Program Information
A Novel Leaf Sequencing Optimization Algorithm Which Considers Previous Underdose and Overdose Events for MLC Tracking Radiotherapy
E Wisotzky1*, R O'Brien2, P Keall2, (1) Charite Berlin, Berlin, Germany (2) University of Sydney, Sydney, Australia
Presentations
TH-AB-303-4 (Thursday, July 16, 2015) 7:30 AM - 9:30 AM Room: 303
Purpose:
MLC-tracking radiotherapy is complex as the beam pattern needs to be modified due to the planned intensity modulation as well as the real-time target motion. The target motion cannot be planned, therefore the modified beam pattern differs from the original plan and the MLC sequence needs to be replanned online. Current MLC-tracking algorithms use a greedy heuristic in that they optimize for a given time, but ignore past errors. To overcome this problem we have developed and improved an algorithm that minimizes large underdose/overdose regions. Additionally previous underdose/overdose events are taken into account to avoid regions with high quantity of underdose/overdose.
Methods:
We improved the existing MLC motion control algorithm by introducing an underdose/overdose map. This map represents the actual projection of the planned tumor shape and logs occurring dose events at its specific regions. These events have an impact on the dose cost calculation and reduce recurrence of underdose/overdose.
We studied the improvement of the new temporal optimization algorithm in terms of the L1-norm minimization of the sum of overdose/underdose compared to the currently used algorithm. For evaluation we simulated the delivery of 5 conformal and 14 IMRT-plans with 7 3D patient measured tumor motion traces.
Results:
Simulations with conformal shapes showed an improvement of L1-norm up to 8.5% after 100 MLC modification steps. The improvement in terms of L1-norm for a single step increased up to 18.2%.
For IMRT, the improvement was 1.45±0.1% after 100 steps and for a single step the average improvement was 3.28±0.4%. This smaller improvement for IMRT is caused by the fact that the plans are simplified in terms of the shape.
Conclusion:
A novel leaf sequencing optimization algorithm which considers previous dose events for MLC-tracking radiotherapy has been developed and investigated. Reductions in underdose/overdose are observed for conformal and IMRT delivery.
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