Efficient interplay effect mitigation for proton pencil beam scanning by spot-adapted layered repainting evenly spread out over the full breathing cycle

Publication date: Available online 4 October 2017
Source:International Journal of Radiation Oncology*Biology*Physics
Author(s): Per Rugaard Poulsen, John Eley, Ulrich Langner, Charles B. Simone, Katja Langen
PurposeTo develop and implement a practical repainting method for efficient interplay effect mitigation in proton pencil beam scanning (PBS).MethodsA new flexible repainting scheme with spot-adapted numbers of repaintings evenly spread out over the whole breathing cycle (here assumed to be 4s) was developed. Twelve fields from five thoracic and upper abdominal PBS plans were delivered three times with the new repainting scheme to an ion chamber array on a motion stage: once static and twice with 4s,3cm peak-to-peak sinusoidal motion with delivery started at maximum inhale and maximum exhale. For comparison, all dose measurements were repeated with no repainting and with 8 repaintings. For each motion experiment, the 3%/3mm gamma pass rate was calculated using the motion-convolved static dose as reference. Simulations were first validated with the experiments and then used to extend the study to 0-5cm motion magnitude, 2-6s motion periods, patient-measured liver tumor motion, and 1-6 fraction treatments. The effect of the proposed method was evaluated for the five clinical cases by 4D dose reconstruction in the planning 4DCT scan. The target homogeneity index, HI=(D2-D98)/Dmean, of a single fraction delivery was reported.ResultsThe gamma pass rates were 59.6±9.7% (no repainting), 76.5±10.8% (8 repaintings), and 92.4±3.8% (new repainting scheme). Simulations reproduced the experimental gamma pass rates with 1.3% root-mean-square error and demonstrated largely improved gamma pass rates with the new repainting scheme for all investigated motion scenarios. One- and two-fraction deliveries with the new repainting scheme had similar gamma pass rates as 3-4 and 6-fraction deliveries with 8 repaintings. The mean HI for the five test cases was 14.2% (no repainting), 13.7% (8 repaintings), 12.0% (new repainting scheme), and 11.6% (4D dose without interplay effects).ConclusionsA novel repainting strategy for efficient interplay effect mitigation was proposed, implemented, and shown to outperform conventional repainting in experiments, simulations and dose reconstructions. This strategy may allow for safe and more optimal clinical delivery of thoracic and abdominal proton PBS and better facilitate hypofractionated and stereotactic treatments.



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