Residual Errors In Pelvic Tumor Patients After Online CBCT Image-guidance

Purpose:To quantify region-specific residual errors after online CBCT corrections was done and total errors without corrections in pelvic tumor patients. Evaluate the effect of correction errors, rotation and deformation on residual errors.Method:The planning CT image and CBCT image before first treatment fraction of218pelvic tumor patients were collected. Then they were registered by computer and confirmed manually, considering translations only. Residual errors were determined by the difference of three-dimensional coordinate of9points of interest on the pelvis. Compare the residual errors when correction errors are corrected to evaluate its effect. The distance between9points of interest is measured to reflect the effect of rotation and deformation. Total errors were defined as residual errors plus couch shift during the first treatment fraction.Result:The SD(Standard deviation) of residual errors are1.17mm-1.94mm,2.39mm-3.16mm and2.85mm-3.91mm in LR, AP and SI direction, which can be significantly reduced after correction errors are corrected. Rotation contribute more to residual errors than deformation. The SD of total errors are2.44mm-2.82mm,3.43mm-4.62mm and4.39mm-5.37mm in LR, AP and SI direction.