of the LPC on the quality of inversely optimized arc radiotherapy plans and the effect of the MLC motion pattern on the dosimetric accuracy of MLC tracking delivery. Specifically, the possibility of predicting the accuracy of MLC tracking delivery based on the plan modulation was investigated. Methods: Inversely optimized arc radiotherapy plans were created on CT-data of three lung cancer patients. For each case, five plans with a single 358° arc were generated with LPC priorities of 0 (no LPC), 0.25, 0.5, 0.75, and 1 (highest possible LPC), respectively. All the plans had a prescribed dose of 2 Gy× 30, used 6 MV, a maximum dose rate of 600 MU/min and a collimator angle of 45° or 315. To quantify the plan modulation, an average adjacent leaf distance (ALD) was calculated by averaging the mean adjacent leaf distance for each control point. The linear relationship between the plan quality [i.e., the calculated dose distributions and the number of monitor units (MU)] and the LPC was investigated, and the linear regression coefficient as well as a two tailed confidence level of 95% was used in the evaluation. The effect of the plan modulation on the performance of MLC tracking was tested by delivering the plans to a cylindrical diode array phantom moving with sinusoidal motion in the superior-inferior direction with a peak-to-peak displacement of 2 cm and a cycle time of 6 s. The delivery was adjusted to the target motion using MLC tracking, guided in real-time by an infrared optical system. The dosimetric results were evaluated using gamma index evaluation with static target measurements as reference. Results: The plan quality parameters did not depend significantly on the LPC (p ≥0.066), whereas the ALD depended significantly on the LPC (p <0.001). The gamma index failure rate depended significantly on the ALD, weighted to the percentage of the beam delivered in each control point of the plan (ALDw) when MLC tracking was used (p <0.001), but not for delivery without MLC tracking (p ≥0.342). The gamma index failure rate with the criteria of 2% and 2 mm was decreased from >33.9% without MLC tracking to <31.4% (LPC 0) and <2.2% (LPC 1) with MLC tracking. Conclusions: The results indicate that the dosimetric robustness of MLC tracking delivery of an inversely optimized arc radiotherapy plan can be improved by incorporating leaf position constraints in the objective function without otherwise affecting the plan quality. The dosimetric robustness may be estimated prior to delivery by evaluating the ALDw of the plan.

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author
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publishing date
type
Contribution to journal
publication status
published
subject
keywords
arc therapy, intra-fraction motion, tumor tracking
in
Medical Physics
volume
39
issue
3
pages
1588 - 1594
publisher
American Association of Physicists in Medicine
external identifiers
  • pmid:22380391
  • scopus:84857804807
ISSN
0094-2405
DOI
10.1118/1.3685583
language
English
LU publication?
no
additional info
Funding Information: The authors wish to thank Pekka Uusitalo, Janne Nord, and Jarkko Peltola from Varian Medical Systems, Helsinki, Finland, for supplying the treatment planning system used in this study. The authors also thank Stephan Erbel and Cornel Schlossbauer from Brainlab, Feldkirchen, Germany for supporting the ExacTrac guidance of the MLC tracking delivery. The authors particularly thank Dan Ruan from Stanford Cancer Center, Stanford, CA for development of the MLC tracking code and Herbert Cattell from Varian Medical Systems, Palo Alto, CA for substantial contributions to the DMLC tracking program. Thanks to Thomas Carlslund and Mikael Olsen from Rigshospitalet, Copenhagen, Denmark, for technical support during the installation of the tracking system at Rigshospitalet. Finally thanks to Ivan Vogelius and Gitte Persson from Rigshospitalet, Copenhagen, Denmark for technical assistance and to Julie Baz University of Sydney, Sydney, Australia, for reviewing the manuscript and improving the clarity. Research support from Varian Medical Systems, the Niels Bohr Institute (University of Copenhagen), US NIH/NCI Grant No. CA93626 and from Snedkermester Sophus Jacobsen og hustru Astrid Jacobsens Fond are gratefully acknowledged. The manuscript was reviewed but not commented on by Varian Medical Systems prior to submission. ®
id
03c27e34-e297-4c48-9cc6-cc78b87b1c4b
date added to LUP
2023-07-19 17:12:15
date last changed
2024-01-05 03:34:13
@article{03c27e34-e297-4c48-9cc6-cc78b87b1c4b,
  abstract     = {{<p>Purpose: Real-time dynamic multileaf collimator (MLC) tracking for management of intrafraction tumor motion can be challenging for highly modulated beams, as the leaves need to travel far to adjust for target motion perpendicular to the leaf travel direction. The plan modulation can be reduced by using a leaf position constraint (LPC) that reduces the difference in the position of adjacent MLC leaves in the plan. The purpose of this study was to investigate the impact of the LPC on the quality of inversely optimized arc radiotherapy plans and the effect of the MLC motion pattern on the dosimetric accuracy of MLC tracking delivery. Specifically, the possibility of predicting the accuracy of MLC tracking delivery based on the plan modulation was investigated. Methods: Inversely optimized arc radiotherapy plans were created on CT-data of three lung cancer patients. For each case, five plans with a single 358° arc were generated with LPC priorities of 0 (no LPC), 0.25, 0.5, 0.75, and 1 (highest possible LPC), respectively. All the plans had a prescribed dose of 2 Gy× 30, used 6 MV, a maximum dose rate of 600 MU/min and a collimator angle of 45° or 315. To quantify the plan modulation, an average adjacent leaf distance (ALD) was calculated by averaging the mean adjacent leaf distance for each control point. The linear relationship between the plan quality [i.e., the calculated dose distributions and the number of monitor units (MU)] and the LPC was investigated, and the linear regression coefficient as well as a two tailed confidence level of 95% was used in the evaluation. The effect of the plan modulation on the performance of MLC tracking was tested by delivering the plans to a cylindrical diode array phantom moving with sinusoidal motion in the superior-inferior direction with a peak-to-peak displacement of 2 cm and a cycle time of 6 s. The delivery was adjusted to the target motion using MLC tracking, guided in real-time by an infrared optical system. The dosimetric results were evaluated using gamma index evaluation with static target measurements as reference. Results: The plan quality parameters did not depend significantly on the LPC (p ≥0.066), whereas the ALD depended significantly on the LPC (p &lt;0.001). The gamma index failure rate depended significantly on the ALD, weighted to the percentage of the beam delivered in each control point of the plan (ALD<sub>w</sub>) when MLC tracking was used (p &lt;0.001), but not for delivery without MLC tracking (p ≥0.342). The gamma index failure rate with the criteria of 2% and 2 mm was decreased from &gt;33.9% without MLC tracking to &lt;31.4% (LPC 0) and &lt;2.2% (LPC 1) with MLC tracking. Conclusions: The results indicate that the dosimetric robustness of MLC tracking delivery of an inversely optimized arc radiotherapy plan can be improved by incorporating leaf position constraints in the objective function without otherwise affecting the plan quality. The dosimetric robustness may be estimated prior to delivery by evaluating the ALD<sub>w</sub> of the plan.</p>}},
  author       = {{Falk, Marianne and Larsson, Tobias and Keall, Paul and Chul Cho, Byung and Aznar, Marianne and Korreman, Stine and Poulsen, Per and Af Rosenschöld, Per Munck}},
  issn         = {{0094-2405}},
  keywords     = {{arc therapy; intra-fraction motion; tumor tracking}},
  language     = {{eng}},
  number       = {{3}},
  pages        = {{1588--1594}},
  publisher    = {{American Association of Physicists in Medicine}},
  series       = {{Medical Physics}},
  title        = {{The dosimetric impact of inversely optimized arc radiotherapy plan modulation for real-time dynamic MLC tracking delivery}},
  url          = {{http://dx.doi.org/10.1118/1.3685583}},
  doi          = {{10.1118/1.3685583}},
  volume       = {{39}},
  year         = {{2012}},
}