A field-size effect of physical doses was studied in scanning irradiation with carbon ions. For the target volumes of , , and , the doses along the beam axis within the spread-out Bragg peaks reduced to 99.4%, 98.2%, and 96.0% of the dose for the target of , respectively. The present study revealed that the observed reductions can be compensated for by adopting the three-Gaussian form of lateral dose distributions for the pencil beam model used in the treatment planning system. The parameters describing the form were determined through the irradiation experiments making flat concentric squared frames with a scanned carbon beam. Since utilizing the three-Gaussian model in the dose optimization loop is at present time consuming, the correction for the field-size effect should be implemented as a “predicted-dose scaling factor.” The validity of this correction method was confirmed through the irradiation of a target of .