To reduce errors in the carbon beam distribution between the treatment planning system and the actual situation, we evaluated the geometrical accuracy, volume accuracy, water equivalent length (WEL), and treatment planning, and compared the evaluation results of axial and helical scan methods with various scan parameters. The results indicated that both scan methods show good geometrical accuracy for thin slice thickness, but for thick slice images it is easier to understand the phantom as a sphere from the helical as compared with the axial scan. Treatment planning with thin slice thickness provided accurate dose distribution for both scan methods, and the dose distribution on the treatment planning system was almost the same as that in the actual situation. Not all institutes, however, can obtain thin slice CT images, and some used thick-slice CT images in planning. For the axial scan, such thick slice images induced differences in the dose distribution between treatment planning and the actual situation. The helical scan with a small reconstructed increment reduced these differences even with relatively thick CT images. To achieve a more accurate dose distribution, radiation therapy planning should be performed using thin slice thickness for both scan methods or the helical scan with a small reconstructed increment. Although we reached this conclusion using the carbon beam, it may also be applied to proton beam therapy
