Improved Multi-Criteria Optimization for Intensity Modulated Proton Therapy Using Iterative Resampling of Randomly Placed Pencil-Beams
S van de Water1*, A Kraan1, S Breedveld1, D Teguh1, T Madden2, H Kooy2, B Heijmen1, M Hoogeman1, (1) Erasmus MC - Daniel den Hoed Cancer Center, Rotterdam, NL, (2) Massachusetts General Hospital, Boston, MATH-A-213AB-10 Thursday 8:00:00 AM - 9:55:00 AM Room: 213AB
Purpose: In treatment planning for spot-scanned intensity modulated proton therapy (IMPT), a fine-grid pencil-beam distribution is used to guarantee high-quality treatment plans. This may lead to very large optimization problems with excessive planning times, especially for large target volumes. To improve the trade-off between plan quality and optimization times, we have developed a new pencil-beam placement method called 'resampling'.
Methods: Resampling is based on repeated multi-criteria optimizations. In each iteration, a new sample of randomly placed pencil-beams is optimized together with favorable (high-weight) pencil-beams of the previous solution. In previous studies, resampling was successfully applied for CyberKnife plan optimization. In this study, IMPT resampling plans for four head-and-neck cancer patients were compared with traditional IMPT plans, generated using a regular grid with typical spacing (5x5x4mm³). For resampling, sample sizes of 3000, 5000, 7000, and 9000 pencil-beams per iteration were tested, storing the (intermediate) plans after each of in total 7 iterations. The same dose prescription and 3-beam arrangement was used in all plans.
Results: For similar optimization times and target coverage, resampling resulted in lower doses to organs-at-risk than the traditional approach. Mean doses were reduced by 0.4Gy on average (1.7%, range: 0Gy-0.9Gy) for both parotid glands, by 2.5Gy (7.1%, range 0.5Gy- 3.8Gy) for both submandibular glands and by 3.3Gy (9.6%, range: 2.3Gy-4Gy) for the swallowing muscles. Maximum doses to spinal cord and brain stem were on average reduced by 4.1Gy (25.7%, range: 2.3Gy-5.8Gy) and 3.4Gy (31.4%, range: -0.4Gy-9.5Gy), respectively. For comparable doses to organs-at-risk, optimization time was reduced by 38.7% on average (range: 6.7%-54.8%), being proportional to the target volume.
Conclusions: Pencil-beam resampling is an efficient method for IMPT plan optimization, allowing for clinically relevant improvements in plan quality and/or planning time, especially for large problem sizes. It opens possibilities for dealing with large-scale problems such as beam-angle optimization.