We are planning to start carbon ion scanning irradiation for pelvic region. The whole pelvic motion mainly cause by breathing induces the interplay effect as much as the target motion. In order to improve this problem, we applied a repainting beam delivery. In this study some treatment plans with repainting for pelvic treatment cases were compared with the measured dose distribution using motion phantom.
Two pelvic region tumor patients were selected for this study. We made the treatment plan with two treatment field for each plan with scanning method with repainting. The number of repainting was mainly used 6 times.  We measured 2D dose distribution in center of target depth orthogonal to beam axis by using 2D ionization chamber array (OCTAVIUS Detector 1500 XDR). It was mounted on a motion phantom (Dynamic platform model 008PL).  The phantom motion was sinusoidal curve with 3 mm amplitude, 3 second cycle in inferior/superior direction. The dose distributions with and without phantom motion were verified by the gamma passing rates by use of gamma analysis with reference to treatment planning.  The acceptance criteria were 2 %/ 2 mm for a dose difference and a distance to agreement. 
Decrease of gamma passing rates with motion and no repainting cases were 4.5 - 12.7%.  While decrease of gamma passing rates with motion and 4-6 repainting cases were 3.5 - 9.6%.
Scanning irradiation with repainting can be improved the dose distribution change due to target motion.

Feasibility study of repainting using scanning beam to improve interplay effect in pelvic region carbon ion radiotherapy