In image-guided radiotherapy (IGRT), the imaging conditions of computed tomography (CT) may impact the positioning uncertainty, but verification methods are currently unavailable. This study aimed to propose a validation method for the imaging conditions of helical CT for IGRT. Predicting the impact of image distortion on image guidance may reduce uncertainty in radiotherapy planning.

Image guidance was performed on the reference images of four Duracon balls by changing the imaging conditions and the positions on the CT images by helical scanning. The predictors of image guidance error and those of the contour mismatch between the reference and cone-beam CT (CBCT) images were analysed.

The image guidance error exceeded 1 mm when the contour centre of the ball was shifted by more than 1 mm. The mismatch between the contours of the reference and CBCT images occurred with the imaging conditions wherein the first slice of the ball was distorted.

Mismatch can be predicted by the coefficient of variation of the radii in the first and centre slices of the ball. Moreover, the image guidance error can be predicted by the contour centre shift of the ball.