Journal
PHYSICS IN MEDICINE AND BIOLOGY
Volume 62, Issue 11, Pages 4254-4272Publisher
IOP PUBLISHING LTD
DOI: 10.1088/1361-6560/aa5c12
Keywords
intensity-modulated proton therapy; prostate cancer; online-adaptive proton therapy; online treatment planning
Funding
- ZonMw, the Netherlands Organization for Health Research and Development [104003012]
- Varian Medical Systems
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Proton therapy is very sensitive to daily density changes along the pencil beam paths. The purpose of this study is to develop and evaluate an automated method for adaptation of IMPT plans to compensate for these daily tissue density variations. A two-step restoration method for 'densities-of-the-day' was created: (1) restoration of spot positions (Bragg peaks) by adapting the energy of each pencil beam to the new water equivalent path length; and (2) re-optimization of pencil beam weights by minimizing the dosimetric difference with the planned dose distribution, using a fast and exact quadratic solver. The method was developed and evaluated using 8-10 repeat CT scans of 10 prostate cancer patients. Experiments demonstrated that giving a high weight to the PTV in the re-optimization resulted in clinically acceptable restorations. For all scans we obtained V-95% >= 98% and V-107% <= 2%. For the bladder, the differences between the restored and the intended treatment plan were below +2 Gy and +2%-point. The rectum differences were below +2 Gy and +2%-point for 90% of the scans. In the remaining scans the rectum was filled with air, which partly overlapped with the PTV. The air cavity distorted the Bragg peak resulting in less favorable rectum doses.
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