Evaluation of dose distributions and respiratory motion tolerance for layer-stacking conformal carbon-ion radiotherapy.

While layer-stacking irradiation provides a conformal dose distribution, it is vulnerable to respiratory motion. Considering that the motion tolerance has not yet been demonstrated, this study aimed to determine the tolerance level for the amount of target motion. Dose distributions considering motion were simulated for a numerical water phantom using in-house software. Comparisons with measured and simulated physical dose distributions confirmed the validity of the simulation, with gamma analysis showing almost 90% or greater agreement under all conditions with a criterion of 3%/3 mm. The variation in physical dose from static conditions followed a similar trend. Based on the evaluation of the simulated clinical dose uniformity, motion tolerance was derived. The acceptable motion amounts in the lateral direction were 11 mm in respiratory-ungated condition and at least 20 mm with 30% lateral gating at 4 Gy (RBE). In the longitudinal (beam upstream) direction, the acceptable target motion amounts were 3 mm without gating and 6 mm with gating. These results employed worst-case scenarios considering multiple respiratory cycles. In both lateral and longitudinal directions, the motion amounts of 3 mm for non-gating and 5 mm for gating were acceptable. The acceptable target motion amounts improved by 1-9 mm with gating and increased prescribed doses. The dose uniformity and motion tolerance under multiple conditions, although based on a simple system, may be useful for treatment involving target motion in layer-stacking irradiation.