Abstract Background and purpose Scanned-beam interplay with the intrafraction target motion may result in dose deterioration in particle therapy. The magnitude of this effect and the possibilities to mitigate it were investigated for carbon ion prostate treatments. Methods and materials For 12 prostate cases, 9 carbon ion treatment plans were prepared using 3 scanned-beam settings (spot sizes of 6, 7 and 9mm and, respectively, raster pitches of 2, 2 and 3mm) for 3 planning margins (3, 6 and 9mm). Plans were recomputed in presence of 5 intrafraction prostate motion scenarios with and without intra-beam motion compensation. Results For 6mm margin and 7mm spot, the median (max) CTV D95% change was −0.2 (−2.6) pp (percentual points) with pure drift motion, −3.8 (−6.0) pp and −2.8 (−3.1) pp in transient motion scenarios and −4.8 (−7.7) pp and −1.8 (−5.7) pp in mixed motion scenarios. No particular raster setting brought distinct advantage, while planning margin expansion showed statistically significant effects for drift-dominated scenarios. Intra-beam motion compensation yielded improved CTV coverage. Conclusion Intrafraction prostate motion can lead to marked target coverage deterioration, dependent on individual motion patterns, which can be only partially avoided through planning-time countermeasures. Among possible delivery-time countermeasures, intra-beam motion compensation is capable of improving target coverage.