Background: In carbon ion radiotherapy (CIR) for esophageal cancer, organ and target motion is a major challenge\nfor treatment planning due to potential range deviations. This study intends to analyze the impact of intrafractional\nvariations on dosimetric parameters and to identify favourable settings for robust treatment plans.\nMethods: We contoured esophageal boost volumes in different organ localizations for four patients and calculated\nCIR-plans with 13 different beam geometries on a free-breathing CT. Forward calculation of these plans was\nperformed on 4D-CT datasets representing seven different phases of the breathing cycle. Plan quality was assessed\nfor each patient and beam configuration.\nResults: Target volume coverage was adequate for all settings in the baseline CIR-plans (V95 > 98% for two-beam\ngeometries, > 94% for one-beam geometries), but reduced on 4D-CT plans (V95 range 50ââ?¬â??95%). Sparing of the\norgans at risk (OAR) was adequate, but range deviations during the breathing cycle partly caused critical, maximum\ndoses to spinal cord up to 3.5x higher than expected. There was at least one beam configuration for each patient\nwith appropriate plan quality.\nConclusions: Despite intrafractional motion, CIR for esophageal cancer is possible with robust treatment plans\nwhen an individually optimized beam setup is selected depending on tumor size and localization.
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