[New method of verification of the position of target points in stereotactic radiosurgery by linear accelerator]

A method for quantitatively checking the position to be used for stereotactic radiosurgery was devised using a digital reconstruction radiogram (DRR) and electric portal images (EPI). On each image, the radiation field and head contour were extracted by binary-coded processing. The center of gravity position vector of the extracted image was calculated, and the vector from the head contour to the radiation field was obtained. By comparing the difference in DRR and EPI, an index was made of positional error. The error in the center of gravity position coordinate in changing 40~80 of 256 image tones in the binary operation was 0.5mm or less. The effect on the center of gravity position vector caused by the image distortion of EPI was within 0.5mm in the 200x200mm(2) region. Statistical processing was carried out on this index value, and a 95% confidence interval was estimated. The index value of the Z component of the lateral image became -1.43+/-0.66, and it shifted to the negative side. Error was evaluated by the verification method devised for the target point position. Results indicate the usefulness of the verification method using center of gravity for the target point position.