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Real-time dynamic MLC tracking for inversely optimised arc radiotherapy

Falk, Marianne (2009)
Medical Physics Programme
Abstract (Swedish)
Purpose: Radiotherapy of moving tumours is a great challenge in cancer treatment. The moving target will lead to “dose-smearing” of the planned dose distribution and a need for larger margins. To reduce the dose to the surrounding healthy tissue, while still ensuring full coverage of the target, other methods for motion compensation is needed. Inversely optimised arc radiotherapy, where the treatment is delivered continuously during one (or several) gantry rotation(s), requires special consideration when choosing a motion compensation technique. Methods that assert beam holds (e.g. clinically used respiratory gating) are not an option since this compromises the delivery accuracy and efficiency. DMLC tracking is a method that uses the... (More)
Purpose: Radiotherapy of moving tumours is a great challenge in cancer treatment. The moving target will lead to “dose-smearing” of the planned dose distribution and a need for larger margins. To reduce the dose to the surrounding healthy tissue, while still ensuring full coverage of the target, other methods for motion compensation is needed. Inversely optimised arc radiotherapy, where the treatment is delivered continuously during one (or several) gantry rotation(s), requires special consideration when choosing a motion compensation technique. Methods that assert beam holds (e.g. clinically used respiratory gating) are not an option since this compromises the delivery accuracy and efficiency. DMLC tracking is a method that uses the multileaf collimator (MLC) leaves to reposition according to the movements of the target. This may be an adequate motion compensation method for arc radiotherapy since it avoids beam holds during the treatment. The purpose of this study was to evaluate the performance of the DMLC tracking method together with inversely optimized arc radiotherapy with special attention on its dependence on varying peak to peak displacement and collimator angles.Methods and materials: To simulate respiratory movements of the target, a 1D motion platform was programmed to form sinusoidal motion with peak to peak distances of 5 mm, 10 mm, 15 mm, 20 mm and 25 mm in the SI direction and a cycle time of 6 seconds. The DMLC-tracking system used 3D target position information from a Real-time Position Management™ (RPM) System (Varian Medical Systems, Palo Alto, CA) to reposition the MLC leaves to account for target displacements. Varian Medical Systems implementation of the inversely optimized arc radiotherapy technique (RapidArcTM) was used for this study. Two RapidArc plans with collimator angle 45° and one plan with collimator angle 90° were created in EclipseTM (Aria ver. 8.5) and delivered to the Delta-4 dosimetric system (Scandidos) with 6 MV using a Varian 2300ix Clinac. The gantry rotation angle was set to run from 210° to 150° to prevent intersection of the incoming beam and the rails of the couch. Measurements were made with the tracking system connected to the MLC as described above and with the tracking system disconnected, not affecting the plan delivery. Gamma index evaluation (3% dose difference, 3 mm DTA and 2% dose difference, 2 mm DTA), with static target measurements as references, were used to evaluate the results.Results: The measurements where DMLC tracking were used showed a significant improvement of the delivery accuracy and a clear reduction of the dose smearing effects compared to the measurements where no motion compensation was used. No significant decrease in the tracking performance for increasing peak to peak displacement was seen. There was a small trend of better tracking performance with 90° collimator angle than for the measurements with 45°. Due to instabilities in the RPM system, some of the dose profiles were shifted which had a worsening effect on the tracking performance.Conclusion: DMLC tracking can improve the accuracy of RapidArc delivery on a moving target. The method evaluated in this work was independent of the size of the peak to peak displacement of the target and works well with both 45° and 90° collimator angles. The use of a more reliable position monitor with DMLC tracking is warranted. (Less)
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author
Falk, Marianne
supervisor
organization
year
type
H2 - Master's Degree (Two Years)
subject
keywords
Strålterapi
language
English
id
2157091
date added to LUP
2011-09-13 15:00:17
date last changed
2011-09-13 15:00:17
@misc{2157091,
  abstract     = {Purpose: Radiotherapy of moving tumours is a great challenge in cancer treatment. The moving target will lead to “dose-smearing” of the planned dose distribution and a need for larger margins. To reduce the dose to the surrounding healthy tissue, while still ensuring full coverage of the target, other methods for motion compensation is needed. Inversely optimised arc radiotherapy, where the treatment is delivered continuously during one (or several) gantry rotation(s), requires special consideration when choosing a motion compensation technique. Methods that assert beam holds (e.g. clinically used respiratory gating) are not an option since this compromises the delivery accuracy and efficiency. DMLC tracking is a method that uses the multileaf collimator (MLC) leaves to reposition according to the movements of the target. This may be an adequate motion compensation method for arc radiotherapy since it avoids beam holds during the treatment. The purpose of this study was to evaluate the performance of the DMLC tracking method together with inversely optimized arc radiotherapy with special attention on its dependence on varying peak to peak displacement and collimator angles.Methods and materials: To simulate respiratory movements of the target, a 1D motion platform was programmed to form sinusoidal motion with peak to peak distances of 5 mm, 10 mm, 15 mm, 20 mm and 25 mm in the SI direction and a cycle time of 6 seconds. The DMLC-tracking system used 3D target position information from a Real-time Position Management™ (RPM) System (Varian Medical Systems, Palo Alto, CA) to reposition the MLC leaves to account for target displacements. Varian Medical Systems implementation of the inversely optimized arc radiotherapy technique (RapidArcTM) was used for this study. Two RapidArc plans with collimator angle 45° and one plan with collimator angle 90° were created in EclipseTM (Aria ver. 8.5) and delivered to the Delta-4 dosimetric system (Scandidos) with 6 MV using a Varian 2300ix Clinac. The gantry rotation angle was set to run from 210° to 150° to prevent intersection of the incoming beam and the rails of the couch. Measurements were made with the tracking system connected to the MLC as described above and with the tracking system disconnected, not affecting the plan delivery. Gamma index evaluation (3% dose difference, 3 mm DTA and 2% dose difference, 2 mm DTA), with static target measurements as references, were used to evaluate the results.Results: The measurements where DMLC tracking were used showed a significant improvement of the delivery accuracy and a clear reduction of the dose smearing effects compared to the measurements where no motion compensation was used. No significant decrease in the tracking performance for increasing peak to peak displacement was seen. There was a small trend of better tracking performance with 90° collimator angle than for the measurements with 45°. Due to instabilities in the RPM system, some of the dose profiles were shifted which had a worsening effect on the tracking performance.Conclusion: DMLC tracking can improve the accuracy of RapidArc delivery on a moving target. The method evaluated in this work was independent of the size of the peak to peak displacement of the target and works well with both 45° and 90° collimator angles. The use of a more reliable position monitor with DMLC tracking is warranted.},
  author       = {Falk, Marianne},
  keyword      = {Strålterapi},
  language     = {eng},
  note         = {Student Paper},
  title        = {Real-time dynamic MLC tracking for inversely optimised arc radiotherapy},
  year         = {2009},
}